Solar Panel Calculator
How big a solar system you need for your electricity use and how many panels that is, or what a number of panels will produce, from your local peak sun hours.
Result…
How it's calculated
Daily output per kW = peak sun hours × (1 − losses) System size (kW) = daily use (kWh) ÷ daily output per kW Panels = size × 1000 ÷ panel watts, rounded up Output = panels × watts ÷ 1000 × peak sun hours × (1 − losses)
Peak sun hours are the hours of full-strength sunlight (1000 W per square metre) your site gets in an average day, and they differ a lot by place and season, so enter your own figure from a solar map, installer or weather service. A panel's watt rating is measured at that strength of sunlight, so a 400 W panel in 4 peak sun hours makes 1.6 kWh before losses. The losses cover the inverter, wiring, heat, dust and shade. A monthly use is turned into a daily one with an average month of 365 ÷ 12 days. The panel count is rounded up, so the installed system is a little larger than the size needed.
Your numbers, step by step
About the Solar Panel calculator
This solar panel calculator works out how large a solar system you need to cover your electricity use, how many panels that takes, and how much energy they should make per day, per month and per year. It also works the other way: enter a number of panels and see their expected output. You give it your use in kWh per day or per month from a bill, the watt rating of the panel you are looking at, your local peak sun hours and an allowance for system losses. Peak sun hours change a lot from place to place and season to season, so the calculator never assumes a figure: use the yearly average for your own location from a solar map, an installer or a weather service. Every step is shown with your numbers. To see what that energy is worth, use the electricity cost calculator.
How to use it
- Under “I want”, choose “System for my use” or “Output of my panels”.
- For a system size, enter your “Electricity use” and pick whether it is per “Day” or per “Month”.
- For output, enter the “Number of panels” you have or plan to fit.
- Enter the “Panel rating” in watts from the data sheet and your local “Peak sun hours per day”.
- Set the “System losses” percentage, using your installer's figure if you have one, and read the size, panel count and output.
Worked example
A home uses 900 kWh a month, which is 900 ÷ (365 ÷ 12) = 29.59 kWh a day. With 4.5 peak sun hours and 14% losses, each kW of panels makes 4.5 × 0.86 = 3.87 kWh a day, so the system needs 29.59 ÷ 3.87 = 7.65 kW. With 400 W panels that is 7646 ÷ 400 = 19.1, rounded up to 20 panels, an 8 kW system. It should make about 30.96 kWh a day, 941.7 kWh in an average month and 11,300 kWh a year, about 105% of the use. Going the other way, 12 panels of 400 W (4.8 kW) in the same conditions make about 18.58 kWh a day.
Updated
Questions people ask
How do I calculate how many solar panels I need?
Divide your daily use in kWh by peak sun hours times (1 − losses) to get the system size in kW. Multiply by 1000 and divide by the panel's watt rating, then round up to a whole panel.
What are peak sun hours?
The number of hours a day of full-strength sunlight, 1000 watts per square metre, that would deliver the same energy your site gets over a whole day. A panel's watt rating is measured at that strength of sunlight, which is why the two multiply together.
What should I enter for system losses?
The share of energy lost to the inverter, wiring, heat, dust and shade. Your installer or a solar design tool can give a figure for your site; the calculator starts at 14% as an example only.
Why does my system make less in winter?
The calculator uses one yearly average. Real peak sun hours are higher in summer and lower in winter, so monthly output swings around the average, and a system sized on the yearly figure will not cover every winter month.
How accurate is this estimate?
It is a first estimate. Roof direction, tilt, shade from trees or buildings and panel temperature all change the output. A site survey from an installer gives the figure to plan and budget with; the solar panel angle calculator helps with the tilt.