(1) DESCRIPTION OF THE PLAN

The following description of the Black Hills Corporation 401(k) Retirement Savings Plan (the “Plan”) is provided for general information purposes only. Participants should refer to the plan document for a more complete description of the Plan’s information.

General — The Plan is a defined-contribution plan for eligible employees of Black Hills Corporation and subsidiary companies (the “Company” or “Plan Sponsor”). The Plan is subject to the provisions of the Employment Retirement Income Security Act of 1974, as amended (ERISA) and is designed to comply with the provisions of Section 401(k) of the Internal Revenue Code (IRC).

Plan Administration — Charles Schwab Trust Bank (Charles Schwab) serves as custodian and trustee. The Plan is administered by the Black Hills Corporation Benefits Committee (the “Committee”). Schwab Retirement Plan Services, Inc. is the recordkeeper of the Plan.

Eligibility and Vesting — Employees meeting certain criteria, as defined in the Plan, are eligible to participate in the Plan one month following their date of employment. The Plan includes an automatic enrollment provision whereby all newly eligible employees are automatically enrolled in the Plan unless they affirmatively elect not to participate in the Plan. Automatically enrolled participants have their deferral rate set at 6% of eligible compensation and their contributions are invested in a target retirement fund appropriate for their age until changed by the participant. The Plan includes an Automatic Savings Increase (ASI) feature where the participant’s savings rate will increase 1% annually until their savings rate reaches 15%, they have elected to change their savings rate, or they have elected to opt-out of ASI.

Participants are vested immediately in their contributions, plus actual earnings thereon. Vesting in the Company’s contribution portion of their accounts is based on years of service at a rate of 20% per year. A participant is 100% vested after five years of service. Participants also become fully vested in Company contributions if their employment with the Company is terminated due to retirement at or after attainment of age 65, total and permanent disability, or death.

Contributions — Contributions to the Plan include (i) salary reduction contributions authorized by participants, (ii) matching contributions made by the Company, (iii) non-elective contributions made by the Company; and (iv) participant rollovers from another plan.

Participants may contribute up to 50% of their eligible compensation to the Plan each year. These contributions are subject to certain IRC limitations with an annual maximum pre-tax contribution of $23,500 for 2025 and $23,000 for 2024. Combined employee pre-tax, employee after-tax, employee Roth and employer contributions per participant (excluding catch-up contributions) were limited to the lesser of $70,000 and $69,000 in 2025 and 2024, respectively or 50% of the employee’s available eligible annual pay. Participants who have attained age 50 before the end of the plan year are eligible to make catch-up contributions of $7,500 for both 2025 and 2024 respectively. There is no limit to how often participants may change their contribution percentages. The Plan provides for Company matching contributions and non-elective Company Retirement Contributions for certain eligible participants.

Company Matching Contributions — The Company makes a standard matching contribution which is equal to 100% of each eligible participant’s pre-tax, Roth and after-tax deferral contributions up to 6% of the participant’s compensation as provided under the Plan. All matching contributions are invested as directed by the participant. Company Matching Contributions of $17,905,334 were made to eligible participants in 2025.

Company Retirement Contributions — The Company Retirement Contributions are non-elective contributions made to eligible participants equal to a certain percent of their eligible compensation based upon their age and years of service at a certain date. All Company Retirement Contributions are invested as directed by the participant. Company Retirement Contributions of $11,531,134 were made to eligible participants in January 2026 for the 2025 plan year.

Rollover Contributions — Participants may also contribute amounts representing distributions from other qualified defined benefit or defined-contribution plans. Contributions are subject to certain IRC limitations. The Plan received $5,012,470 in rollover contributions from other qualified plans in 2025.

Participant Accounts — Individual accounts are maintained for each Plan participant. As applicable, each participant’s account is credited with (i) the participant’s contribution, (ii) Company contributions, and (iii) Plan earnings. Participant accounts are also charged an allocation of Plan losses and administrative expenses. Allocations are based on participant earnings or account

(2) SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

(3) FAIR VALUE MEASUREMENTS

ASC 820, Fair Value Measurements and Disclosures, provides a framework for measuring fair value. That framework provides a fair value hierarchy that prioritizes the inputs to valuation methodologies used to measure fair value, as follows:

Assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

Additional information about plan assets, including methods and assumptions used to estimate the fair value of these assets, is as follows:

Mutual funds are categorized as Level 1 and are valued at the closing price reported on the active market on which the individual securities are traded.

Common stock is valued at the closing price reported on the active market on which the securities are traded. Common stock is categorized as Level 1. See Note 7 for further discussion of Black Hills Corporation common stock.

Self-directed brokerage accounts consist of common stocks, mutual fund, and short-term investments that are valued on the basis of readily determinable market prices and are categorized as Level 1. These accounts also consist of bonds, which are valued by evaluated pricing from observable market prices, and are categorized as Level 2.

Common collective trust funds are funds based upon the redemption price of units held by the Plan, which is based on the current NAV of the common collective trust fund’s underlying assets. The NAV, as provided by the custodian, is used as a practical expedient to estimate fair value. Unit values are determined by the financial institution sponsoring such funds by dividing the net assets by its units outstanding at the valuation dates.

Except for the Invesco Stable Value Fund, there are no unfunded commitments, redemption restrictions, or redemption notification requirements. For the Invesco Stable Value Fund, there are no unfunded commitments. There is a 12 month redemption notification period for a plan level withdrawal. There are no participant level redemption restrictions or redemption notice period.

(4) PLAN TERMINATION

Although it has not expressed any intention to do so, the Company has the right under the Plan to amend or discontinue its contributions at any time and to terminate the Plan subject to the provisions set forth in ERISA. In the event that the Plan is terminated, participants would become 100% vested in their accounts.

(5) FEDERAL INCOME TAX STATUS

The Internal Revenue Service (“IRS”) has determined and informed the Company by a letter dated March 31, 2014, that the Plan and related trust were designed in accordance with the applicable provisions of the IRC and regulations thereunder. The Company and Plan management believe that the Plan is currently designed and operated in compliance with the applicable requirements of the IRC, and the Plan and related trust continue to be tax-exempt.

GAAP requires Plan management to evaluate tax positions taken by the Plan and recognize a tax liability (or asset) if the Plan has taken an uncertain position that more likely than not would not be sustained upon examination by the IRS. No uncertain positions were taken by the Plan that would require recognizing a tax liability or asset.

(6) RELATED PARTY TRANSACTIONS AND EXEMPT PARTY-IN-INTEREST TRANSACTIONS

The Plan invests in a fund offered by Charles Schwab and Black Hills Corporation common stock. These transactions qualify as exempt party-in-interest transactions.

At December 31, 2025 and 2024, the Plan held 253,135 and 250,787 shares, respectively, of common stock of Black Hills Corporation, the sponsoring employer, with a cost basis of $13,771,753 and $13,525,925 respectively. The market value of these shares totaled $17,572,606 and $14,676,035 at December 31, 2025 and 2024, respectively. During the year ended December 31, 2025, the Plan recorded dividend income from this investment of $681,267. The plan sponsor has the right to vote the shares of the Company’s common stock held by the Plan.

The Plan also issues loans to participants, which are secured by the vested balances in the participants’ accounts.

At December 31, 2025 and 2024, the Plan held Schwab U.S. Treasury Money Fund of $2,784 and $23,425 respectively.

The Plan also pays certain fees to Charles Schwab on behalf of the Plan for trust and record keeping services. Fees paid to Charles Schwab for trust and record keeping services were $21,450 and $21,825 for the years ended December 31, 2025 and 2024, respectively. These transactions qualify as exempt party-in-interest transactions.

(7) SUBSEQUENT EVENTS

The Plan has evaluated the impact of events that have occurred subsequent to December 31, 2025, through the date the Plan’s financial statements were filed with the SEC. Based on this evaluation, the Plan has determined none of these events were required to be recognized or disclosed.

BLACK HILLS CORPORATION 401(k) RETIREMENT SAVINGS PLAN

(EIN: 46-0458824) (Plan No. 003)

FORM 5500, SCHEDULE H, PART IV, LINE 4i —

SCHEDULE OF ASSETS (held at end of year)

As of December 31, 2025