Understand the size in your proposal
What size solar system do I need?
Start with your annual electricity use in kWh and an annual production estimate for your roof. This calculator translates those assumptions into array capacity and a rounded panel count.
Estimate system size ↓
Solar system size and panel count calculator
Enter energy use, not the dollar amount of your bill. The production input must come from a model or installer estimate for your site; we do not assume every California roof produces the same amount.
Your planning estimate
This is an arithmetic planning estimate, not a roof design, production guarantee or financial recommendation. Ask an installer to model the final layout.
Next: compare the quoted cash price and scope →Calculations stay in your browser. No address, contact information or bill upload is needed. Inputs are not transferred to the partner form. Tool limits support household planning; they are not recommended system ranges.
Where to find the annual solar production assumption
Ask your installer for the proposed array size in DC kW and estimated annual output in AC kWh, plus the location, layout and loss assumptions behind the model. Divide annual output by DC size to obtain the production input used here.
You can also explore PVWatts, which estimates grid-connected solar production. Record the modeled capacity and annual energy result. Review tilt, orientation, shading-related loss assumptions and inverter settings rather than accepting a generic location-only estimate. Our calculator does not call or reproduce the PVWatts model.
If you change the layout or inverter configuration, the output per kW may also change. Scaling an earlier result is a starting point; rerun the production model for the final design. PVWatts itself notes uncertainty and limitations in site-specific predictions.
How to calculate solar system size from annual kWh
Target annual energy = annual use × target percentage ÷ 100.
Estimated DC kW = target annual energy ÷ annual production per DC kW.
Panel count = estimated DC kW × 1,000 ÷ panel watts, rounded up.
Illustrative inputs: 9,600 kWh annual use, an 80% energy target, 1,500 kWh per DC kW per year, and 400 W panels. The production figure is hypothetical, not a California average.
- Target: 9,600 × 0.80 = 7,680 kWh/year.
- Array size: 7,680 ÷ 1,500 = 5.12 kW DC.
- Panel count: 5,120 ÷ 400 = 12.8, rounded up to 13 panels.
- Rounded capacity: 13 × 400 ÷ 1,000 = 5.2 kW DC.
At the same assumed yield, that rounded array corresponds to 7,800 kWh/year. It does not mean the house avoids buying 7,800 kWh from the grid: generation and consumption occur at different times.
Why roof area and future electricity use matter
A calculated panel count does not show whether those panels fit. Panel dimensions, access clearances, shading and roof condition need a site-specific layout. Use the roof planning checklist before treating a capacity estimate as an installation plan.
Ask for separate scenarios for today's consumption and planned changes such as EV charging. Keep the additional annual kWh assumption visible instead of silently inflating historical bills. Give every installer the same brief using the itemized quote request template.
Solar sizing questions
What is the difference between kW and kWh?
kW describes power or rated capacity; kWh describes energy over time. A 5 kW array does not necessarily produce 5 kWh every hour. The production estimate accounts for changing conditions and system assumptions.
Does a 100% energy target eliminate my electricity bill?
No. Matching annual generation to annual consumption does not match their timing or their dollar value. Grid imports, exports and charges depend on the applicable tariff. Review the bill-savings assumptions in the solar paperwork.
Does this calculator size a solar battery?
No. Storage sizing requires separate consideration of usable kWh, output kW, operating mode and the loads you want to support. Compare solar with and without a battery for the same array.
Can I size solar using only my monthly bill in dollars?
A bill total alone does not tell you how many kWh the home used. Locate the usage figures and collect a full year where possible. Ask an installer how any missing months or future changes were estimated.
Method reviewed November 11, 2026. Technical reference: PVWatts input and output definitions. Arithmetic examples are CaSolar illustrations. Images are illustrative.