Solar Panel Calculator
This solar panel calculator sizes a home solar system from your electricity use and the sun where you live. Enter your monthly kWh, the peak sun hours, system losses, and the panel wattage, and the solar panel calculator returns the system size in kW, the number of panels, the roof area they need, and the energy the system should produce in a year.
Results
The Solar Panel Calculator is part of our Solar collection. Browse more solar to size, convert, and check your work alongside this tool.
What the solar panel calculator does
A solar system must produce, over a year, the energy you want to cover. The solar panel calculator turns your monthly use into a daily figure, divides it by the peak sun hours at your location, and allows for losses in wiring, heat, dust, and the inverter. The result is the system size in kilowatts, which it then splits into whole panels of the wattage you choose.
How to calculate solar panel size
The calculator follows three steps:
Peak sun hours are the hours a day the sun would deliver 1,000 W per square meter, a standard way to express local sunshine. Roof area uses about 20 sq ft per 400 W panel, and yearly output is the system size times peak sun hours times 365, less losses.
Solar panel calculator example
A home using 900 kWh a month, with 4.5 peak sun hours and 14% losses:
With 400 W panels that is 20 panels, an 8 kW system on about 400 sq ft of roof, producing roughly 10,800 kWh a year.
How to use the solar system size calculator
Find your monthly kWh on your electric bill; an average over 12 months is best. Look up the peak sun hours for your area, usually 3.5 to 6 in most of the world. Keep 14% for losses unless your installer gives another figure, enter the wattage of the panels you are considering, and set the share of your use to cover. Press Calculate.
Reading your solar panel output calculator result
The system size is what you need on average across the year. Output is higher in summer and lower in winter, and net metering or a battery smooths the difference. If your roof has limited space, choose higher wattage panels to fit the same kW. Shade, a north-facing roof in the northern hemisphere, or a steep tilt can cut output, so ask an installer for a site survey.
When to use a solar panels for home calculator
- Getting a first estimate before installer quotes
- Checking whether a quote is sized to your use
- Planning partial coverage to fit a budget
- Comparing panel wattages for a small roof
- Sizing for a future EV or heat pump
Common solar panel calculator mistakes
- Using one summer bill. Average a full year.
- Using daylight hours. Use peak sun hours.
- Ignoring losses. 10 to 20% is normal.
- Forgetting shade. Partial shade cuts output sharply.
Tips for a solar calculator estimate
- Get peak sun hours from a solar map for your exact area.
- Plan for future loads now.
- Compare at least three installer quotes.
How to calculate solar panel size for a whole house
Start with a year of bills, since use swings with the seasons. If you plan to add an electric car or a heat pump, add their expected use first: an EV driven 12,000 miles a year adds about 3,400 kWh. Then size the array, and choose an inverter for it with the solar inverter size calculator. Most grid-tied systems aim for 80 to 100% of yearly use, since covering more than you use often earns little.
Solar array size, cost, and storage
Bigger is not always better. Each extra kW costs money, so check the payback of different sizes with the solar payback calculator. If you want backup power or live off the grid, the array must also charge batteries, and the battery bank calculator sizes the storage for your daily use and days of autonomy.
Solar array size by use and peak sun hours
System kW and number of 400 W panels at 14% losses:
| Monthly use | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours |
|---|---|---|---|
| 500 kWh | 5.5 kW (14) | 4.2 kW (11) | 3.5 kW (9) |
| 750 kWh | 8.2 kW (21) | 6.4 kW (16) | 5.2 kW (14) |
| 1,000 kWh | 10.9 kW (28) | 8.5 kW (22) | 7.0 kW (18) |
| 1,500 kWh | 16.4 kW (41) | 12.7 kW (32) | 10.4 kW (27) |
Solar Panel Calculator FAQ
How many solar panels do I need?
Divide your daily kWh by peak sun hours and losses to get kW, then by panel watts. 900 kWh a month at 4.5 sun hours needs about 7.6 kW, or 20 panels of 400 W.
What are peak sun hours?
The number of hours a day that sunlight averages 1,000 W per square meter. Most locations get 3.5 to 6.
How much roof space do solar panels need?
About 18 to 22 sq ft per panel. Twenty 400 W panels need roughly 400 sq ft.
How much does a kW solar system produce?
About peak sun hours × 365 × 0.86 kWh a year per kW, around 1,400 kWh at 4.5 sun hours.