What changed in this filing

/stocks — the workspaceLOADING

Item 1. Business

2026-02-12 compared with 2025-02-13 · 136 added, 100 removed, 329 unchanged (42% of the section changed)

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We provide electricity and / or natural gas to approximately 850,300 customers in Montana, South Dakota, Nebraska, and Yellowstone National Park.

Upon the completion of the holding company reorganization in 2023, NW Corp became a subsidiary of NorthWestern Energy Group.

Our operations in Montana and Yellowstone National Park are conducted through our subsidiary, NW Corp, and our operations in South Dakota and Nebraska are conducted through our subsidiary, NWE Public Service.

We have provided service in South Dakota and Nebraska since 1923 and in Montana since 2002.

On January 1, 2024, we completed the second and final phase of our holding company reorganization.

NW Corp contributed the assets and liabilities of its South Dakota and Nebraska regulated utilities to NWE Public Service, and then distributed its equity interest in NWE Public Service and certain other subsidiaries to NorthWestern Energy Group, resulting in NW Corp owning and operating the Montana regulated utility and NWE Public Service owning and operating the Nebraska and South Dakota utilities, each as a direct subsidiary of NorthWestern Energy Group.

On August 18, 2025, we entered into the Merger Agreement with Black Hills and Merger Sub, that provides for an all-stock merger of equals between NorthWestern and Black Hills (the Merger).

The Merger Agreement provides for Merger Sub to merge with and into NorthWestern, with NorthWestern continuing as the surviving entity and a direct wholly owned subsidiary of Black Hills, which would assume the new corporate name of Bright Horizon Energy as the resulting parent company of the combined corporate group.

The Merger will combine the strengths of both companies, resulting in an organization with greater scale, financial stability, and operational expertise.

It is designed to create a stronger, more resilient energy company focused on delivering safe, reliable, and affordable energy solutions to customers.

Pursuant to the Merger Agreement, at the effective time of the Merger, each share of common stock of NorthWestern issued and outstanding as of immediately prior to closing will be converted into the right to receive 0.98 validly issued, fully paid and non-assessable shares of common stock of Black Hills, par value $1.00 per share (Black Hills Common Stock).

See Note 3 - Pending Merger with Black Hills Corporation to the Consolidated Financial Statements included herein for additional information regarding this pending Merger.

We manage our businesses by the nature of services provided, and operate principally in two operating segments:

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Because natural gas is used primarily for residential and commercial heating, the demand for this product depends heavily upon weather patterns throughout our service territory, and a significant amount of natural gas revenues are recognized in the first and fourth quarters related to the heating season.

For over a century, we have upheld our commitment to providing reliable and affordable electric and natural gas services while also being good stewards of the environment.

Our approach to sustainability encompasses not only environmental care but also strong corporate governance, meaningful community engagement, and a dedication to delivering long-term value for all stakeholders.

Our energy portfolio reflects a balanced mix of clean and traditional resources, ensuring we can deliver reliable electricity to customers around the clock.

In 2024, approximately 58 percent of our electric portfolio—comprising owned and long-term resources—was carbon-free, exceeding the U.S.

electric power industry average of 41 percent in 2023 (source:

Energy Information Administration).

However, our resource mix may not fully represent the actual energy delivered to customers due to market-based energy exchanges.

We do not receive all of the related Renewable Energy Credits (RECs) from our contracted electric supply resources and periodically sell RECs produced by our own carbon-free energy resources.

The owner of the RECs claims the renewable attributes of the energy.

We have a social responsibility to our customers to deliver reliable energy at an affordable price.

Our reliability standards and results are typically in the first or second quartile compared to our peers and our typical residential bills are significantly lower than the national average.

We are engaged with the communities we serve, hosting town meetings to provide updates on important matters relating to both NorthWestern and our residents.

For our employees, we offer a career with competitive pay and great benefits, providing a balance between work and life.

We have been recognized by Newsweek as one of the “Great Places to Work” three years in a row.

Over time, we have increased our environmental sustainability efforts and our access to carbon-free energy resources.

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Our Scope 2 emissions are primarily from the electric and natural gas utilized to heat, cool and power our offices, warehouses and other facilities.

Delivering affordable, reliable energy is central to our mission and reflects our responsibility to customers and the communities we serve.

Our reliability standards consistently rank in the first or second quartile compared to industry peers, and our typical residential energy bills remain significantly below the national average.

We recognize that our employees are key to our success, and we are committed to providing fulfilling careers with competitive pay, excellent benefits, and a strong work-life balance.

This commitment has earned us recognition from Newsweek as one of the "Great Places to Work" in 2023 and 2024.

Our governance practices underscore our dedication to ethical conduct, transparency, and accountability.

We adhere to a robust Code of Conduct and Ethics that applies to employees, board members, vendors, and contractors, with additional requirements for senior leaders to ensure the highest standards of integrity in financial reporting and decision-making.

We currently own a mix of clean and carbon-free energy resources balanced with traditional energy sources that are necessary for us to deliver affordable and reliable electricity to our customers 24/7.

In 2025, approximately 52 percent of our electric portfolio, from owned and long-term resources, was carbon-free, compared to approximately 41 percent (Source:

Energy Information Administration, Annual Energy Review, Electricity Net Generation:

Electric Power Sector) for the total U.S.

electric power industry in 2024.

We do not receive all of the related Renewable Energy Credits (RECs) from our contracted electric supply resources.

The owner of the RECs claims the renewable attributes of the energy.

Our resource mix does not represent the actual energy delivered to our customers.

Market purchases and sales fill the gaps between customer demand and economic dispatch of our supply portfolio resources.

In 2025, we completed construction of the 175 MW YCGS electric generation facility and in 2026, we expect to complete construction of Aberdeen Generating Station Units 3 & 4, a 33 MW natural gas fired electric generating facility located in Aberdeen, South Dakota.

On January 1, 2026, we acquired an additional 592 MW of ownership interest in Colstrip Units 3 & 4.

As discussed further below, in order to meet the SPP's additional accredited capacity needs by 2030, we submitted a project with the SPP under their Expedited Resource Adequacy Study program for the construction of a 131 MW natural gas generating facility located in Aberdeen, South Dakota.

While still striving to meet our environmental goal of Net Zero by 2050, these generation facilities are needed to support our efforts to provide reliability and affordability to our customers.

Since 2011, we have added 1,096 MW of carbon-free generation (both owned and long-term contract) in Montana and South Dakota.

We believe that technological advancements, along with decreasing costs of carbon-free generation and the regionalization of intermittent generation, will significantly contribute to our Net-Zero goal.

The pace of transition to Net-Zero will depend on the timing of technological advancements, costs, and retirement of our existing coal fleet.

We are committed to conducting business with integrity, while ensuring transparency and accountability, and meeting our responsibilities to all our stakeholders.

We adhere to a strict code of ethics regarding corporate governance.

Our Code of Conduct and Ethics applies to all employees, board members, vendors and contractors, with additional Code of Ethics for the Chief Executive Officer and Senior Financial Officers concerning financial reporting and other related matters.

MONTANA ELECTRIC OPERATIONS

Our regulated electric utility business in Montana, which is conducted through NW Corp, includes generation, transmission and distribution.

Our regulated electric utility business in Montana, which is primarily conducted through NW Corp, includes generation, transmission and distribution.

Our service territory covers approximately 107,600 square miles, representing approximately 73 percent of Montana's land area.

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In addition to delivering energy to distribution systems to serve customers, we also transmit electricity for nonregulated entities owning generation, and utilities, cooperatives, and power marketers serving the Montana electricity market.

Our total control area peak demand reached a peak of approximately 2,079 MWs on January 13, 2024.

Our total control area peak demand reached a peak of approximately 1,976 MWs on February 13, 2025.

Our control area average demand for 2025 was approximately 1,345 MWs per hour, with total energy delivered of approximately 11.78 million MWHs.

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Our annual retail electric supply load requirement averages approximately 767 MWs per day, with a peak load of approximately 1,300 MWs, and are supplied by owned and contracted resources and market purchases with multiple counterparties.

Owned generation resources supplied approximately 65 percent of our retail load requirements for 2024.

We expect that approximately 81 percent of our retail obligations will be met by owned generation resources in 2025, reflecting the full year addition of YCGS which was placed into service in October 2024.

In 2025, our owned generation resources generated electric supply representing approximately 67 percent of our retail load.

We expect that our owned generation resources will generate electric supply representing approximately 95 percent of our forecasted retail load for 2026.

The increase relative to the prior year is due to the acquisition of additional interests in Colstrip Units 3 & 4 from Avista.

In addition, we have contracts with QFs totaling 569 MWs of nameplate capacity, including 107 MWs from waste petroleum coke and waste coal, 268 MWs from wind, 17 MWs from hydro, and 177 MWs from solar projects.

We have several other long-term power purchase agreements including contracts for 135 MWs nameplate capacity from wind generation, 310 MWs from unspecified resources, 52 MWs of natural gas generation, and 13 MWs of hydro supply.

On average, our owned and long-term contracted resources are expected to provide enough energy to meet our retail energy load requirements in 2026.

Load requirements during peak demand in excess of our owned and long-term contracted resources will be satisfied with market purchases.

Our owned and contracted resources will be economically dispatched to meet load requirements and any excess supply will be sold into the market.

Owned Generation Facilities

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Morony 1930 Missouri 2040 49

Mystic (Rowe Dam)

1925 West Rosebud Creek 2050 12

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Thompson Falls 1915/1995 Clark Fork 2026 (1)

(1) The FERC Project is still called Mystic Lake, one of the 3 dams is now called Rowe Dam

(1) We are in the process of relicensing the Thompson Falls hydro facility.

We anticipate that the FERC will issue an annual license for 2026, and that this annual license will renew on an annual basis until our relicensing proceeding is complete.

The FERC extended our current license for 2026, and we anticipate that they will continue to extend on an annual basis until our relicensing proceeding is complete.

(2) The Hebgen facility (0 MW net capacity) is excluded from the figures above.

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Interest Owned

Colstrip Unit 4, located near Colstrip in southeastern Montana Sub-bituminous coal 30% 222

Colstrip Units 3 & 4, located near Colstrip in southeastern Montana (1)

Sub-bituminous coal 55% 814

DGGS, located near Anaconda, Montana Natural Gas & Liquid Fuel 100% 150

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Two Dot Wind, located in Wheatland County in Montana Wind 100% 11

Colstrip Unit 4 provides base-load supply and is operated by Talen Montana, LLC (Talen).

(1) We have three separate ownership interests in Colstrip facility, a 30 percent ownership interest in Unit 4, a 15 percent ownership interest in Colstrip Units 3 & 4, and a 25 percent ownership interest in Colstrip Units 3 & 4, totaling to a 55 percent ownership interest in Colstrip Units 3 & 4.

Colstrip Units 3 & 4 provide base-load supply and are operated by Talen Montana, LLC (Talen).

Talen has a 30 percent ownership interest in Colstrip Unit 3.

We have a reciprocal sharing agreement with Talen regarding the operation of Colstrip Units 3 and 4, in which each party receives 15 percent of the respective combined output and is responsible for 15 percent of the respective operating and construction costs, regardless of whether a particular cost is specified to Colstrip Unit 3 or 4.

Associated with our 30 percent ownership interest in Colstrip Unit 4, we have a reciprocal sharing agreement with Talen regarding the operation of Colstrip Units 3 and 4, in which each party receives 15 percent of the respective combined output and is responsible for 15 percent of the respective operating and construction costs, regardless of whether a particular cost is specified to Colstrip Unit 3 or 4.

However, each party is responsible for its own fuel-related costs.

Colstrip Unit 4 is supplied with fuel from adjacent coal reserves under a coal supply agreement in effect through 2025.

See Item 1A Risk Factors "Regulatory, Legislative and Legal Risks" for further discussion regarding the service life of generation facilities.

On January 1, 2026, we acquired an additional 40 percent ownership interest in Colstrip Units 3 & 4.

This includes the acquisition of ownership interests previously owned by Avista Corporation (Avista) (222 MW) and Puget Sound Energy (Puget) (370 MW) that brought our total share of Colstrip Units 3 & 4 up to 814 MW, or 55 percent of the plant capacity.

We have a coal supply agreement to supply fuel from adjacent coal reserves for our ownership interests in Colstrip Units 3 & 4 that is effective through 2033.

The 222 MW interest in Colstrip Units 3 & 4 acquired from Avista is owned by NW Corp and is included in our MPSC regulated supply portfolio.

The 370 MW interest in Colstrip Units 3 & 4 acquired from Puget is owned by NW Colstrip 370.

This resource is not included within the MPSC regulated resource supply portfolio at this time.

We expect our future opportunity to serve growing utility customer demand, including large-load customers, may be supported by this resource.

Resource Planning

Resource planning is an important function necessary to meet our customers' future energy needs and is used to guide resource acquisition activities.

We filed our latest resource plan with the MPSC in April of 2023.

The filing showed a capacity adequate resource portfolio in the near term that included an online date of the YCGS in 2024 and the acquisition of an additional 222 MW of capacity in Colstrip in 2026.

In July of 2024, NorthWestern announced that an additional 370 MW would be acquired at Colstrip in 2026.

These two additional Colstrip acquisitions would increase NorthWestern’s current Colstrip ownership from 222 MW to 814 MW, effective January 1, 2026.

For additional information regarding the acquisition of additional Colstrip capacity, see Note 5 - Property , Plant, and Equ ipment , to the Consolidated Financial Statements.

In addition to our responsibility to meet peak demand, national NERC reliability standards increased the need for us to have greater dispatchable generation capacity available and be capable of increasing or decreasing output to address intermittent generation such as wind and solar.

Our generation portfolio is a balanced mix of energy and capacity resources having different operating characteristics and fuel sources designed to provide energy at the lowest possible cost to meet our obligation to serve retail customers while maintaining reliability.

Our Draft 2026 IRP is publicly available and is scheduled to be filed with the Commission in April 2026.

This IRP is our first plan developed under the 2023 Montana administrative rules, which transitioned the planning process from a traditional resource adequacy focus to an integrated evaluation of generation, transmission, fuel supply, and reliability.

Our previous resource plan was filed with the MPSC in April 2023.

While 2026 capacity adequacy is maintained due to the recent additions of YCGS and Avista’s 222 MW share of Colstrip Units 3 & 4, the 2026 IRP identifies emerging capacity needs beginning in 2027 driven by load growth, capacity contract expirations, and resource retirements.

Consistent with regional practice, NorthWestern evaluates resource adequacy through the Western Resource Adequacy Program, which establishes planning reserve margin requirements based on probabilistic reliability metrics and seasonal system conditions.

In addition to our responsibility to meet peak demand, national NERC reliability standards continue to require sufficient dispatchable generation capable of increasing or decreasing output to manage system variability, including the growing demand of intermittent generation such as wind and solar.

Our generation portfolio remains a balanced mix of energy and capacity resources having different operating characteristics and fuel sources designed to reliably serve retail customers at the lowest possible cost while maintaining reliability.

Western Energy Imbalance Market

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and West Central Electric Cooperative.

We are a transmission-owning member in the SPP, with our transmission facilities residing in zone 19 of the SPP footprint.

We are a member in the SPP, with our transmission facilities residing in zone 19 of the SPP footprint.

Each year, we review all new or modified transmission assets and transfer functional control of assets that qualify under the SPP Tariff to the SPP.

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To date, we have transferred control of 333 line miles of 115 kV facilities and over 158 line miles of 69 kV facilities.

While we have transferred functional control of these facilities to the SPP, they are still owned by us and reflected in the table above.

Along with SPP, our South Dakota facilities have ties to MISO.

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Miscellaneous combustion turbine units and small diesel units (used only during peak periods) Combination of fuel oil and natural gas 100.0% 12

(1) We are currently upgrading Aberdeen Generating Unit No.

Refer to the Resource Planning section below for further information.

The Big Stone, Coyote and Neal plants are owned jointly with unaffiliated parties.

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Resource Planning

We have a resource plan that includes estimates of customer usage and programs to provide for the economic, reliable and timely supply of energy.

We continue to update our load forecast to identify the future electric energy needs of our customers, and we evaluate additional generating capacity requirements on an ongoing basis.

We submitted a plan to the SDPUC in December of 2024 that provided for the modernization of our generating fleet.

That plan focused on improving reliability and flexibility.

We are currently upgrading Aberdeen Generating Unit No.

1 to a 28 MW natural gas fired generating facility.

This generating facility includes two simple cycle natural gas combustion turbines and we anticipate it being in service in 2026 for a total projected cost of $65.0 million.

We maintain an integrated resource planning process that includes forecasts of customer energy usage and evaluates a range of options to provide for the economic, reliable, and timely supply of energy.

We regularly update our load forecasts to reflect changes in customer demand, industrial growth, and regional planning requirements, and we assess future generating capacity needs on an ongoing basis.

In September 2024, we submitted an updated 2024 IRP to the South Dakota Public Utilities Commission that incorporates updated load forecasts, revised planning reserve margin requirements, updated resource cost and performance assumptions, and an evaluation of a range of resource portfolios over a twenty-year planning horizon.

Consistent with the 2024 IRP, we are replacing older generation resources at the Aberdeen Generating Station, with construction of Aberdeen Generating Station Units 3 & 4 underway and an expected in-service date in 2026.

This project is expected to provide approximately 33 MW of nameplate capacity at a total projected cost of approximately $65.0 million, of which approximately $29.0 million is expected to be incurred in 2026.

The SPP has recently updated its resource accreditation and Planning Reserve Margin (PRM) requirements in response to growing reliability concerns.

As a result, SPP is requiring additional accredited capacity by 2030 to meet the updated PRM targets.

In October 2025, we submitted a project with the SPP under their Expedited Resource Adequacy Study program for the construction of a 131 MW natural gas generating facility located in Aberdeen, South Dakota, to meet regional capacity needs by 2030.

Anticipated costs for this project are approximately $300.0 million.

NATURAL GAS OPERATIONS

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During 2025, we distributed natural gas to approximately 249,400 customers in 123 Montana communities over a system that consists of approximately 5,900 miles of underground distribution pipelines.

We also serve several smaller distribution companies that provide service to approximately 34,000 customers.

We also serve a smaller distribution company that provides service to approximately 1,417 customers.

We transmit natural gas in Montana from production receipt points and storage facilities to distribution points and other nonaffiliated transmission systems.

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City Gate Stations 134

We have connections in Montana with four major, unaffiliated transmission systems:

Williston Basin Interstate Pipeline, NOVA Gas Transmission Ltd., Colorado Interstate Gas, and Spur Energy.

Twelve compressor sites provide more than 48,600 horsepower on the transmission line and an additional 15,900 horsepower at our storage fields, capable of moving more than 400,000 dekatherms per day.

We have connections in Montana with five major, unaffiliated transmission systems:

Williston Basin Interstate Pipeline, NOVA Gas Transmission Ltd., Colorado Interstate Gas, Black Hills Energy, and Many Islands Pipeline.

Thirteen compressor sites provide more than 51,400 horsepower on the transmission line and an additional 16,586 horsepower at our storage fields, capable of moving more than 400,000 dekatherms per day.

In addition, we own and operate two transmission pipelines through our subsidiaries, Canadian-Montana Pipe Line Corporation and Havre Pipeline Company, LLC.

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Our Montana natural gas supply requirements for electric generation fuel for the year ended December 31, 2025, were approximately 10.8 Bcf.

We have contracted with several major producers and marketers with varying contract durations to provide the anticipated supply to meet ongoing requirements.

We have contracted with several major

producers and marketers with varying contract durations to provide the anticipated supply to meet ongoing requirements.

Our natural gas supply requirements are fulfilled through third-party fixed-term purchase contracts, short-term market purchases and owned production.

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Owned Production and Storage - Since 2010, we have acquired gas production and gathering system assets as a part of an overall strategy to provide rate stability and customer value:

as we own these assets, which are regulated, our customers are protected from potential price spikes in the market.

as we own these assets, which are regulated, our customers are better protected from potential price spikes in the market.

As of December 31, 2025, these owned reserves totaled approximately 25.3 Bcf and are estimated to provide approximately 2.76 Bcf in 2026, or approximately 14 percent of our expected annual retail natural gas load in Montana.

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Municipal Natural Gas Franchise Agreements

We have municipal franchises to provide natural gas service in the communities we serve.

We have municipal franchises to provide natural gas service in the communities we serve in Nebraska and South Dakota.

The terms of the franchises vary by community.

Our Montana franchises typically have a fixed 10-year term and continue for additional 10-year terms unless and until canceled, with 5 years notice.

The maximum term permitted under Nebraska law for these franchises is 25 years while the maximum term permitted under South Dakota law is 20 years.

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We continue to serve those customers while we obtain formal renewals.

During the next five years, eleven of our Montana franchises could expire by action taken by the franchises' city or town, which account for approximately 79,362 or 37.95 percent of our Montana natural gas customers.

Three of our South Dakota franchises and two franchises in Nebraska, which account for approximately 39, 496 or 42 percent of o ur South Dakota and Nebraska natural gas customers, are scheduled to reach the end of their fixed term during the next five years.

In the 2025 Montana legislative session, new language was enacted eliminating the requirement for gas franchise agreements.

Under this law, utility providers may construct and maintain natural gas lines within the public rights-of-way without obtaining a franchise agreement.

Two of our South Dakota franchises and two of our franchises in Nebraska, which account for approximately 32,613 or 34 percent of o ur South Dakota and Nebraska natural gas customers, are scheduled to reach the end of their fixed term during the next five years.

We do not anticipate termination of any of these franchises.

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Montana electric delivery and production (1)

November 2023 $2,565.5 $3,330.9 6.72% 9.65% 48.02%

Montana - Colstrip Unit 4 November 2023 276.9 257.8 8.25% 10.00% 50.00%

February 2026 $3,176.2 $3,425.6 7.00% 9.65% 47.84%

Montana - Colstrip Unit 4 February 2026 256.7 256.0 8.25% 10.00% 50.00%

Montana natural gas delivery and production (2)

November 2023 582.8 814.8 6.67% 9.55% 48.02%

February 2026 757.3 886.6 6.97% 9.60% 47.84%

Montana natural gas delivery - Great Falls Gas

October 2018 (3)

17.5 27.4 6.91% 9.20% 50.97%

Total Montana (4)

$4,207.7 $4,595.6

South Dakota electric (5)

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Nebraska natural gas (5)

December 2007 $24.3 $52.5 8.49% 10.40% n/a

$46.0 $54.7 7.09% 9.55% n/a

$5,141.7 $5,569.7

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(2) The Montana gas revenue requirement includes a step-down which approximates annual depletion of our natural gas production assets included in rate base.

(3) This jurisdiction was acquired in 2025 as part of the acquisition of Energy West Operations.

For additional information regarding this acquisition, see Note 4 - Acquisition of Energy West Operations to the Consolidated Financial Statements included herein.

(4) This table excludes insignificant jurisdictions for Montana propane delivery, Havre Pipeline Company, and Cut Bank Gas natural gas delivery.

(5) For those items marked as "n/a," the respective settlement and/or order was not specific as to these terms.

(4) In June 2024, we filed a South Dakota natural gas rate review filing (2023 test year) with the SDPUC and a Nebraska natural gas rate review filing.

(2023 test year) with the NEPSC.

See Note 3 - Regulatory Matters , to the Consolidated Financial Statements for additional discussion of this rate review filing.

MPSC Regulation

Our Montana operations are subject to the jurisdiction of the MPSC with respect to rates, terms and conditions of service, accounting records, electric service territorial issues and other aspects of our operations, including when we issue, assume, or guarantee securities in Montana, or when we create liens on our regulated Montana properties.

Nearly all of our Montana operations are subject to the jurisdiction of the MPSC with respect to rates, terms and conditions of service, accounting records, electric service territorial issues and other aspects of our operations, including when we issue, assume, or guarantee securities in Montana, or when we create liens on our regulated Montana properties.

We have an obligation to provide service to our customers with an opportunity to earn a regulated rate of return.

Electric Supply Tracking Mechanism - The PCCAM tracks, for recovery through utility rates, the cost of power purchased and fuel used to generate electricity.

The PCCAM incorporates sharing of a portion of the business risk or benefit associated with the energy supply costs with 90 percent of the variance above or below the established base revenues and actual costs collected from or refunded to customers.

Certain PCCAM rates are adjusted on a quarterly basis for volumes and costs during

each July to June 12-month tracking period based on the established base revenues and actual costs collected from or refunded to customers.

The PCCAM incorporates sharing of a portion of the business risk or benefit associated

with the energy supply costs with 90 percent of the variance above or below the established base revenues and actual costs collected from or refunded to customers.

As part of its December 2025 order, the MPSC has suspended the 90/10 cost sharing mechanism of the PCCAM on a temporary basis pending further review by the MPSC.

See Note 5 - Regulatory Matters within the Consolidated Financial Statements for further information.

Certain PCCAM rates are adjusted on a quarterly basis for volumes and costs during each July to June 12-month tracking period based on the established base revenues and actual costs collected from or refunded to customers.

Customer prices may be adjusted annually to absorb the difference for the annual tracking period.

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The adjustment clauses for both electric and gas utilities go into effect upon filing, and are deemed approved within 10 days after the information filing unless the SDPUC Staff requests changes during that period.

Phase In Rate Plan Rider - Effective January 2024, we received approval of a Phase in Rate Plan Rider, which may allow recovery of capital investments without filing a general electric rate review.

Phase In Rate Plan Rider - Effective July 1, 2025, we received approval to begin recovering costs for Aberdeen Generating Station Units 3 & 4 through our Phase in Rate Plan Rider.

This tariff allows recovery of capital investments without filing a general electric rate review.

SDPUC approval of the plan and associated project cost recovery are required.

An update to this plan is required to be filed with the SDPUC by June 1 of each year.

NPSC Regulation

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We are subject to FERC's jurisdiction and regulations with respect to rates for electric transmission service and electricity sold at wholesale, hydro licensing and operations, the issuance of certain securities, incurrence of certain long-term debt, and compliance with mandatory reliability standards, among other things.

Under FERC's open access transmission policy, as

owners of transmission facilities, we are required to provide open access to our transmission facilities under filed tariffs at cost-based rates.

Under FERC's open access transmission policy, as owners of transmission facilities, we are required to provide open access to our transmission facilities under filed tariffs at cost-based rates.

In addition, we are required to comply with FERC's Standards of Conduct for Transmission Providers.

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These transmission rates are adjusted annually through formula rates.

The electricity sold from NW Colstrip 370, which has a 25 percent ownership interest in Colstrip Units 3 & 4, is subject to the FERC's jurisdiction and regulations with respect to rates for electricity sold at wholesale.

We have submitted a request to the FERC for approval of cost-based rates and expect this rate approval to be effective in the first quarter of 2026.

We have signed a contract to sell the dispatchable capacity and associated energy from NW Colstrip 370 through late 2027.

This contract is subject to the approval by the FERC.

Our natural gas transportation pipelines are generally not subject to FERC's jurisdiction, although we are subject to state regulation.

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We have capacity agreements in South Dakota and Nebraska with interstate pipelines that are also subject to FERC jurisdiction.

Our hydroelectric generating facilities are licensed by the FERC and operated under the terms of those licenses and FERC regulations.

In connection with the relicensing of these generating facilities, applicable law permits the FERC to issue a new license to the existing licensee, to a new licensee, or alternatively allows the U.S.

government to take over the facility.

If the existing licensee is not relicensed, it is compensated for its net investment in the facility, not to exceed the fair value of the property taken, plus reasonable severance damages to other property affected by the lack of relicensing.

Our hydroelectric generating facilities are licensed by the FERC and operated in accordance with the terms of those licenses and applicable FERC regulations.

As part of the relicensing process, federal law authorizes FERC to issue a new license to the current licensee, to a different licensee, or, alternatively, permits the U.S.

government to assume ownership and operation of the facility.

If the existing licensee is not granted a new license, it is entitled to compensation equal to its net investment in the facility, not to exceed the fair value of the property taken, plus reasonable severance damages for other property adversely affected by the loss of the license.

Reliability Standards - We must comply with the standards and requirements that apply to the NERC functions for which we have registered in both the MRO for our South Dakota operations and the WECC for our Montana operations.

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ENVIRONMENTAL

The operation of electric generating, transmission and distribution facilities, and gas gathering, storage, transportation and distribution facilities, along with the development (involving site selection, environmental assessments, and permitting) and construction of these assets, are subject to extensive federal, state, and local environmental and land use laws and regulations.

construction of these assets, are subject to extensive federal, state, and local environmental and land use laws and regulations.

Our activities involve compliance with diverse laws and regulations that address emissions and impacts to the environment, including air and water, and protection of natural resources and wildlife.

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As new laws or regulations are issued, we assess their applicability and implement the necessary modifications to our facilities or their operation to maintain ongoing compliance.

President Trump issued an Executive Order on January 20, 2025, "Unleashing American Energy," directing all federal executive agency heads to review all agency actions implicating energy reliability and affordability or potentially burdening the development of domestic energy resources.

It is not yet clear how existing regulations affecting existing fossil fuel assets will be reconsidered or repealed.

If the MATS Rules remain enforced, between now and 2028 total compliance costs for the Colstrip plant is estimated to range from $350 million to $665 million, of which we would be responsible for our proportionate share.

In 2024, the EPA released final rules that will potentially impose requirements on fossil fuel assets, however, in 2025, the EPA issued multiple Notices of Proposed Rulemaking that would remove these additional requirements on fossil fuel assets.

There is no mandated timeline for final action on these rules.

If the MATS Rules are implemented and enforced as currently written, between now and 2028 total compliance costs for the Colstrip plant are estimated to range from $350 million to $665 million, of which we would be responsible for our proportionate share.

This estimate has been developed by the Colstrip operator and represents an initial high-level scoping estimate that will require significant refinement to narrow the range of costs, which is currently underway.

Without change, implementation of existing climate related regulations have the potential to limit or curtail our operations, including the burning of fossil fuels at our coal-fired and some natural gas power plants.

Without change, implementation of existing environmental regulations have the potential to limit or curtail our operations, including the burning of fossil fuels at our coal-fired and some natural gas power plants.

While we strive to comply with all environmental regulations applicable to our operations, it is not possible to determine when or to what extent additional facilities or modifications of existing or planned facilities will be required as a result of changes to energy and environmental laws and regulations, or new administrative or judicial interpretations or enforcement decisions regarding them.

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Of these, 1,353 employees were in Montana and 314 were in South Dakota or Nebraska.

Of our Montana employees, 462, or 36 percent, were covered by seven collective bargaining agreements involving five unions, which were renegotiated in 2022.

Each of the seven Montana collective bargaining agreements negotiated in 2022 will expire in 2026.

Of our South Dakota and Nebraska employees, 165, or 54 percent, are covered by a collective bargaining agreement renegotiated in 2021 that expires in 2025.

Additionally, there are ten employees who are in the process of negotiating a collective bargaining agreement.

Of our Montana employees, 489, or 36 percent are covered by five unions under eight collective bargaining agreements.

All eight of these collective bargaining agreements are set to expire in 2026 and are currently under renegotiation.

Of our South Dakota and Nebraska employees, 167, or 53 percent, are covered by a collective bargaining agreement that is set to expire in 2026 and is now being renegotiated.

We consider our relations with employees and union leadership to be positive and respectful.

3 unchanged sentences

This includes an integrated learning and performance management system which includes annual performance reviews that link goals and competencies together so that managers are able to provide a holistic view to employees in regards to their performance against goals as well as key competencies as they relate to their role in the organization.

This process provides opportunities to develop and enhance skills and knowledge, and enables our employees to grow professionally and perform their duties in a safe and efficient manner.

This process provides opportunities to develop and enhance skills and knowledge, and enables our

employees to grow professionally and perform their duties in a safe and efficient manner.

This structured training and development is intended to provide employees a consistent learning experience, and maximizes learning retention and background knowledge.

25 unchanged sentences

During the years ended December 31, 2025 and 2024, our recordable incident rates were 1.73 and 1.65, respectively, and lost time incident rates were 0.54 and 0.57, respectively, on a company wide basis.

As of December 31, 2024, our five-year average safety performance exceeded the five-year average of our industry peers.

INFORMATION ABOUT OUR EXECUTIVE OFFICERS

Executive Officer Current Title and Prior Employment Age (1)

Bird President and Chief Executive Officer and Director of NorthWestern Energy Group, Inc., since October 2, 2023, and of NorthWestern Energy Public Service Corporation since January 1, 2024, and of NorthWestern Corporation since January 2023;

formerly President and Chief Operating Officer of NorthWestern Corporation since February 2021 and Chief Financial Officer from December 2003 to February 2021.

Lail Vice President and Chief Financial Officer of NorthWestern Energy Group, Inc., since October 2, 2023, and of NorthWestern Energy Public Service Corporation since January 1, 2024, and of NorthWestern Corporation since February 2021;

formerly Vice President and Chief Accounting Officer of NorthWestern Corporation since April 2020;

Bird President and Chief Executive Officer and Director of NorthWestern Energy Group, Inc., since October 2, 2023, and of NWE Public Service since January 1, 2024, and of NW Corp since January 2023;

formerly President and Chief Operating Officer of NW Corp since February 2021 and Chief Financial Officer from December 2003 to February 2021.

Lail Vice President and Chief Financial Officer of NorthWestern Energy Group, Inc., since October 2, 2023, and of NWE Public Service since January 1, 2024, and of NW Corp since February 2021;

formerly Vice President and Chief Accounting Officer of NW Corp since April 2020;

and Vice President and Controller from October 2015 to April 2020.

Heim Vice President - General Counsel and Federal Government Affairs of NorthWestern Energy Group, Inc., since October 2, 2023, and of NorthWestern Energy Public Service Corporation since January 1, 2024, and of NorthWestern Corporation since January 2023;

formerly Director, Regulatory Corporate Counsel of NorthWestern Corporation since June 2020;

Heim Vice President - General Counsel and Federal Government Affairs of NorthWestern Energy Group, Inc., since October 2, 2023, and of NWE Public Service since January 1, 2024, and of NW Corp since January 2023;

formerly Director, Regulatory Corporate Counsel of NW Corp since June 2020;

and formerly Equity Shareholder at the law firm of Moss & Barnett, P.A.

from 2017 to 2020.

Lafave Vice President - Asset Management & Business Development of NorthWestern Corporation since June 2023 and of NorthWestern Energy Public Service Corporation since January 1, 2024;

formerly Director of Long-Term Resources of NorthWestern Corporation since 2003.

Schroeppel Vice President - Customer Care, Communications and Human Resources of NorthWestern Corporation since May 2009 and of NorthWestern Energy Public Service Corporation since January 1, 2024.

Merkel Vice President - Distribution of NorthWestern Corporation since September 2022 and of NorthWestern Energy Public Service Corporation since January 1, 2024;

formerly General Manager - Operations and Construction of NorthWestern Corporation since 2007.

Vold Vice President - Technology of NorthWestern Corporation since February 2021 and of NorthWestern Energy Public Service Corporation since January 1, 2024;

formerly Business Technology Officer of NorthWestern Corporation since 2012.

Hines Vice President - Supply and Montana Government Affairs of NorthWestern Corporation since January 2018 and of NorthWestern Energy Public Service Corporation since January 1, 2024;

formerly Vice President - Supply of NorthWestern Corporation since May 2011.

Cashell Vice President - Transmission of NorthWestern Corporation since May 2011 and of NorthWestern Energy Public Service Corporation since January 1, 2024.

Lafave Vice President - Asset Management & Business Development of NW Corp since June 2023 and of NWE Public Service since January 1, 2024;

formerly Director of Long-Term Resources of NW Corp since 2003.

Schroeppel Vice President - Customer Care, Communications and Human Resources of NW Corp since May 2009 and of NWE Public Service since January 1, 2024.

Merkel Vice President - Distribution of NW Corp since September 2022 and of NWE Public Service since January 1, 2024;

formerly General Manager - Operations and Construction of NW Corp since 2007.

Vold Vice President - Technology of NW Corp since February 2021 and of NWE Public Service since January 1, 2024;

formerly Business Technology Officer of NW Corp since 2012.

Vice President - Transmission of NW Corp since May 2011 and of NWE Public Service since January 1, 2024.

(1) As of February 6, 2026.

(2) Michael Cashell has announced that he will retire from the Company in April of 2026.

Officers are elected annually by, and hold office at the pleasure of, the Board of Directors (Board), and do not serve a “term of office” as such.

Compared sentence by sentence after normalising whitespace, quotation marks, case and digits, so re-formatting and restated figures do not read as changed language. Wording changes appear as one removal and one addition. The current filing and the prior one are authoritative.

What changed 10-K

Item 1. Business