With the default inputs (Monthly electricity use 900; Share of usage to cover 100%; Peak sun hours per day 4.5; System losses 14%; Panel size 400; Installed cost $2.75; Incentives and rebates 0%), this calculator returns System Size Needed 7.65 kW, Panels Needed 20 panels and Installed Cost $21,025.80.
Every row is computed with this page's own calculator. Change the inputs above to model your own numbers.
| Monthly electricity use | System Size Needed | Cost After Incentives |
|---|---|---|
| 500 kWh | 4.25 kW | $11,681.00 |
| 750 kWh | 6.37 kW | $17,521.50 |
| 900 kWh | 7.65 kW | $21,025.80 |
| 1,200 kWh | 10.19 kW | $28,034.41 |
| 1,500 kWh | 12.74 kW | $35,043.01 |
| 2,000 kWh | 16.99 kW | $46,724.01 |
Enter the household's average monthly electricity use in kilowatt-hours. Choose the share of that usage the solar system should cover, from partial offset to full offset. Then enter the site's average peak sun hours per day and an allowance for system losses.
Provide the rated wattage of one panel, the installed cost per watt, and a combined percentage for applicable incentives and rebates. The calculator estimates the system capacity, number of panels, annual production, installed cost, and cost after incentives. Residential panels are commonly rated at 400 to 450 watts each.
Using the shipped defaults, monthly electricity use is 900 kWh and the target offset is 100%. Peak sun hours are 4.5 per day, with system losses of 14%. The panel size is 400 watts, installed cost is $2.75 per watt, and incentives and rebates are 0%.
The annual electricity requirement is based on twelve months of consumption. Production capacity is adjusted for the entered sunlight and losses. PVWatts, the NREL production model, also uses 14% as its default for system losses. The resulting panel requirement is rounded up because a fraction of a panel cannot be installed.
The displayed results are System Size Needed: 7.65 kW, Panels Needed: 20 panels, Installed Cost: $21,025.80, Cost After Incentives: $21,025.80, and Estimated Annual Production: 10,800 kWh/yr.
This is a planning estimate, not a roof design or contractor quote. Roof geometry, shade, panel orientation, equipment selection, labor, permitting, and electrical work can affect feasible capacity and final pricing. Rounding the panel count upward can also make the panel total's nameplate capacity slightly larger than the calculated requirement.
The 30% federal Residential Clean Energy Credit ended for expenditures after December 31, 2025. A homeowner-purchased system placed in service in 2026 receives no federal residential credit. State, utility, and local incentives may still apply and vary widely. Third-party-owned systems may qualify for a commercial credit through the installer for systems placed in service by the end of 2027, but the installer claims that credit.
Use the Solar Savings Calculator to project savings and payback from a system. Use the Solar Financing Calculator to examine payments and financing costs.