Solar Battery Calculator
A solar battery calculator sizes storage as daily kWh × days ÷ 0.8 depth of discharge: 10 kWh a day for 2 days needs 25 kWh, which is five 5 kWh batteries.
Solar battery calculator: storage and battery count
A solar battery calculator multiplies your daily energy use by the days of backup you want, then divides by the usable share of each battery, which is 0.8 here. At 10 kWh a day for 2 days, the bank must be 10 × 2 ÷ 0.8 = 25 kWh. With 5 kWh batteries, that is 5 batteries.
The bank is bigger than the energy you want to use, because the batteries are not drained to zero.
The formula in plain words
Storage needed = daily kWh × days of autonomy ÷ 0.8. Batteries = storage needed ÷ the capacity of one battery, rounded up. The calculator returns both the storage and the count.
Worked example, a cabin
The cabin uses 10 kWh a day. You want to ride out 2 sunless days. Each battery is 5 kWh.
- Energy for 2 days: 10 × 2 = 20 kWh
- Storage with reserve: 20 ÷ 0.8 = 25 kWh
- Batteries: 25 ÷ 5 = 5
Worked example, essential loads only
A home backs up the fridge, lights and internet, not the whole house. Those loads come to 8 kWh a day, and the owner wants 3 days, with 5 kWh batteries.
- Energy: 8 × 3 = 24 kWh
- Storage: 24 ÷ 0.8 = 30 kWh
- Batteries: 30 ÷ 5 = 6
Cutting the backup list from the whole house to a few loads is the biggest lever on cost.
Batteries needed with 5 kWh units
| Daily use | Days | Storage needed | Batteries |
|---|---|---|---|
| 5 kWh | 1 | 6.25 kWh | 2 |
| 5 kWh | 2 | 12.5 kWh | 3 |
| 10 kWh | 1 | 12.5 kWh | 3 |
| 10 kWh | 2 | 25 kWh | 5 |
| 10 kWh | 3 | 37.5 kWh | 8 |
| 20 kWh | 2 | 50 kWh | 10 |
Finding your daily kWh
- Read it from the bill. A 300 kWh month over 30 days is 10 kWh a day.
- For backup only, list each load. A 500 W device running 10 hours uses 500 × 10 ÷ 1,000 = 5 kWh.
- Include the loads that run at night, since the bank carries them when the panels are off.
- Remember that starting surges on motors can be higher than their running watts. The inverter must handle them.
Common mistakes
- Sizing for the whole house when only a few circuits need backup.
- Forgetting the reserve. A 25 kWh need is not met by 20 kWh of batteries.
- Using the nameplate capacity when the battery’s usable capacity is lower. Check the data sheet.
- Skipping the recharge side. The panels have to put back what you take out, and a cloudy week may not.
- Ignoring losses in the inverter and wiring. Leave a margin of safety.
Battery banks involve high current and fire risk. Have a licensed electrician or qualified solar installer design and wire the system, and confirm local code and utility rules.
Checking the result by hand
You can rerun the arithmetic in a minute. Take the 10 kWh cabin. Two days is 20 kWh. Dividing by 0.8 gives 25 kWh. Five batteries of 5 kWh hold 25 kWh in total, and 80% of that, 20 kWh, is what you can use.
If your batteries are rated for less than 80% use, scale by hand. At 50%, the same cabin needs 20 ÷ 0.5 = 40 kWh, which is eight 5 kWh batteries instead of five. Always read the battery maker’s data sheet for the recommended usable share, and size to that. The calculator keeps the 80% setting fixed, so it matches lithium-style banks and not lead-acid ones.
Related pages
Size the panels with the solar panel calculator. A generator is the other backup option, covered in the generator size calculator, and the solar panel angle calculator helps you set the tilt.
Frequently asked questions
What is depth of discharge?
It is the share of a battery's rated capacity you can use before recharging. This calculator assumes 80%. A 5 kWh battery then gives 4 kWh of usable energy, and the last 20% stays in reserve to protect the battery's life.
What are days of autonomy?
How many days the bank must carry your loads with no sun to recharge it. One day covers a night. Two or three days cover a stretch of cloudy weather or an outage. More days means more batteries and more cost.
Do lead-acid and lithium batteries size differently?
Yes. Lithium banks are often used down to about 80%, while lead-acid banks are usually kept to about 50% to protect them. This calculator is fixed at 80%. For a lead-acid bank, divide by 0.5 by hand: 10 kWh × 2 days ÷ 0.5 = 40 kWh.
How do I find my daily kWh?
Look at the kWh on your electric bill and divide by the days in the billing period. For a backup system, add up only the loads you want to keep running. Watts × hours of use ÷ 1,000 gives kWh for each device.
Does this include the solar panels or inverter?
No. It sizes storage only. You still need panels to recharge the batteries, and an inverter and charge controller that match the system. Inverter losses mean you should leave some margin.
Who should install a battery bank?
A licensed electrician or a qualified solar installer. Battery systems carry high current and can start fires if wired wrong, and local code and the utility have rules for interconnection.
Estimates only. Verify quantities with your supplier before purchasing.