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Solar Panel Size Calculator
How many watts of panels and how much battery you need for your daily use.
Free tools that run in your browser. Nothing you type, open or create is uploaded to NerdBible or anyone else.
1 kWh is 1,000 Wh. A household bill shows kWh per month: divide by 30 and multiply by 1,000 for Wh per day.
What are peak sun hours?
Peak sun hours are the day's sunlight expressed as hours of full sun (1,000 watts per square metre). Five hours of weak morning and evening light might add up to two peak sun hours. The figure depends on where you are, the season, and the tilt and direction of the panels, so it is not filled in for you. Free solar maps such as the European Commission's PVGIS or NREL's PVWatts give it for your exact location. Size for the worst month you need power in, usually midwinter, not the yearly average.
| Battery voltage | Capacity needed |
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System losses cover wiring, dust, heat, the charge controller, the inverter and charging the battery, and 20 to 30% is a common allowance. Battery usable depth depends on the chemistry: lithium (LiFePO4) batteries are often run to 80 to 90%, lead acid batteries to about 50% to keep them healthy. This is a sizing estimate, not a design for a grid connected installation, which needs a qualified installer.
How to use it
- Enter how many watt hours you use per day, or switch to listing your devices with their watts and hours.
- Enter the peak sun hours for your location and the worst month you need power in.
- Adjust system losses, days of autonomy and the battery's usable depth if you need to.
- Read the panel power, the number of panels of your chosen size, and the battery capacity in Wh and in amp hours at 12, 24 and 48 V.
Questions
How is the panel size worked out?
Daily use divided by peak sun hours gives the panel power you would need with no losses. That is then divided by what is left after losses: with 25% losses, by 0.75.
What are days of autonomy?
How many days the battery should keep you going with no sun at all, for example during a run of cloudy days. One day suits a camper or a cabin used in summer; off-grid homes often plan for two to three.
Why does the battery get bigger when I lower the usable depth?
If you only use 50% of a battery, it needs twice the capacity to deliver the same energy. That is why lead acid banks are sized much larger than lithium ones.
Which battery voltage should I choose?
Small systems such as vans and boats usually run at 12 V. Larger systems use 24 or 48 V because the same power then flows at a lower current, which allows thinner cables and smaller losses. Amp hours are shown for each.
Will the panels charge the battery fully in one day?
Only if the panel size covers both your daily use and the recharge. This tool sizes the panels for your daily use; after a run of sunless days the battery refills only from any surplus, so add panel power if you need faster recovery.