Solar Panel Calculator
This solar panel calculator estimates how big a solar system you need to cover your electricity use. Enter your monthly or annual kWh from your utility bill, the peak sun hours for your area, your panel wattage and a system efficiency factor, and it returns the system size in kilowatts and the number of panels.
It also estimates how many kWh the installed panels would produce in a year and what share of your usage that covers. Set the offset goal below 100% if you only want to cover part of your bill. Treat the result as a planning number to compare against installer quotes, not a final design.
Solar Panel Calculator
How to Use the Solar Panel Calculator
- 1
Enter your electricity use
Choose Monthly kWh or Annual kWh and type the figure from your utility bill. A 12-month average smooths out seasonal swings.
- 2
Set your peak sun hours
Enter the average daily peak sun hours for your location. The default is 4.5; use the regional table under the calculator or NREL PVWatts for a better number.
- 3
Adjust panel wattage, derate and offset
Keep 400 W panels and 80% efficiency or change them to match a quote. Lower the offset goal to size a system that covers only part of your use.
- 4
Read the results
See the system size in kW, the number of panels, the installed kW, the estimated annual production and the percentage of your usage covered.
How the solar sizing formula works
Peak sun hours describe how much solar energy your location receives, expressed as hours of full-strength sunlight (1,000 watts per square meter) per day. A 1 kW array in full sun for one hour produces about 1 kWh before losses.
Real systems lose some output to inverter conversion, wiring, heat, dust and shading. The derate factor accounts for that. An 80% derate is a common conservative planning value; NREL PVWatts, for comparison, uses 14% system losses by default before inverter efficiency.
Daily kWh = annual kWh x offset / 365; System size (kW) = daily kWh / (peak sun hours x derate); Panels = system kW x 1,000 / panel watts, rounded up; Annual production = installed kW x peak sun hours x derate x 365Worked example: 900 kWh a month
Say your bills average 900 kWh per month, you get about 4.5 peak sun hours, and you plan to use 400 W panels with an 80% derate and a 100% offset goal.
- Annual use: 900 x 12 = 10,800 kWh
- Daily target: 10,800 / 365 = 29.6 kWh per day
- System size: 29.59 / (4.5 x 0.8) = 8.22 kW
- Panels: 8,219 W / 400 W = 20.5, rounded up to 21 panels (8.4 kW installed)
- Annual production: 8.4 x 4.5 x 0.8 x 365 = about 11,038 kWh, or roughly 102% of your use
System size by monthly usage and sun hours
Results from this calculator with 400 W panels, an 80% derate and a 100% offset. The number in brackets is the panel count.
| Monthly use | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 500 kWh | 5.87 kW (15) | 4.57 kW (12) | 3.74 kW (10) |
| 750 kWh | 8.81 kW (23) | 6.85 kW (18) | 5.60 kW (15) |
| 1,000 kWh | 11.74 kW (30) | 9.13 kW (23) | 7.47 kW (19) |
| 1,250 kWh | 14.68 kW (37) | 11.42 kW (29) | 9.34 kW (24) |
| 1,500 kWh | 17.61 kW (45) | 13.70 kW (35) | 11.21 kW (29) |
Tips and common mistakes
- Use a full year of bills. A single summer or winter month can overstate or understate your needs by a wide margin.
- Peak sun hours are not daylight hours. A place with 12 hours of daylight may still average only 4 to 5 peak sun hours.
- Plan for future loads. An electric vehicle, heat pump or induction range can add thousands of kWh a year.
- Roof space limits panel count. Orientation, shade from trees and chimneys, and fire setbacks all reduce usable area.
- Production varies by season. Your system may overproduce in summer and underproduce in winter, which is how net metering or battery storage comes into play.
Get a site assessment before you buy
This estimate assumes an unshaded, well-oriented array and an average year. A qualified solar installer will assess your roof angle and direction, shading, roof condition and structure, electrical panel capacity, local permitting and utility interconnection rules, and will model production for your exact address. Use this calculator to check that quotes are in a sensible range.
Features
- Enter monthly or annual kWh usage
- Editable peak sun hours with a US regional reference table
- Adjustable panel wattage (default 400 W) and system derate (default 80%)
- Offset goal to cover part or all of your usage
- System size in kW, panel count, and estimated annual production
Frequently Asked Questions
How many solar panels do I need for 1,000 kWh per month?
With 4.5 peak sun hours, 400 W panels and an 80% derate, you need about a 9.13 kW system, which rounds up to 23 panels. In a sunnier area with 5.5 peak sun hours, it drops to about 7.47 kW or 19 panels. In a cloudier area with 3.5 peak sun hours, it rises to about 11.74 kW or 30 panels.
How much electricity does a 400 watt solar panel produce?
Using this calculator's method, a 400 W panel produces about 400 x 4.5 x 0.8 = 1.44 kWh per day with 4.5 peak sun hours and an 80% derate, or about 526 kWh per year. With 5.5 peak sun hours, that rises to about 1.76 kWh per day or 642 kWh per year. Actual output depends on tilt, shade, temperature and weather.
What are peak sun hours?
Peak sun hours are the average number of hours per day that sunlight would need to shine at 1,000 watts per square meter to deliver the day's total solar energy. They are lower than daylight hours because morning, evening and cloudy light is weaker. Much of the US falls between roughly 3.5 and 6 peak sun hours per day on an annual average, with the Southwest at the high end.
What derate or system efficiency should I use?
An 80% derate is a common conservative planning value. It covers inverter conversion, wiring losses, heat, dust and minor shading. A clean, unshaded system with a modern inverter may do better, around 85%, while a partly shaded or dusty array may do worse. You can change the derate in the calculator to see how much it moves the system size.
Should I size solar to cover 100% of my electricity use?
Not always. It depends on your utility's net metering rules, roof space and budget. If excess production earns little credit, a system that covers 70 to 90% of your use may pay back faster. Set the offset goal in the calculator to compare. For example, a 50% offset for 900 kWh a month needs about 4.11 kW, or 11 panels at 400 W with 4.5 sun hours.
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