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The number of solar panels you need comes down to three numbers: how much electricity you use per day, how many peak sun hours your location gets, and the wattage of the panels you buy. For an average US home using about 30 kWh per day in a 4.5 peak-sun-hour location, that works out to roughly 8.3 kW of solar, or about 21 panels of 400 watts.
Below is the exact method, so you can run it for your own home, cabin or RV in a few minutes.
Solar panel calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
The formula
Number of panels = Daily kWh × 1,000 ÷ (Peak sun hours × 0.80) ÷ Panel wattage
- Daily kWh is your energy use per day. Take the monthly kWh from your electric bill and divide by 30.
- Peak sun hours is the number of hours per day your location receives the equivalent of full, 1,000 W/m² sunshine. Most of the US, UK and Europe falls between 2.5 and 6. See our guide to peak sun hours.
- 0.80 is a system efficiency factor. Real systems lose around 20% to heat, dust, wiring resistance and inverter conversion.
- Panel wattage is the rated power on the panel’s label, usually 300 to 450 W for modern panels.
Step 1: Find your daily energy use
Look at your last 12 electric bills and add up the kWh. Dividing the annual total by 365 gives a far better number than a single month, because heating and cooling change your usage a lot through the year.
| Monthly bill usage | Daily usage |
|---|---|
| 300 kWh | 10 kWh/day |
| 600 kWh | 20 kWh/day |
| 900 kWh | 30 kWh/day |
| 1,200 kWh | 40 kWh/day |
If you are planning an off-grid cabin or RV with no bill to look at, list your appliances instead and use our off-grid load calculation worksheet.
Step 2: Convert to required array size
Divide your daily watt-hours by your peak sun hours multiplied by 0.80.
Example: 30 kWh/day in a location with 4.5 peak sun hours.
- 30 kWh = 30,000 Wh
- 4.5 × 0.80 = 3.6
- 30,000 ÷ 3.6 = 8,333 W (8.3 kW)
Step 3: Divide by panel wattage
8,333 W ÷ 400 W = 20.8, so you round up to 21 panels.
Always round up. Rounding down means the array falls short every day.
Quick reference table
These figures assume 400 W panels and the 0.80 efficiency factor.
| Daily use | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 5 kWh | 5 panels | 4 panels | 3 panels |
| 10 kWh | 9 panels | 7 panels | 6 panels |
| 20 kWh | 18 panels | 14 panels | 12 panels |
| 30 kWh | 27 panels | 21 panels | 18 panels |
| 40 kWh | 36 panels | 28 panels | 23 panels |
Notice how much location matters. The same 30 kWh home needs 27 panels in a cloudy northern climate but only 18 in a sunny desert state.
Grid-tied vs off-grid: does the answer change?
For a grid-tied home, the formula above sizes a system that offsets 100% of your usage over a year. Many homeowners deliberately install less, for example 70 to 80%, depending on net metering rules and budget.
For an off-grid system, size for your worst month rather than the yearly average. If December gives you 2.5 peak sun hours, use 2.5, not your annual figure. Off-grid systems also need a battery bank; see how many batteries you need for solar.
Will my roof fit that many panels?
A typical 400 W residential panel measures roughly 1.7 m × 1.1 m (about 21 square feet). Twenty-one panels need around 440 square feet of usable, unshaded roof, before allowing for fire setbacks and spacing. Our guide on roof space for solar panels covers this in detail.
Common mistakes
- Using daylight hours instead of peak sun hours. Twelve hours of daylight might only be 4 peak sun hours.
- Forgetting system losses. Sizing without the 0.80 factor leaves you about 20% short.
- Using one summer bill. Air conditioning months can be double your spring usage.
- Ignoring shade. A chimney shadow on one panel can cut a whole string’s output unless you use optimizers or microinverters.
Run your own numbers
Enter your daily kWh and peak sun hours in our free solar calculator to get your array size, panel count, battery capacity and inverter size at the same time.
Worked example: a family home in Dallas
A household in Dallas, Texas has these bills over a year: about 700 kWh in mild winter months, 1,500 kWh in peak summer months, 12,000 kWh in total.
- Daily average: 12,000 ÷ 365 = 32.9 kWh per day.
- Peak sun hours: Dallas averages about 5.3.
- Array size: 32,900 ÷ (5.3 × 0.80) = 7,759 W.
- Panels: 7,759 ÷ 400 W = 19.4, so 20 panels of 400 W (8 kW).
If the family’s retail plan credits exports at a fair buyback rate, spring and autumn surplus offsets part of the summer shortfall. If they want to cover peak summer months in full, they would size for 1,500 kWh ÷ 30 = 50 kWh per day: 50,000 ÷ 4.24 = 11.8 kW, about 30 panels of 400 W.
FAQ
How many solar panels do I need for a 2,000 sq ft house?
House size is a weak guide because usage varies so much. A 2,000 sq ft home typically uses 25 to 35 kWh per day, which needs about 17 to 25 panels of 400 W at 4.5 peak sun hours.
Can I use fewer, higher-wattage panels?
Yes. The total array wattage is what matters. Ten 450 W panels (4,500 W) produce the same as fifteen 300 W panels, and the higher-wattage option uses less roof space.
Does a battery change how many panels I need?
Batteries store energy but do not create it. You still need enough panels to produce your full daily usage, plus a little extra to cover battery charging losses.
How many solar panels do I need for a 1.5 ton AC?
About 6 to 7 panels of 450 W for daytime running of an inverter AC. See solar panels for a 1.5 ton AC.
How many solar panels do I need for 5 kW?
At 400 W each, 13 panels (5.2 kW); at 550 W, 10 panels (5.5 kW). Choose the panel size that fits your roof best.
Related guides
- What Size Solar System Do I Need for 30 kWh Per Day?
- How Many Solar Panels Do I Need for 1,000 kWh Per Month?
- How Much Power Does a 400W Solar Panel Produce Per Day?
- Monocrystalline vs Polycrystalline Solar Panels: Which Is Better?
Sources and further reading
- NREL PVWatts Calculator: solar output estimate for any address.
- U.S. EIA: How Much Electricity Does an American Home Use?: average household use, about 10,800 kWh a year.
- European Commission JRC: PVGIS: free solar output and optimum-tilt tool for Europe and most of the world.
- Global Solar Atlas (World Bank): sun data and solar potential for any location.