Petroleum County, Montana
What that is equal to
🏠 52
homes powered for a year
at the US average of 10,500 kWh per household
🌍 203 t
CO₂ avoided annually
displacing grid power at 0.371 kg CO₂e/kWh (EPA eGRID)
🚗 44
cars taken off the road
an average passenger vehicle emits 4.6 t CO₂e a year
⚡ 1.56 million
electric vehicle miles
at roughly 0.35 kWh per mile
The pay-off
It pays for itself here. A panel on this roof delivers electricity at 9.1¢ a kWh against a local price of 14.7¢ — 38% cheaper than local power. The install is repaid after 15.1 years, inside the panel's 25-year life.
9.1¢ vs 14.7¢
cost per kWh: this panel vs the grid
levelised over 25 years including 0.5%/yr degradation
$1.17 million
to fit every building here
1,301 panels at $900 installed each
15.1 years
to break even
saving $80.3 thousand a year area-wide at today's price
Assumes $900 to install one 325 W panel — hardware, mounting,
microinverter, electrical work, labour and permitting — repaid only by the electricity it
avoids buying. No subsidies, tax credits, net-metering premium, financing cost or future
price rises are counted, and the figure is undiscounted. Electricity price is the
MT residential average (EIA, May 2026). A single panel is unusually expensive per watt because it
carries a whole array's fixed costs, which is why these numbers look worse than rooftop
solar generally does.
How this is calculated
| Buildings with roofs | 1,301 |
| × Panel rating | 325 W |
| × Annual yield per panel here | 420.7 kWh |
| Annual generation | 547,315 kWh |
Assumptions and limitations
- Yield is specific to this location: 420.7 kWh per panel per
year, from NASA POWER climatology transposed onto a tilted roof plane.
The same panel makes roughly 596 kWh in Phoenix and 265 kWh in Anchorage.
- Assumes a south-facing roof at a typical residential pitch, with a
~22% total system loss for soiling, shading, wiring, inverter, and heat.
- Counts every above-ground structure with a footprint of at least 20 m²,
so garages and outbuildings are excluded but barns and sheds of real
size are not. Building data is machine-derived and imperfect.
- No account is taken of roof orientation, obstructions, structural
condition, or whether the owner wants a panel.
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