About the Solar Panel Calculator
This solar panel calculator answers the first question every homeowner asks: how many panels do I need? Enter your monthly electricity use, the peak sun hours where you live and the wattage of the panels you are considering, and it sizes the system in kilowatts, rounds up to a whole number of panels, and estimates yearly production, roof area and installed cost.
It is useful before you request installer quotes, so you can sanity-check the system sizes they propose, or when planning an off-grid cabin, RV or shed. Set the offset below 100% if you only want to cover part of your bill — for example when roof space is limited or net metering pays little for exported power.
Peak sun hours are the daily equivalent of full 1,000 W/m² sunshine averaged over the year — roughly 3–4 in cloudy northern regions, 4–5 across much of the US and 5.5–7 in deserts. The system-loss setting covers inverter, wiring, heat, soiling and shading losses; 14% is the default used by NREL’s PVWatts. For a site-specific estimate that accounts for roof tilt and direction, cross-check with PVWatts.
With the default inputs, the panels needed is 20. Change any value above to recalculate instantly.
How to use the solar panel calculator
- 1Add up 12 months of electricity use from your bills and enter the monthly average.
- 2Enter the peak sun hours for your location (from a solar map or PVWatts).
- 3Choose the panel wattage you are considering and keep losses at 14% unless you know better.
- 4Set how much of your usage you want to cover and your installed cost per watt.
- 5Read the panel count, system size, roof area and cost.
Formula and method
Each kilowatt of panels produces roughly sun hours × 365 kWh per year before losses, then the system-loss percentage (inverter, wiring, heat, dirt, shade) is removed. Dividing the energy you want to cover by that per-kW yield gives the required system size.
The panel count is the system size in watts divided by one panel’s wattage, rounded up to a whole panel. Because of rounding, the real system is slightly larger than required, so production and the share of usage covered are recalculated from the whole-panel size. Cost is system watts × installed cost per watt, before tax credits or rebates.
- kW
- DC system size in kilowatts
- sun hours
- Average daily peak sun hours at your location
- losses
- Total system losses as a fraction (0.14 = 14%)
- offset
- Share of your usage you want solar to cover
Worked examples
900 kWh/month home, 4.5 sun hours
Covering 10,800 kWh a year where each kW yields 4.5 × 365 × 0.86 ≈ 1,412.6 kWh needs 7.65 kW. With 400 W panels that rounds up to 20 panels (8.0 kW), producing about 11,300 kWh — 105% of usage — on roughly 430 sq ft of roof, costing about $23,200 before incentives.
Sunny Arizona home, 1,500 kWh/month
At 6.5 sun hours each kW makes about 2,040 kWh a year, so 18,000 kWh needs 8.82 kW. With 430 W panels that is 21 panels, a 9.03 kW system producing roughly 18,424 kWh.
Cover 70% of a 600 kWh/month bill in a cloudy climate
Seventy percent of 7,200 kWh is 5,040 kWh. At 3.5 sun hours each kW yields about 1,098.6 kWh, so 4.59 kW is needed — 12 panels of 400 W (4.8 kW), covering about 73% of usage.
Frequently asked questions
How many solar panels do I need for my house?+
Divide your yearly kWh use by the yearly output of one panel. A 400 W panel in a location with 4.5 peak sun hours and 14% losses makes about 565 kWh a year, so a home using 10,800 kWh needs about 20 panels.
What are peak sun hours?+
Peak sun hours are the number of hours per day that sunlight averages 1,000 watts per square metre. A location with 5 peak sun hours gets the same energy as 5 hours of full midday sun, even though the sun is up much longer.
How much roof space do solar panels need?+
A typical 400 W residential panel is about 21–22 sq ft, so a 20-panel system covers around 430 sq ft. Fire-code setbacks from ridges and edges and gaps for obstructions mean you usually need more total roof area than the panels alone.
Does roof direction affect how many panels I need?+
Yes. In the northern hemisphere south-facing roofs produce the most; east- or west-facing roofs typically produce around 10–20% less, which means more panels for the same output. Use a site-specific tool such as NREL PVWatts to model tilt and azimuth.
Should I size solar to cover 100% of my usage?+
It depends on your utility’s net metering rules. Where exported power is credited at the retail rate, covering 100% often makes sense. Where exports earn much less, sizing closer to your daytime usage or adding a battery can give a better return.