Calculators / Energy and insulation
How many solar panels do I need?
Look up your ZIP, then put in what you use from your bill. You get the system size, the number of panels, what they'd make and what that's worth.
Updated · How we check our numbers
Your solar system
20 panels, 8 kW
To cover 100% of 10,800 kWh a year where each kW makes 1,370 kWh, you need about 7.9 kW: 20 × 400 W panels. They'd make about 10,960 kWh, worth $1,565 a year at 14.3¢.
- System
- 8 kW20 × 400 W
- Makes
- 10,960 kWha year
- Worth
- $1,565a year
Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
yearly use = 900 kWh × 12 = 10,800 kWh
system = 10,800 kWh ÷ 1,370 kWh per kW = 7.9 kW
panels = 7.9 kW × 1000 ÷ 400 W = 20, rounded up
worth = 10,960 kWh × 14.3¢ = $1,565
Other sizes
400 W panels- Cover 50%10 panels, 4 kW$783/yr
- Cover 75%15 panels, 6 kW$1,174/yr
- Cover 100%20 panels, 8 kW$1,565/yr
- Cover 125%25 panels, 10 kW$1,956/yr
The output per kW assumes a south-facing roof at 20° with no shade. East or west faces, steep or flat roofs, and shade all cut it; run your own address and roof in PVWatts (pvwatts.nlr.gov) or ask an installer for a shade report.
Savings assume every kWh is worth your full electricity price. Many utilities credit power you send back to the grid at less than that, which stretches the payback. Check your utility's solar export rate.
We haven't applied any tax credits, state incentives or utility rebates. Take them off the cost if you qualify.
How we work it out
system kW = kWh you want to cover ÷ kWh a year per kW where you live
Output per kW comes from PVWatts, the National Laboratory of the Rockies' (formerly NREL) solar model, run for every county: 1 kW of standard panels on a south-facing roof at 20°, with its default 14% for wiring, soiling and other losses, using typical-year weather. Your ZIP gets the nearest county center.
Savings value every kWh at your state's average price (EIA). Utilities that credit exported power at less than retail make the real payback longer. We don't apply tax credits or rebates, and roofs that face east or west, sit steeper or flatter, or get shade make less: PVWatts itself models your exact roof.
Solar output by ZIP comes from PVWatts v8, from the National Laboratory of the Rockies (formerly NREL). The laboratory provides the data as is, without warranty, and doesn't endorse this calculator.
Where the numbers come from
Energy
Storage and off-grid
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