Solar PV Sizing Calculator
Size a grid-tied solar array from usable roof area or a target kW size, then estimate annual production, bill offset, cost, and simple payback.
With 400 sq ft of usable California roof, this planning estimate sizes about 6.8 kW and produces about 10,880 kWh per year.
How to use it
- Choose roof area or target kW. Size from usable roof area, or enter a system size you are already considering.
- Pick the state. Use the regional PV yield closest to the project location.
- Enter use and rate. Use annual kWh, or estimate use from monthly bill and utility rate.
- Read production and payback. Use array size, annual kWh, offset, cost, and simple payback as a planning estimate.
How this estimate works
Roof-area mode uses about 17 W per usable square foot. Annual production is array kW times the regional kWh/kWp yield. That NREL PVWatts yield figure already accounts for typical system losses, so it is not reduced a second time. The offset compares production with annual use, and simple payback divides installed cost by annual bill savings.
Sources
Regional PV yield framing follows NREL's PVWatts calculator and the National Solar Radiation Database. For an exact figure, run PVWatts for your address. This is a planning estimate, not an engineered design. Permits, utility rules, roof structure, and a site assessment are required.
Solar PV sizing questions
How much solar fits on my roof?
This calculator uses about 17 watts of usable PV per square foot of suitable roof area. Real designs lose area to setbacks, vents, shade, hips, and orientation.
What does kWh per kWp mean?
It is annual energy produced per kilowatt of installed solar capacity. The calculator uses regional defaults from NREL PVWatts data; that figure already includes typical system losses.
Does this replace an engineered solar design?
No. It is a planning estimate. A solar contractor must check roof structure, electrical capacity, utility rules, permits, tilt, azimuth, shade, and inverter sizing.
Why is bill offset capped by annual use?
Simple savings are calculated on the kWh you can use or credit at your entered rate. Net-metering credits, time-of-use rates, and export rules can change the real value.
Is this the same as the solar savings by state tool?
No. This page sizes an array from roof area or target kW. The solar savings page focuses on state-level economics and local solar value.