Where the numbers come from
Sun and production
We ran PVWatts version 8 on Nov. 10, 2026 for a 1 kW standard-module system on a fixed roof mount, 20° tilt, facing south (azimuth 180°), with PVWatts' default 14% losses. Weather data: NSRDB typical-year (TMY) files, as reported by PVWatts. We also ran Seattle facing east (90°) and west (270°) to get the direction ratios: east 83.8% and west 86.7% of south.
| City | Lat, lon used | kWh per kW per year |
|---|---|---|
| Seattle, WA | 47.6, -122.33 | 1,088 |
| Bellingham, WA | 48.75, -122.48 | 1,057 |
| Spokane, WA | 47.66, -117.43 | 1,256 |
| Yakima, WA | 46.6, -120.51 | 1,374 |
| Portland, OR / Vancouver, WA | 45.52, -122.68 | 1,128 |
| Eugene, OR | 44.05, -123.09 | 1,176 |
| Bend, OR | 44.06, -121.31 | 1,476 |
| Medford, OR | 42.33, -122.87 | 1,357 |
Our raw PVWatts results are kept with the site's source files.
Roof fit
Panels are rated at 1,000 watts of sunlight per square meter, so size (kW) = area (m²) × 1 kW/m² × efficiency. One square foot is 0.0929 m². The 20% efficiency default is our assumption; check the spec sheet of the panels you're quoted.
Prices
Electricity: EIA Table 5.6.A, residential average for August 2026 (preliminary). Install: Berkeley Lab's 2026 data update reports a median of about $3.60/W for host-owned home systems installed in 2025, about $3.00/W for cash purchases and about $4.50/W for loan-financed systems, all before incentives, and a wide range between installers. Source: Berkeley Lab 2026 Data Update
What we leave out
- Incentives, financing costs, and future rate changes.
- Panel wear over time, snow, and local shade (enter your own shade loss).
- Utility fees, export-credit limits, and battery storage.