How Many Solar Panels Do I Need to Power My House?
Part of the project planner: Planning backup power, batteries and solar.
A home using 900 kWh a month needs about 21 solar panels of 400 W (8.4 kW), which make about 11,038 kWh a year and take about 420 square feet of roof. That assumes 4.5 peak sun hours a day, the US average, and 80% system efficiency. The chart shows other homes and places.
Panels needed by monthly use and sun hours
400 W panels, 80% system efficiency, covering all of your use:
| 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 |
| 1,200 kWh | 42 | 31 | 28 | 23 | 19 |
| 1,500 kWh | 52 | 39 | 35 | 29 | 24 |
| 2,000 kWh | 69 | 52 | 46 | 38 | 32 |
Peak sun hours are the equivalent hours of full-strength sunshine, averaged over the year. About 3 is cloudy and far north, about 4.5 is the US average, and 6 or more is the desert Southwest. Put your own numbers into the solar panel calculator.
System size, output and roof area at the US average
| Monthly use | Panels (400 W) | System size | Yearly output | Roof area |
|---|---|---|---|---|
| 400 kWh | 10 | 4.0 kW | 5,256 kWh | 200 ft² |
| 600 kWh | 14 | 5.6 kW | 7,358 kWh | 280 ft² |
| 900 kWh | 21 | 8.4 kW | 11,038 kWh | 420 ft² |
| 1,200 kWh | 28 | 11.2 kW | 14,717 kWh | 560 ft² |
| 1,500 kWh | 35 | 14.0 kW | 18,396 kWh | 700 ft² |
How the numbers are worked out
- Your monthly kWh becomes a daily figure: 900 kWh a month is 29.6 kWh a day.
- Each kW of panels makes the sun hours times the efficiency in kWh a day: 4.5 × 0.8 = 3.6 kWh per kW.
- The system size is the daily use divided by that: 29.6 ÷ 3.6 = 8.2 kW.
- Divide by the panel wattage and round up: 8.2 kW ÷ 0.4 kW = 20.6, so 21 panels.
What to check before you buy
- The roof. A south-facing, unshaded roof is best. East and west roofs make 15 to 20% less, and shade can cut the output a great deal. A roof near the end of its life should be replaced first.
- How your utility pays. If it pays less than the retail rate for extra power, a system sized to your use is better than a bigger one.
- Incentives. The federal tax credit and state and utility rebates change, so check the current rules.
- Quotes. Get at least three, and compare the price per watt, the equipment and warranties, and what is included.
Mistakes that cost people money
- Sizing from the roof instead of from your use.
- Ignoring shading from trees and nearby buildings, which a site survey picks up.
- Buying a battery without working out which loads you want to keep on in an outage.
- Comparing quotes by total price and not by price per watt.
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 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 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), which is about 525 kWh a year.
More guides
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.