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System size, panel count and roof area from your electric usage and sun hours, with equipment list, installed cost, yearly savings and payback.
Average daily equivalent full-sun hours. Your installer's software will refine this for your roof.
Figures include an 80% derate for inverter, wiring, heat and soiling losses.
Midwest sun (4.2 hours), south-facing, 420 W panels, 100% offset.
Add about 250–350 kWh a month for each EV driven 12,000 miles a year.
Who owns the system changes the economics.
Ask every installer for the cash price per watt, even if you plan to finance.
Divide your yearly kWh by (peak sun hours × 365 × 0.8) to get system kW, then divide by panel wattage. A home using 900 kWh a month in the Midwest needs about 9 kW, or 22 panels at 420 W.
About 18 sq ft per modern residential panel, plus setbacks from ridges and edges required by fire code. A 22-panel system needs roughly 400 sq ft of usable roof.
Typical US installed prices are about $2.50–$3.50 per watt for roof-mounted systems, so $22,000–$32,000 for 9 kW, before any incentives.
The 30% residential credit for homeowner-owned systems ended for systems installed after 2025. Leases and PPAs may still pass through a business credit; state and utility rebates vary. Check current rules for your state.
The equivalent hours per day of full-strength sunlight. They range from about 3.5 in the Pacific Northwest to nearly 6 in the desert Southwest.
Yes, at about 15% less production than south. West-facing panels can be worth more on time-of-use rates that peak in the evening.
Not for savings under full net metering. Batteries make sense for backup power or where the utility pays little for exported power.
Panels usually carry 25-year warranties and lose about 0.5% output a year. Microinverters often carry 25 years; string inverters 10–12.