Solar Panel Calculator
A solar panel calculator sizes an array as daily kWh × 1.2 ÷ peak sun hours: 30 kWh a day at 5 sun hours needs 7.2 kW, or 18 panels at 400 W.
How many solar panels you need
A solar panel calculator estimates the size of the array that would cover your daily electricity use. Enter your daily use in kilowatt-hours, the peak sun hours at your site, and the rating of the panel you plan to use in watts. The tool returns the array size in kilowatts and the number of panels.
Think of panels as buckets set out in the rain. Peak sun hours are how long the rain falls at full strength. You need enough buckets to fill your daily water tank, and a few extra to cover the splash that gets lost.
The formula in plain words
- Array size (kW) = daily kWh × 1.2 ÷ peak sun hours.
- Panels = array size in watts ÷ panel watts, rounded up.
The 1.2 multiplier is a 20 percent cushion for system losses. Panels never deliver their full rated power all day, so you size up.
Worked example: a larger home
A home uses 30 kWh a day. The site gets 5 peak sun hours, and the panels are 400 W.
- Array size: 30 × 1.2 ÷ 5 = 7.2 kW
- In watts: 7,200
- Panels: 7,200 ÷ 400 = 18
Worked example: a smaller home in a cloudier spot
A home uses 18 kWh a day. The site gets 4 sun hours, and panels are 430 W.
- Array size: 18 × 1.2 ÷ 4 = 5.4 kW
- Panels: 5,400 ÷ 430 = 12.56, rounded up to 13
Array size by daily use
All rows assume 5 sun hours and 400 W panels.
| Daily use | Array size | Panels |
|---|---|---|
| 10 kWh | 2.4 kW | 6 |
| 20 kWh | 4.8 kW | 12 |
| 30 kWh | 7.2 kW | 18 |
| 40 kWh | 9.6 kW | 24 |
| 50 kWh | 12.0 kW | 30 |
Sun hours change the answer a lot
At 30 kWh a day with 400 W panels:
| Peak sun hours | Array size | Panels |
|---|---|---|
| 3.5 | 10.29 kW | 26 |
| 5 | 7.20 kW | 18 |
| 6 | 6.00 kW | 15 |
A sunnier site needs far fewer panels for the same use, so check your actual number before you size anything.
How to find your inputs
For daily use, read your utility bill. It lists kilowatt-hours for the month. Divide by the number of days in the billing period.
For peak sun hours, ask an installer or check a public solar resource map for your region. Orientation and tilt affect it too. A south-facing roof in the northern hemisphere gets the most sun.
For panel wattage, read the spec sheet of the panel you are considering. Common residential panels today have ratings in the several-hundred-watt range, but ratings vary by model.
Panels need roof or ground area, roughly on the order of 20 sq ft each for a modern residential panel. Confirm the size on the spec sheet.
What this does not cover
The result is a size for an array that makes the energy. It does not cover batteries, net metering rules, the cost, or how well your roof can host the panels. For backup power during outages, see the generator size calculator. To see what you pay for the power you use now, try the electricity cost calculator.
Common mistakes
- Using daylight hours in place of peak sun hours. Peak sun hours are fewer.
- Using monthly use as daily use. Divide by the days.
- Ignoring shade. Even a small shadow on a string of panels can cut output.
- Forgetting panel degradation. Output slowly declines over the years.
- Sizing only for a low-use month. Look at your highest season too.
- Skipping the permit. Solar needs electrical work that should be done by a licensed professional and inspected.
Related calculators
Estimate monthly bills with the electricity cost calculator. The generator size calculator is for backup power, and the kVA to amps calculator helps when you look at inverter or service sizes.
Frequently asked questions
How many solar panels do I need?
Multiply your daily energy use in kWh by 1.2, divide by your peak sun hours to get kW, then divide by the panel wattage. At 30 kWh a day, 5 sun hours, and 400 W panels, the answer is 18 panels.
What is the 1.2 factor for?
It adds 20 percent to cover real-world losses such as wiring, inverter efficiency, heat, and dirt on the panels. Different designers use different allowances.
What are peak sun hours?
They are the hours per day of full-strength sunlight your location gets on average. It is not the same as daylight hours. Look up your area on a solar resource map or ask an installer.
How do I find my daily use?
Look at your electric bill for the monthly kWh and divide by about 30. For a more careful number, use the highest and lowest months across the year.
Is the result enough to order a system?
No. It is a first estimate. Roof direction, shading, local rules, and the utility's connection terms all matter. A licensed installer or electrician should design and install the system.
Estimates only. Verify quantities with your supplier before purchasing.