This free solar panel calculator answers the big question: how many solar panels do I need? Enter the kWh from your electric bill and your local sun hours to get the solar system size in kW, the number of panels, the roof area, the cost and payback period, and the battery bank for off-grid or backup power. It works for homes, cabins, RVs and off-grid systems anywhere in the world.
How the solar size is calculated
Daily energy (kWh) = monthly kWh ÷ 30.4
System size (kW) = daily kWh × coverage ÷ (sun hours × (1 − losses))
Panels = system size × 1000 ÷ panel watts (rounded up)
Battery (kWh) = daily kWh × days of autonomy ÷ depth of discharge
Worked example
Your bill shows 900 kWh a month, which is 900 ÷ 30.4 ≈ 29.6 kWh a day. With 4.5 peak sun hours and 20% losses, each kW of panels makes 4.5 × 0.8 = 3.6 kWh a day, so you need 29.6 ÷ 3.6 ≈ 8.2 kW. With 400 W panels that is 8,200 ÷ 400 = 20.6, rounded up to 21 panels.
How many solar panels do I need? Quick table
Number of 400 W panels needed to cover 100% of your monthly usage, with 20% system losses. Find your usage on your electric bill and your sun hours in the table further down.
| Monthly usage | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 500 kWh | 15 panels (5.9 kW) | 12 panels (4.6 kW) | 10 panels (3.7 kW) |
| 750 kWh | 23 panels (8.8 kW) | 18 panels (6.8 kW) | 15 panels (5.6 kW) |
| 1,000 kWh | 30 panels (11.7 kW) | 23 panels (9.1 kW) | 19 panels (7.5 kW) |
| 1,250 kWh | 37 panels (14.7 kW) | 29 panels (11.4 kW) | 24 panels (9.3 kW) |
| 1,500 kWh | 45 panels (17.6 kW) | 35 panels (13.7 kW) | 29 panels (11.2 kW) |
| 2,000 kWh | 59 panels (23.5 kW) | 46 panels (18.3 kW) | 38 panels (14.9 kW) |
How much electricity does a solar panel produce?
Daily and monthly output per panel after 20% system losses.
| Panel | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 300 W | 0.84 kWh/day 26 kWh/month | 1.08 kWh/day 33 kWh/month | 1.32 kWh/day 40 kWh/month |
| 400 W | 1.12 kWh/day 34 kWh/month | 1.44 kWh/day 44 kWh/month | 1.76 kWh/day 54 kWh/month |
| 450 W | 1.26 kWh/day 38 kWh/month | 1.62 kWh/day 49 kWh/month | 1.98 kWh/day 60 kWh/month |
| 550 W | 1.54 kWh/day 47 kWh/month | 1.98 kWh/day 60 kWh/month | 2.42 kWh/day 74 kWh/month |
Solar panel cost and payback period
System cost = system watts × cost per watt × (1 − incentives)
Savings per year = yearly production × electricity price
Payback (years) = system cost ÷ savings per year
For the 900 kWh example above, 21 panels make a 8.4 kW system producing about 11,038 kWh a year. At $2.80 per watt it costs about $23,520 before incentives. Saving $0.17 on each kWh gives about $1,876 a year, so it pays for itself in about 12.5 years.
The savings assume every kWh your panels produce replaces a kWh you would have bought at your current price, which is true with full net metering. If your utility pays less for exported power, savings will be lower. The 25-year figure ignores panel ageing (about 0.5% a year) and future price rises, which usually more than make up for it.
Peak sun hours by region (approximate yearly average)
| Region | Peak sun hours |
|---|---|
| US Southwest (Arizona, Nevada, S. California) | 6–7 |
| US South & Southeast, Texas, Florida | 4.5–5.5 |
| US Midwest & Northeast | 3.5–4.5 |
| US Pacific Northwest | 3–4 |
| UK, Ireland, Northern Europe | 2.5–3.5 |
| Southern Europe, Mediterranean | 4.5–5.5 |
| Middle East, North Africa, India, Pakistan | 5–6.5 |
| Australia (most cities) | 4.5–6 |
For a precise figure for your exact location and roof angle, use NREL's free PVWatts tool.
Related calculations
- Not sure how many kWh you use? Add up your appliances with the electricity cost calculator.
- Charging an electric car with solar? The EV charging calculator shows how many kWh each charge needs.
- Sizing a portable solar generator? Use the battery runtime calculator.
- Running cables from the panels? Check them with the wire size calculator.