PV Calculator
Size a photovoltaic system from your electricity use: enter monthly kWh, your local sun hours, and panel wattage to get system size, panel count, and roof area.
What your result means
System size is the kilowatts of panels needed to cover your usage, after a real-world efficiency derate of about 20% for inverter losses, heat, wiring, and dust. Panels needed divides that by your panel wattage, and roof area estimates the space at about 17.5 sq ft per panel. Next, price it with our solar panel cost calculator and set the tilt with the panel angle calculator.
How to use this calculator
- Find your average monthly usage in kWh on your electric bill.
- Look up your area's peak sun hours (4-5 is typical; check a sun-hours map).
- Enter your panel wattage — modern panels are 350 to 450 W.
- Read system size in kW, the panel count, and the rough roof area.
- Add a margin if you plan to add an EV or electric heating later.
The formula
Daily usage divided by usable sun hours gives the kilowatts you need, where the 0.80 factor covers real-world losses. Divide watts by panel wattage for the count, and figure about 17.5 sq ft of roof per panel.
Worked example
At 900 kWh a month that's 30 kWh a day. With 4.5 peak sun hours and a 0.80 derate, you need 30 / (4.5 × 0.8) = 8.3 kW. At 400 W per panel that's 21 panels, taking up roughly 370 sq ft of roof.
Peak sun hours by region (rough)
| Region | Peak sun hours/day |
|---|---|
| Pacific Northwest | 3.0 - 4.0 |
| Northeast / Midwest | 3.5 - 4.5 |
| Mid-Atlantic / Plains | 4.5 - 5.0 |
| Southeast | 4.5 - 5.5 |
| Southwest | 5.5 - 6.5 |
Tips & gotchas
- Size to your actual annual kWh, not a single high month — averages smooth out seasons.
- Add headroom (10-25%) if an EV, heat pump, or pool pump is coming.
- Cloudier regions need more panels for the same output — sun hours drive everything.
- Roof space, not budget, is often the real limit — check the area estimate against your roof.
- Net metering rules affect whether oversizing pays off; check your utility.
Frequently asked questions
How many solar panels do I need?
Divide your daily kWh by your peak sun hours and a 0.8 efficiency factor for system size, then divide by panel wattage. A typical home needs about 15 to 25 panels.
What are peak sun hours?
The number of hours per day that sunlight hits the intensity used to rate panels. It varies by region and season, commonly 4 to 5 hours in much of the US.
Why the 0.8 efficiency factor?
Real systems lose energy to inverter conversion, heat, wiring, soiling, and shading. Multiplying by about 0.8 keeps the estimate realistic instead of best-case.
How much roof space do solar panels need?
Roughly 17 to 18 sq ft per residential panel. A 20-panel system needs about 350 sq ft of unshaded, well-oriented roof.
Should I oversize my system?
A little headroom helps if you'll add an EV or electric heating, but net-metering rules determine whether extra production pays off. Check your utility first.
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Estimates only — see our full disclaimer.