Panel count
How many solar panels do I need?
For the 900 kWh a month the average US home uses, about 22 panels at 400 watts, an 8.8 kW system. The same bill needs 15 panels in Phoenix and 27 in Seattle, because the sun does not read your electric bill. Put in your usage and your state; the tally updates as you type.
How the tally works
Three numbers decide everything. Your daily usage: the monthly kWh on your bill divided by 30.4. Your sun: the peak-sun-hours figure for your state, which is just total daily solar energy restated as hours of full-strength sun (explainer here). And your panel's real-world output: rated watts times those hours times 0.77, the NREL quick-estimate derate that pays for wiring losses, dirt, mismatch, inverter conversion, and heat. Divide usage by per-panel output, round up, done.
Worked example, the national averages: 900 kWh a month is 29.6 kWh a day. A 400W panel under 4.5 peak sun hours makes 0.4 × 4.5 × 0.77 = 1.39 kWh a day. 29.6 ÷ 1.39 = 21.4, so 22 panels. Every table on this site is computed by that same formula, and the methodology page lists every constant in it.
One honest caveat, once: this is a planning estimate built on statewide sun averages and a default derate. Your roof's direction, pitch, and shade can move output 20% either way, which is exactly what an installer's site survey and a PVWatts run on your actual address settle. Use the tally to size your expectations and to smell oversized quotes, not to order racking.
Quick reference: panels by bill and by sun
Computed with the same engine as the tally above, at 400W a panel. Find your bill row, then the column closest to your state's sun.
| Usage | Cloudy states (4.0 h) | US average (4.5 h) | Sun Belt (5.3 h) | Desert Southwest (6.5 h) |
|---|---|---|---|---|
| 500 kWh/mo | 14 (5.6 kW) | 12 (4.8 kW) | 11 (4.4 kW) | 9 (3.6 kW) |
| 750 kWh/mo | 21 (8.4 kW) | 18 (7.2 kW) | 16 (6.4 kW) | 13 (5.2 kW) |
| 900 kWh/mo | 25 (10 kW) | 22 (8.8 kW) | 19 (7.6 kW) | 15 (6 kW) |
| 1,000 kWh/mo | 27 (10.8 kW) | 24 (9.6 kW) | 21 (8.4 kW) | 17 (6.8 kW) |
| 1,500 kWh/mo | 41 (16.4 kW) | 36 (14.4 kW) | 31 (12.4 kW) | 25 (10 kW) |
| 2,000 kWh/mo | 54 (21.6 kW) | 48 (19.2 kW) | 41 (16.4 kW) | 33 (13.2 kW) |
Questions people ask
How many solar panels does the average house need?
About 22 panels at 400 watts each, an 8.8 kW system, to cover the roughly 900 kWh a month EIA says the average US home uses. Local sun moves that number hard: the same bill takes about 15 panels in Phoenix and about 27 in Seattle. Set your own usage and state above and read the real count.
How many panels do I need for 1,000 kWh per month?
Twenty-four 400-watt panels (9.6 kW) at the national-average 4.5 peak sun hours. In strong sun (5.3 hours) it drops to 21, and in cloudy states (4.0 hours) it rises to 27. The 1,000 kWh page has the full table by state.
Does panel wattage change the count much?
It changes the count, not the system. The 900 kWh example needs 25 panels at 350W, 22 at 400W, or 19 at 450W, but all three land within a few percent of the same 8.8 kW system size, because what you are really buying is kilowatts of capacity. Wattage matters most when roof space is tight: higher-watt panels put more system on fewer square feet.
Why do you multiply by 0.77 instead of using the panel’s rated watts?
Because rated watts are a lab number (STC: 25°C cell, perfect light, no wiring). On a real roof you lose about 14% to soiling, wiring, mismatch, and inverter conversion, and more to heat on summer afternoons. NREL’s own quick-estimate convention is DC watts times peak sun hours times 0.77 for AC output, so that is what every number on this site uses. Our methodology page shows the arithmetic.
How much roof do the panels take?
Plan about 21 sq ft per 400-watt panel once racking gaps are counted, so the 22-panel average system wants roughly 460 sq ft of usable roof. Usable is the operative word: fire-code setbacks from ridges and edges, vents, and shaded strips all come out of the gross area, and a south-facing rectangle is worth more than its square footage suggests.