How this calculator works
Your panels have to produce your daily electricity use. Divide the monthly kWh by about 30.4 to get kWh per day. A panel makes its rated power for the "peak sun hours" of full-strength sunshine a day, less system losses (inverter, wiring, heat, dirt), so the system size in kW is the daily use divided by the sun hours and the efficiency. Divide by the panel wattage to get the number of panels.
Worked example: a home using 900 kWh a month uses 29.6 kWh a day. With 4.5 peak sun hours and 80% efficiency, each kW of panels makes 3.6 kWh a day, so you need 8.2 kW, which is 21 panels of 400 W (8.4 kW). That makes about 11,000 kWh a year and takes about 420 ft² of roof.
Panels needed by monthly use and sun hours (400 W panels, 80% efficiency, 100% offset)
Find your use on a bill and your sun hours in the picker above.
| Monthly use | 3 sun hours | 4 sun hours | 4.5 sun hours | 5.5 sun hours | 6.5 sun hours |
|---|---|---|---|---|---|
| 400 kWh | 14 | 11 | 10 | 8 | 7 |
| 600 kWh | 21 | 16 | 14 | 12 | 10 |
| 900 kWh | 31 | 24 | 21 | 17 | 15 |
| 1200 kWh | 42 | 31 | 28 | 23 | 19 |
| 1500 kWh | 52 | 39 | 35 | 29 | 24 |
| 2000 kWh | 69 | 52 | 46 | 38 | 32 |
Tips before you buy
- Roof direction and shading matter as much as the panel count: a south-facing, unshaded roof at a tilt near your latitude is best. East and west roofs make about 15 to 20% less.
- Size the system to your use, not your roof. Many utilities pay much less for extra power than they charge, and some limit the size of the system.
- Get several quotes. Compare the price per watt, the panel and inverter brand and warranty, and what is included, such as permits and the inspection.
- The federal tax credit and state or utility incentives change over time, so check the current rules before you buy.
- A battery adds backup power but a lot of cost. Size it separately from the panels, for the loads you want to keep on in an outage.
- Check that your roof is in good shape first. Replacing a roof under existing panels means taking them off and putting them back.
Tools & materials for this project
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Frequently asked questions
How many solar panels do I need to power a house?
For a typical US home using 900 kWh a month, about 21 panels of 400 W (8.4 kW), with 4.5 peak sun hours and 80% system efficiency. A sunnier location needs fewer, and a cloudier one needs more.
How many solar panels do I need for 1,000 kWh a month?
About 23 panels of 400 W at 4.5 peak sun hours, or 16 panels in a very sunny place with 6.5 hours.
How much roof area do I need for solar panels?
About 20 square feet per 400 W panel, so about 420 square feet for 21 panels. Allow for access paths and for parts of the roof that are shaded or face the wrong way.
How many kWh does a solar panel produce per day?
A 400 W panel makes about 1.4 kWh a day at 4.5 peak sun hours and 80% efficiency (0.4 kW × 4.5 × 0.8), or about 525 kWh a year.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.