Solar Generator and Battery Size Calculator
This calculator works out how big a battery or portable power station you need for the devices you plan to run, and how many watts of solar panels it takes to refill that battery in a day. List each device with its watts and hours of use. The result is in watt-hours (Wh), the unit power station and battery listings use for capacity, and in watts for the panels and the inverter.
Turn on JavaScript to use the calculator. The formula and a worked example are below, so you can also work it out by hand.
0 Wh of battery or power station capacity
and about 0 watts of solar panels to refill one day's use.
| Energy your devices use per day | Wh |
| Energy drawn from the battery per day, after inverter loss | Wh |
| Watts if every device runs at once | W |
Pick an inverter or power station whose continuous AC output is at least 0 W. Motors and compressors also need a higher surge rating, so check each device's manual for its starting watts.
A smaller portable power station fits this. Good next reads:
This is mid-size power station territory, around 1,000 to 1,500 Wh. Good next reads:
You need a large power station, around 2,000 to 3,000 Wh, or one that takes an extra battery. Good next reads:
This is more than most single power stations hold. Look at expandable home backup units or a LiFePO4 battery bank. Good next reads:
For the panels: Best Portable Power Station with Solar Panels for Camping: 5 Compared · Best Solar Powered Generator with Panels (2026): 4 Compared · Best Solar Panel Kit for RV: 5 Kits Compared by Spec
How the calculator works
1. Daily energy. For each device: watts × hours per day × how many. Add the rows together to get watt-hours per day.
2. Inverter loss. Devices that plug into an AC outlet run through an inverter, which loses some energy as heat. The calculator divides the daily energy by the inverter efficiency to get the energy actually drawn from the battery. If every device runs on DC or USB, set the efficiency to 100.
3. Battery capacity. Energy drawn per day × days without sun ÷ the usable share of the battery. Batteries are not meant to be run from full to completely empty every day, so the listed capacity has to be larger than what you use. The usable share is the depth of discharge the battery or power station maker states.
4. Solar panel watts. Energy drawn per day ÷ (peak sun hours × (1 − system losses)). A peak sun hour is one hour of sunlight at full strength, so a site with 4 peak sun hours collects about as much energy as 4 hours of full sun, even though the sun is up longer. System losses cover wiring, dirt on the panels, heat and other real-world losses.
5. Inverter output. The watts of every device running at the same time. A power station or inverter needs a continuous AC rating at least that high. Motors and compressors, such as fridges, pumps and power tools, draw more for a moment when they start, so check their manuals for starting watts and compare with the surge rating.
Where the starting values come from
| Input | Starting value | Where it comes from |
|---|---|---|
| Solar system losses | 14% | The default in NREL’s PVWatts calculator |
| Peak sun hours | 4 | A placeholder. Look up your location on NREL PVWatts |
| Usable share of the battery | 80% | A placeholder. Use the depth of discharge on your battery or power station listing |
| Inverter efficiency | 90% | A placeholder. Use the efficiency on your inverter or power station listing |
Only the 14% loss figure is a published default. The other three change a lot from one location or product to the next, so replace them with your own numbers before you buy. Peak sun hours in particular are much lower in winter than in summer in most of the US.
Worked example
Say a weekend camping setup runs a CPAP rated 40 watts for 8 hours, a laptop rated 60 watts for 4 hours, and four LED lights rated 10 watts each for 5 hours. These are example label figures, not measurements of any product.
- Daily energy: 40 × 8 + 60 × 4 + 10 × 5 × 4 = 320 + 240 + 200 = 760 Wh.
- After a 90% efficient inverter: 760 ÷ 0.90 = 844 Wh drawn from the battery.
- Battery for one day at 80% usable: 844 × 1 ÷ 0.80 = 1,056 Wh.
- Panels at 4 peak sun hours and 14% losses: 844 ÷ (4 × 0.86) = 245 watts.
- Everything at once: 40 + 60 + 4 × 10 = 140 watts of continuous inverter output.
So this setup points to a power station of roughly 1,000 Wh or more with at least 140 watts of AC output, and about 250 watts of panels. Covering two cloudy days doubles the battery figure to about 2,110 Wh, while the panel figure stays the same.
Using the numbers when you shop
- Capacity is listed in Wh on power stations. Some batteries list only amp-hours (Ah) and a voltage; the listing’s own Wh figure, where given, is the one to compare.
- Continuous and surge output are listed separately. Compare continuous output with the “every device at once” figure, and surge with your largest motor’s starting watts.
- Solar input limit. A power station accepts only so many watts of solar, and only within a stated voltage range. Check that limit before you buy more panel watts than it can take.
- Cutting use shrinks everything: fewer hours, lower-watt devices or DC versions of AC devices all lower the result. Run the calculator again to see the difference.
Questions
Where do I find a device’s watts?
On the label, the power adapter or in the manual. If the label gives only volts and amps, multiply them: 120 volts × 1 amp is 120 watts. Labels show the most a device can draw, so devices that cycle on and off, such as fridges, use less over a day than watts × 24 hours.
Why is the battery bigger than my daily use?
Three reasons: the inverter loses some energy, the battery is not meant to be emptied completely, and you may want to cover days without sun. Each one adds to the capacity you need.
Can I size the panels for cloudy days too?
The panel figure refills one day’s use in the sun hours you entered. To recover from cloudy days faster, enter fewer sun hours or add panel watts, within the solar input limit of your power station or charge controller.
Does this work for wiring a house?
No. It sizes portable and off-grid gear such as power stations, RV and van systems and camping setups. Anything connected to a home’s wiring needs a licensed electrician.
Sources
- Aron P. Dobos, “PVWatts Version 5 Manual“, National Renewable Energy Laboratory, 2014: total system losses with a default of 14%.
- NREL PVWatts Calculator: look up the solar resource for your address.