How Much Does a Kilowatt of Solar Produce in a Year, and How Many Panels Does That Add Up To?
Summary
It depends heavily on where you live. Running NLR's PVWatts for a south-facing, 20-degree roof with 14% losses, one kilowatt produces about 1,755 kWh a year in Phoenix, 1,291 kWh in New York, and 1,088 kWh in Seattle. To hit the same 8,000 kWh target with 400 W panels, that works out to 12 panels in Phoenix and 19 in Seattle.
Setting the target
Take a home that uses 10,000 kWh a year and wants solar to cover 80% of it. The target is 8,000 kWh. If an EV or heat pump is on the way, add its expected use before you do the math.
Same roof, three cities
Divide 8,000 kWh by the per-kilowatt estimate to get the system size, then divide by 400 W and round up to get the panel count. Phoenix needs about 4.6 kW, or 12 panels. New York needs about 6.2 kW, or 16 panels. Seattle needs about 7.4 kW, or 19 panels. The only thing that changed between runs was the location, and Seattle still needs 1.6 times as many panels as Phoenix. Your own roof's direction, pitch, and shade will move these numbers again.
How firm the estimate is
PVWatts cautions that its predictions include many inherent assumptions and uncertainties. Its expected range is based on 30 years of actual weather data at the location, so a system sized for exactly 8,000 kWh will land a little above or below that in any given year.
Sources
- National Laboratory of the Rockies — PVWatts Calculator
- NLR Developer Network — PVWatts API Version 8 (queried by the editors on September 23, 2026: 1 kW, south-facing, 20° tilt, fixed roof mount, standard module, 14% losses)
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