Solar System Size Calculator
To size a solar system, convert your annual electricity use into a required kW using peak sun hours and a loss factor, then divide by your panel wattage and round up. For a typical 10,800 kWh home aiming to cover all its use, that is about an 8 kW system — roughly 20 panels and 420 square feet of roof.
Using US averages. Set your state or ZIP for local electricity, fuel, sun and grid figures. Stays on your device.
From a year of bills; US average is ~10,800 kWh.
100% aims to match your annual use.
Modern residential panels are ~400 W.
Daily average for your location; US ~4.5.
Assumptions
Production after losses; PVWatts default ~84%.
A typical 400 W panel is ~21 sq ft.
Gross, before incentives; benchmark ~$3.00/W.
Recommended system size
8.0 kW
20 panels at 400 W. Panel count is rounded up to a whole panel.
Verdict: You'd need about 20 panels (~8.0 kW) on roughly 420 sq ft of roof to cover 102% of your electricity use.
Get a shareable image- Panels needed
- 20
- Roof area
- 420 sq ft
- Annual production
- 11,038 kWh
- Est. gross cost
- $24,000Before incentives
- Bill offset
- 102%
- Annual production11,038 kWh
- Your usage10,800 kWh
A planning estimate — it ignores roof orientation, tilt and shading, and rounds the panel count up. Estimate only; not financial advice.
Save & compare
Save the current inputs and compare up to 4 side by side. Stored on this device only.
How it works
This calculator turns an energy target into hardware: how big a system, how many panels and how much roof you need to cover a chosen share of your yearly electricity use.
- Annual electricity use and the share to cover set the energy target in kWh.
- Peak sun hours and the derate factor convert that target into a required system size in kW.
- Panel wattage turns the size into a whole number of panels (rounded up), and the per-panel area gives the roof space.
The headline is the actual system size after rounding up to whole panels. The chart compares the system’s estimated annual production against your usage so you can see the offset you’d achieve.
Methodology & assumptions
Results use the inputs you provide plus these representative defaults. Each is editable in the calculator; the location-dependent and cost figures are tied to a dated source.
| Assumption | Default | Source |
|---|---|---|
| Peak sun hoursUS-wide representative; varies strongly by location. | 4.5 h/day | National Renewable Energy Laboratory (NREL) |
| System derate (losses)PVWatts default system losses are ~14%. | 84% | National Renewable Energy Laboratory (NREL) |
| Installed cost (benchmark)Gross, before incentives; varies widely. | $3.00 /W | National Renewable Energy Laboratory (NREL) |
| Panel wattage (default)User input — typical modern residential panel. | 400 W | — |
| Area per panel (default)User input — footprint of a typical 400 W panel. | 21 sq ft | — |
| Annual electricity use (default)User input — replace with a year of your own bills. | 10,800 kWh | — |
Full formula, every default and its source: Solar system size methodology.
Worked example
Take a home using 10,800 kWh a year that wants to cover 100% of it, with 400 W panels, 4.5 peak sun hours and an 84% derate:
- Production per kW: 4.5 × 365 × 0.84 ≈ 1,380 kWh per kW each year.
- Required size: 10,800 ÷ 1,380 ≈ 7.83 kW.
- Panels: 7,830 W ÷ 400 W = 19.6 → 20 panels (rounded up) → an 8.0 kW system.
- Roof area: 20 × 21 ≈ 420 sq ft; production ≈ 11,040 kWh (about 102% of use).
- Rough cost: 8,000 W × $3.00 ≈ $24,000 before incentives.
Because the panel count rounds up, the installed system slightly exceeds the target. Put in your own usage and location above.
Frequently asked questions
How many solar panels do I need?
Divide the system size your home needs by your panel wattage and round up. With the defaults — about an 8 kW system using 400 W panels — that is 20 panels. Higher electricity use, lower panel wattage or fewer peak sun hours all push the count up.
How much roof space does that take?
Multiply the panel count by the footprint of one panel. A typical 400 W panel is about 21 square feet, so 20 panels need roughly 420 square feet of unshaded roof — plus setbacks and walkways, which this estimate does not add.
What size system in kW do I need?
We turn your annual energy target into a required DC size using peak sun hours and a derate factor, then round up to whole panels. With the defaults that is about 7.8 kW ideally, rounded to an 8.0 kW installed system once panels are whole units.
Does roof orientation or shading matter?
Yes, a lot. This calculator assumes a generic, well-sited array and folds losses into a single derate factor. A south-facing, unshaded roof at a good tilt will beat an east/west or shaded roof, which would need more panels for the same output. Treat the result as a starting point.
What panel wattage should I assume?
Most current residential panels are 380–450 W; 400 W is a reasonable default. Higher-wattage panels mean fewer panels and less roof area for the same system size. You can edit the wattage and the per-panel area to match a specific product.
Will this system cover my whole electricity bill?
It is sized to cover the share you choose (100% by default) of your annual kWh. Because the panel count rounds up, the actual system can produce a little more than your target. Whether that zeroes your bill also depends on your utility’s net-metering rules.
What about future usage growth, like an EV or heat pump?
Plan for it by raising the annual use figure or the offset percentage before sizing. Adding an EV or heat pump can increase consumption substantially, and it is usually cheaper to size the array for it up front than to expand later.
Related calculators
Related guides
- How many solar panels to power a house?How many solar panels to power a house? About 20 for a typical home. Use your kWh, sun hours and panel wattage to size your own array — with worked numbers.
- Are solar panels worth it?Solar usually pays back in 7-12 years, then powers your home near-free for ~20 more. See when it is worth it, when it is not, and how to get your own number.
By EnergyTally Team · Editorial & analysis team
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