Solar Panel Calculator
How many solar panels it takes to cover your electricity use, from your kWh, the peak sun hours where you live, the panel rating and the system losses.
Result
Enter electricity use and peak sun hours a day to see the result.
https://calcgallery.com/energy-utilities/solar-panel-calculator/
Results are estimates based on the ratings, usage and prices you enter. Actual consumption, losses, tariffs and equipment performance vary. Check your bill, meter and manufacturer information before relying on the result. Read the full disclaimer
How to use the Solar Panel Calculator
- Choose whether your figure is for a day, a month or a year, and enter the kWh from your bills.
- Enter the peak sun hours for your location and the way the panels will face, from NREL PVWatts or a solar resource map.
- Enter the panel rating in watts, and how much of your use you want the panels to cover.
- Check the system losses and inverter efficiency, which open on the PVWatts defaults, and read the number of panels.
How it is calculated
daily use = kWh for the period ÷ days in the periodderate = (1 − system losses) × inverter efficiencyarray needed = daily use × share to cover ÷ (peak sun hours × derate)panels = ⌈array needed ÷ panel rating⌉A month is taken as 365 ÷ 12 days and a year as 365. Panels are whole, so the count is rounded up, and the system you would install is a little larger than the array strictly needed. The page shows what those whole panels produce in an average day and year, and what share of your use that is.
Where the loss figures come from
The system losses and inverter efficiency open on the defaults of PVWatts, the free calculator from the US National Renewable Energy Laboratory, as its version 5 manual sets them out. They are starting values, not measurements of your system: a panel shaded for part of the day, or a roof that is rarely cleaned, loses more.
Getting a better number
Use a year of bills rather than one month, since use swings with the seasons. For peak sun hours, PVWatts gives a month-by-month figure for any address and tilt; the page works with an average, so a system sized on the yearly average makes more than you use in summer and less in winter. If your tariff pays little for exported power, sizing to cover less than all of your use can make sense; the solar payback calculator shows what the panels are worth to you.
With your numbers
- Derate = (1 − 14%) × 96% = 82.56%
- Array needed = 29.59 kWh × 100% ÷ (5 h × 82.56%) = 7.168 kW
- Panels = ⌈7.168 kW ÷ 400 W⌉ = 18
Quick reference
A panel count from average sunlight, not a system design: it does not size an inverter, check a roof, plan wiring or answer permit and grid-connection rules, which vary by place.
| Example | Panels |
|---|---|
| 900 kWh a month, 5 sun hours, 400 W panels | 18 |
| 900 kWh a month, 3 sun hours, 400 W panels | 30 |
| 30 kWh a day, 4 sun hours, 350 W, 80% of use | 21 |
| 10,000 kWh a year, 5.5 sun hours, 450 W | 14 |
| 500 kWh a month, 4.5 sun hours, 430 W | 11 |
| 3,500 kWh a year, 2.8 sun hours, 400 W | 11 |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
Dobos, A. P. PVWatts Version 5 Manual, National Renewable Energy Laboratory, Technical Report NREL/TP-6A20-62641, September 2014, Table 2, p. 3: "System losses % 14"; section 11, System Losses, p. 10: "Losses represented by this number include the impacts of soiling, shading, snow cover, mismatch, wiring, connections, light induced degradation, nameplate rating, system age, and operational availability"; Table 6, "Total losses 14 %"; "the inverter is explicitly modeled". https://www.nrel.gov/docs/fy14osti/62641.pdf (read from the Internet Archive copy, because nrel.gov did not resolve from the build machine)
Used for
- NREL PVWatts default total system losses: 14% of DC energy for soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate rating, age and availability. It leaves out the inverter and module heating, which PVWatts models separately.
Dobos, A. P. PVWatts Version 5 Manual, National Renewable Energy Laboratory, Technical Report NREL/TP-6A20-62641, September 2014, Table 2, p. 3, Advanced inputs: "Inverter efficiency % 96"; p. 3: "The nominal inverter efficiency can be entered by the user." https://www.nrel.gov/docs/fy14osti/62641.pdf (read from the Internet Archive copy, because nrel.gov did not resolve from the build machine)
Used for
- NREL PVWatts default nominal inverter efficiency: 96%, the share of DC energy the inverter delivers as AC.
Common questions about the Solar Panel Calculator
- How many solar panels do I need?
- Divide the energy you use in a day by what one panel makes in a day, and round up. One panel makes its rating in kW times the peak sun hours times the derate, the share of rated output that reaches your sockets after losses. The page does that division with your own numbers, so the answer changes most with your use and your sunlight.
- What are peak sun hours?
- The sunlight a surface receives in a day, written as the number of hours it would take at full sun, 1,000 watts per square metre, which is the brightness a panel is rated at. A day with 5 kWh per square metre of sunlight is 5 peak sun hours, even though the sun was up far longer. The figure depends on where you are, the season, and the tilt and direction of the panels, so use one for your own site.
- Why is 14% the default loss?
- It is the default of NREL's PVWatts calculator, which adds up allowances for dirt, shading, snow, mismatch between panels, wiring, connections, early light-induced degradation, nameplate tolerance and downtime. PVWatts treats the inverter separately, at 96% by default, and so does this page. Change either if you know your site is shadier or your equipment differs.
- Does this include losses from hot panels?
- No. Panels make less power when they are hot, and PVWatts models that hour by hour from the weather, which a single daily figure cannot. In a hot climate, add a few percent to the system losses to allow for it.
- Is this how an installer sizes a system?
- It is the first step. An installer also checks roof space and orientation, shading through the year, the inverter and its ratings, the wiring, the roof structure, and the permit and grid-connection rules where you live, which vary by place. This page gives a panel count to start that conversation, not a design.