Energy Calculators

Solar panel size calculator

Find the solar system size and number of panels needed to cover your electricity use.

An average Iowa household uses about 10,332 kWh a year (EIA, 2025); with 1,370 kWh per kW (PVWatts, Des Moines) that needs a 7.5 kW system, or 19 panels of 400 W.

Production for Des Moines (PVWatts); typical use from EIA.

kWh

Average Iowa household: 10,332 kWh (2025, EIA). Add up 12 months of bills for yours.

%
kWh/kW

South-facing, 20° tilt, 14% losses (PVWatts). Shade and orientation change this.

W
System size
7.54 kW
Panels
19400 W each = 7.60 kW
Expected production
10,412 kWh/yr

Formula: system kW = annual use × share to cover ÷ production per kW; panels = system watts ÷ panel watts, rounded up.

Inputs used: use: 10,332 kWh/yr; cover: 100%; production: 1,370 kWh/kW; panel: 400 W.

These are estimates, not financial advice.

Roof space depends on your panels’ dimensions (on the spec sheet) and on setbacks required by your local fire code.

Production: PVWatts® v8, National Laboratory of the Rockies (U.S. Department of Energy) (PVWatts 8.5.0). Usage: U.S. Energy Information Administration, 2025 annual data.

Worked examples in three states

StateUse/yrkWh/kWSystemPanels
Hawaii6,180 kWh1,6223.8 kW10
Alabama13,740 kWh1,37610.0 kW25
Nevada10,416 kWh1,7535.9 kW15

How we calculate

system kW = annual kWh × share to cover ÷ kWh per kW; panels = system watts ÷ panel watts, rounded up. Production per kW: PVWatts v8 for each state’s largest city (south-facing, 20° tilt, 14% losses). Usage: EIA annual residential sales ÷ customers.

Frequently asked questions

How many solar panels do I need?

Divide the electricity you want to cover each year by the production per kW for your location to get the system size, then divide by the panel rating. Twelve months of bills give the best usage figure.

Should I cover 100% of my use?

Not necessarily. Where exported electricity is credited below the retail price, a system that covers less than all of your use can pay back faster. Check the solar payback calculator.

Why does production differ by state?

Sunshine, temperature and weather differ. PVWatts uses typical-year weather for each location; shading and roof orientation also matter.

Data last updated: