Solar System Size Calculator
Convert annual electricity use into a planning-size solar array using local solar resource and an overall performance factor. Confirm the result with address-specific modeling and utility rules.
Quick answerAt the defaults, 900 kWh/month equals 29.59 kWh/day. With 4.5 peak sun hours and an 80% overall performance factor, the estimated array is 8.22 kW—about 21 panels rated 400 W each. This is an annual-energy estimate, not a roof layout or interconnection approval.
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How it works
Daily use = monthly use × 12 ÷ 365. System size = daily use ÷ (peak sun hours × performance ratio ÷ 100). Panels = system size ÷ panel wattage. This sizes a system to roughly offset your annual use; exact sizing also depends on roof space, shading and net-metering rules.
Solar size examples at the default sun and loss assumptions
Uses 4.5 peak sun hours/day, an 80% performance factor and 400 W modules.
| Average use / month | Average use / day | Estimated array | 400 W panels |
|---|---|---|---|
| 300 kWh | 9.86 kWh | 2.74 kW | 7 |
| 600 kWh | 19.73 kWh | 5.48 kW | 14 |
| 900 kWh | 29.59 kWh | 8.22 kW | 21 |
| 1,200 kWh | 39.45 kWh | 10.96 kW | 28 |
| 1,500 kWh | 49.32 kWh | 13.70 kW | 35 |
Planning examples only. Roof geometry, fire setbacks, module dimensions, shade, inverter sizing and utility limits can change the buildable system.
Worked example with the default inputs
Daily electricity use = 900 kWh/month × 12 ÷ 365 = 29.59 kWh/day. Effective full-output hours = 4.5 × 0.80 = 3.6 hours/day. Array size = 29.59 ÷ 3.6 = 8.22 kW.
Panel count = 8,219 W ÷ 400 W = 20.55, rounded up to 21 panels. The rounded array is 8.4 kW DC, slightly above the mathematical minimum because panels come in whole units.
Peak sun hours are an energy equivalent
One peak sun hour represents the daily solar energy that would arrive during one hour at 1,000 W/m²; it is not one clock hour of sunshine. Weather, latitude, tilt and orientation change the annual average.
Use NREL PVWatts or another authoritative local dataset rather than choosing a broad climate label. PVWatts uses historical weather and site inputs and reports a range because year-to-year production varies.
What belongs in the performance factor
The factor reduces ideal nameplate output for real losses such as temperature, inverter conversion, soiling, mismatch, wiring, shading and downtime. NREL PVWatts models several items explicitly and its documentation lists a 14% default for system losses that are not otherwise modeled; that is not automatically the correct factor for this simpler formula.
Use the performance factor supplied by a site model when possible. If shade or roof orientation is poor, lowering this input makes the planning estimate larger, but it cannot tell you whether the roof layout is feasible.
Why annual offset is not the same as bill elimination
A system sized to annual kWh can export energy at midday and import energy at night or in another season. Fixed charges, time-of-use rates, export compensation, interconnection limits and utility sizing rules can leave a bill even when annual production matches annual consumption.
DOE recommends reviewing previous bills and considering future electricity needs. Model EV charging, electrified heating or planned efficiency changes before signing a system size, and compare self-use and export value with the payback calculator.
Sources used for assumptions: NREL — PVWatts Calculator; NREL — PVWatts V8 API and model inputs; U.S. Department of Energy — Step-by-step guide for consumers going solar.
Frequently asked questions
What are peak sun hours?
They are daily solar energy expressed as equivalent hours at 1,000 W/m², not daylight duration. Use a location-specific annual value.
Should I use one month or a full year of bills?
Use 12 months when possible because seasonal consumption varies. Divide annual kWh by 12 for the monthly input.
Should I size solar for 100% of annual use?
Not automatically. Export compensation, interconnection limits, daytime self-use, future loads, roof area and budget can make a smaller or larger target more appropriate.
Why is panel count rounded up?
Panels are whole modules. The calculator rounds the mathematical count upward so the selected modules meet or slightly exceed the estimated DC size.
Does this account for shade and roof direction?
Only through the performance factor. Use address-specific PVWatts modeling and a professional site assessment for shade, orientation, roof condition and layout.
How do I estimate roof space?
Multiply the exact module length by width and panel count, then add required gaps, access paths and fire setbacks. Do not use a generic square-foot estimate for permitting.
Will this system make my electricity bill zero?
Not necessarily. Fixed charges, timing, exports, seasonal mismatch and utility rules can leave charges even when annual generation equals annual use.
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