Solar panel calculator
Find your average monthly kWh on your electric bill and enter your area’s peak sun hours for a first estimate.
System size
7.4 kW
- Panels
- 19 400 W panels
- Yearly production
- 11,096 kWh
- Roof space
- 380 sq ft, roughly
How it works
The calculator turns your monthly use into a daily average, then divides by your peak sun hours (the equivalent hours of full-strength sun per day) and by the system efficiency. That gives the system size in kilowatts. Divide by the panel size for the panel count.
This is a first estimate. Roof direction, tilt and shade change real production a lot. For your actual address, the free PVWatts calculator, which models production for a specific location, is a good next step.
Example
With monthly electricity use 900 kWh, how much to cover 100%, peak sun hours per day 5 hours, panel size 400 watts, system efficiency 80%, the answer is 7.4 kW.
Tips before you buy
- Use a full year of bills to get your average. Summer and winter use can differ a lot.
- South-facing roofs produce the most in the US. East and west roofs produce less, and shade from trees can cut output sharply.
- Ask installers for quotes with their production estimate, and compare those to this number.
Common questions
How many solar panels for 900 kWh a month?
With 5 peak sun hours and 80% efficiency, about a 7.4 kW system, or 19 panels of 400 W.
How much roof space do solar panels need?
Roughly 18 to 21 square feet per typical 400 W residential panel, plus spacing for access.
What are peak sun hours?
The number of hours per day when sunlight averages 1,000 watts per square meter. It varies by location and season.