| System size | 7 kW |
|---|---|
| Cost before incentives | $17,500 – $24,500 |
| Panels at 400 W | 18 |
| Roof area needed | 387 sq ft |
| Annual production | ≈ 9,800 kWh |
| Offsets a bill of about | $131/month |
How that is worked out
Panels are counted at 400 W each, which is where most residential modules sit today. Production assumes about 1,400 kWh per kW per year — a national planning figure that runs high in the Southwest and low in the Pacific Northwest, so check your own state before treating it as yours. Cost uses the $2.50–$3.50 per watt band most US homeowners see before incentives.
Where this goes wrong
Floor area is a weak proxy for consumption. Two homes the same size can differ by a factor of two depending on heating fuel, whether there is an EV, and how the thermostat is run. If you have twelve months of bills, use those instead — they are the only number that describes your house rather than an average of houses.
Roof area is a hard constraint that catches people out. So does the electrical panel: if it cannot take a solar backfeed breaker, an upgrade is not optional. And under an export credit below retail, a system sized to offset 100% of consumption can be worse value than a smaller one — see net metering explained.
What to do with this
Treat it as a sanity check on a quote, not as a specification. The guide to comparing quotes covers what to hold installers to, and whether a battery is worth it matters more than sizing in states that credit exports below retail.
Planning figures, not an engineering estimate. Your own quote depends on your roof, your consumption and your utility.