Solar Panel Calculator: System Size
This solar panel calculator estimates the home solar system that would cover your electricity use. Enter your monthly usage in kWh from your utility bill, or your bill amount and electricity rate, plus the peak sun hours where you live. It works out the system size in kW, how many panels you need at your chosen wattage, the roof space in square feet, yearly production, the estimated installed cost at a price per watt, and a simple payback period from your bill savings. Defaults use typical US values, and every number can be changed for your quote.
Solar Panel Calculator
How many solar panels you need, system size in kW, roof space, cost and payback.
How to use the Solar Panel Calculator
- Enter your monthly kWh, or your bill and rate per kWh.
- Enter peak sun hours for your area.
- Set panel wattage, the share of use to cover, and price per watt.
- Read the panels needed, system size, roof space, cost and payback.
Solar sizing formula
System kW = yearly kWh to cover ÷ 365 ÷ peak sun hours ÷ efficiency. Panels = kW × 1,000 ÷ panel watts, rounded up. Payback = installed cost ÷ yearly savings.
Features
- From kWh or from your bill
- Peak sun hours and system losses
- Panel count and roof area in sq ft
- Installed cost at $ per watt
- Simple payback in years
- Runs in your browser
Frequently asked questions
How many solar panels do I need?
Divide your daily kWh by peak sun hours and about 0.8 for system losses to get kW, then divide by panel watts. A home using 900 kWh a month with 4.5 sun hours needs about 8.2 kW, or 21 panels of 400 W.
What are peak sun hours?
The number of hours a day when sunlight averages 1,000 watts per square meter. Most of the US gets 3.5 to 6, with the Southwest highest.
How much roof space do solar panels need?
A typical 400 W residential panel is about 21 square feet, so a 20-panel system needs roughly 420 square feet plus spacing.
How much does a home solar system cost?
US installed prices commonly run about $2.50 to $3.50 per watt before incentives, so an 8 kW system is about $20,000 to $28,000.
Is there still a federal solar tax credit?
The federal residential clean energy credit ended for systems placed in service after December 31, 2025. Check state and utility rebates, which still exist in some areas.