How to size a solar system
- Find your usage. Your electric bill shows kilowatt-hours (kWh) used each month. Average a full year if you can — air conditioning and heating make usage swing a lot by season.
- Choose how much to cover. 100% aims to offset your whole bill. Choose less if roof space or budget is limited, or if your utility credits exported power at a low rate.
- Enter peak sun hours for your area — the daily average number of hours of full-strength sunlight (1,000 W per m²). This is the biggest factor in how much each panel produces.
- Set your panel size. Most new residential panels are roughly 350–450 W and about 17–21 sq ft each.
Solar panel formula
System size (kW) = Annual kWh to cover ÷ Production per kW
Panels = System size (W) ÷ Panel wattage, rounded up
System efficiency accounts for real-world losses — inverter conversion, wiring, heat, dust and minor shading. 86% (14% losses) is the default used by NREL's PVWatts tool; use a lower figure for shaded or hot sites.
Worked example
A home using 900 kWh a month uses 10,800 kWh a year. With 4.5 peak sun hours and 86% efficiency, each kW of panels produces about 1,413 kWh a year, so the home needs a 7.65 kW system. With 400 W panels that is 20 panels (8.0 kW), producing roughly 11,300 kWh a year and needing about 360 sq ft of roof.
System size chart
kW needed to cover 100% of monthly usage at different peak sun hours (86% efficiency):
| Monthly use | 3.5 sun hrs | 4.5 sun hrs | 5.5 sun hrs | 6.5 sun hrs |
|---|---|---|---|---|
| 500 kWh | 5.5 kW | 4.2 kW | 3.5 kW | 2.9 kW |
| 750 kWh | 8.2 kW | 6.4 kW | 5.2 kW | 4.4 kW |
| 1,000 kWh | 10.9 kW | 8.5 kW | 7.0 kW | 5.9 kW |
| 1,250 kWh | 13.7 kW | 10.6 kW | 8.7 kW | 7.4 kW |
| 1,500 kWh | 16.4 kW | 12.7 kW | 10.4 kW | 8.8 kW |
| 2,000 kWh | 21.8 kW | 17.0 kW | 13.9 kW | 11.8 kW |
Peak sun hours by region
| US region | Typical peak sun hours |
|---|---|
| Desert Southwest (AZ, NV, southern CA, NM) | 6–7 |
| Texas, Southeast, Florida | 4.5–5.5 |
| Mid-Atlantic and Midwest | 4–5 |
| Northeast and Great Lakes | 3.5–4.5 |
| Pacific Northwest | 3–4 |
These are broad annual averages. For your exact address, NREL's free PVWatts calculator uses local weather data, roof tilt and orientation.
Roof space and orientation
- In the US, south-facing roofs produce the most; east- and west-facing roofs typically produce somewhat less.
- Shade from trees, chimneys or neighboring buildings can cut output sharply — even partial shade on one panel affects a string unless you use microinverters or optimizers.
- Fire codes often require clear pathways along ridges and edges, so the usable area is smaller than the total roof.
Cost and payback
Installed solar is usually quoted in dollars per watt. Multiply by the system size in watts for the total, then subtract any rebates or incentives. Federal, state and utility incentives change often, so check what currently applies where you live. Simple payback is the net cost divided by the yearly bill savings; net-metering rules, rate increases and panel degradation (typically around 0.5% per year) all affect the real figure.
Frequently asked questions
How many solar panels do I need?
Divide your yearly kWh by (peak sun hours × 365 × 0.86) to get the system size in kW, then divide by your panel wattage. A home using 900 kWh a month with 4.5 sun hours needs about 7.6 kW — 20 panels of 400 W.
How much electricity does a solar panel produce?
A 400 W panel with 4.5 peak sun hours and 86% system efficiency produces about 565 kWh a year, or roughly 47 kWh a month.
What size solar system do I need for 1,000 kWh a month?
About 8.5 kW at 4.5 peak sun hours, 7.0 kW at 5.5 hours, or 10.9 kW at 3.5 hours.
How much roof space do solar panels need?
Each residential panel takes roughly 17–21 sq ft. A 20-panel system needs about 350–420 sq ft of usable, unshaded roof, plus any setbacks your local fire code requires.
What are peak sun hours?
The number of hours per day when sunlight averages 1,000 watts per square meter. Five peak sun hours means the panels receive the same energy as five hours of full midday sun, spread across the whole day.