Montana Solar Estimate

Methodology

Last updated November 1, 2026

Montana Solar Estimate is an independent, Montana-owned estimate service. We are not an installer, a utility or a lender, and nobody pays to change a number. This page says where each figure comes from, what we assume, and what we do not model, so you can check our work.

Utility rates and net-metering rules

Each of Montana's 26 retail electric utilities has an entry read from its own published residential rate schedule and net-metering or distributed-generation policy, or from its tariff on file with the Montana Public Service Commission for the two investor-owned utilities. 23 of 26 are read directly from those documents; the rest publish no schedule online and are marked unverified on their pages. Every utility page links the source document, prints the schedule's effective date and the date we last read it, and lists what the utility did not publish. Full review: November 1, 2026, repeated each spring. The index is Solar in Montana by utility.

How the rules enter the estimate: savings are valued at the marginal energy rate for each month (seasonal schedules included). A demand charge every member pays is not counted as savings, because rooftop solar rarely lowers a home's billed peak; a demand charge or surcharge that only net-metered accounts pay is subtracted. Surplus production is forfeited or paid out exactly as the policy says, including monthly true-ups. When you upload a bill, the estimate uses the energy price that bill actually paid instead of the schedule.

Which utility serves an address

Service territories come from the Homeland Infrastructure Foundation-Level Data “Electric Retail Service Territories” layer (U.S. Department of Homeland Security and Oak Ridge National Laboratory), Montana subset, 2020 vintage. The layer is coarse at town edges, so the estimate always lets you confirm or change the utility, and corrections are logged to improve the tie-breakers.

Roof and production

Roof planes, pitch, orientation and shading come from Google's Solar API where imagery exists; we fill south-facing planes first (south, southeast, southwest, then east and west) and never due north. Annual production per kilowatt is a Montana reference table built from the PVGIS model on NREL NSRDB irradiance for Montana cities, adjusted for orientation and tilt, with a snow loss derate and 14% system losses. Each estimate names its reference city. The production range is ±8%.

System size

The recommended system offsets 100% of annual use (net zero). Annual use comes from your bill's twelve-month history when you upload one (±5%), otherwise from one month's figure scaled with a Montana load profile for your heating type. Roof area and the utility's net-metering cap are reported as notes rather than used to shrink the size, because ranches and businesses can ground-mount and larger systems can run without export credit.

Installed cost

Cash prices before any incentive, from a Montana size ladder (Oct 2026) built from SolarReviews' Montana size data, EnergySage's Montana averages and published Montana quotes: $3.40 per watt at 4 kW down to $2.45 at 20 kW and above, with a ±10% range. Systems above 20 kW use the top of the ladder; commercial and ground-mount pricing is not separately modeled yet.

Incentives

The federal residential clean energy credit (Section 25D) ended for systems completed after December 31, 2025, so estimates for homes show no federal credit. Business-owned systems, including ranches, farms and rental property, use the 30% investment credit (Section 48E) for systems placed in service by the end of 2027, plus 100% bonus depreciation valued at an assumed combined marginal tax rate; tax-exempt owners receive the 30% as a cash payment (elective pay). No Montana state credit is assumed. Adders, state programs and utility rebates are not assumed. None of this is tax advice.

Savings and payback

First-year savings follow the utility's rule above. Payback is the year cumulative savings pass the installed cost, with electricity prices rising 2.7% a year, an assumption rather than a forecast, and panel output declining 0.5% a year. Where exports earn less than retail, we assume 35% of production is used on site without a battery. Headline figures are the best case for the recommended system; the full range is always shown beneath them.

What we do not model

Corrections

If a utility changes a rate or we have something wrong, email mtsolarestimate@gmail.com. Corrections go into the same table the estimator and these pages read from, so they take effect everywhere at once.