Solar Battery Size Calculator
Enter the energy you want to keep running and for how many days. The calculator estimates the battery capacity you need in kWh and how many batteries of a chosen size that takes.
How to use this Solar Battery Size Calculator
- Estimate the energy in kWh per day of the loads you want to run during an outage or at night.
- Enter how many days you want the battery to cover without recharging.
- Enter the usable depth of discharge from the battery data sheet.
- Enter the efficiency of the battery and inverter together.
- Enter the capacity of one battery and press Calculate.
How to size a solar battery
The battery must hold the energy you need, plus the part you cannot use and the part lost in conversion.
- Energy to cover = daily load x days
- Capacity needed = energy / (depth of discharge x efficiency)
- Batteries = capacity needed / capacity of one battery, rounded up
Worked example
You want to back up 10 kWh a day for one day. With a 90% depth of discharge and 90% efficiency the battery bank must hold 10 / (0.90 x 0.90) = 12.35 kWh. If each battery stores 5 kWh, you need 3 batteries, which gives 15.0 kWh installed and 2.65 kWh of spare capacity.
If you want two days of backup the capacity doubles to 24.69 kWh, which means 5 batteries. Longer backup raises the cost fast, so many people size for a few critical loads such as the fridge, lights and internet instead of the whole house. Also check the power rating of the battery. A battery can hold enough energy but still be unable to start a large motor or run an air conditioner and an oven at once.
Assumptions & limits
- Depth of discharge and efficiency are inputs. Use the numbers from the data sheet of the product you plan to buy.
- The load is a steady daily average. Peaks and surges are not modelled.
- Battery capacity fades with age and with temperature.
- The solar array also needs to recharge the battery. Use the solar size calculator for that.
- It is a planning estimate and not a system design. Ask a licensed installer before buying.
Frequently asked questions
How do I size a battery for solar?
Work out the daily energy to back up, multiply by the days you want, then divide by the usable depth of discharge and the efficiency.
What is depth of discharge?
It is how much of the battery capacity you can use without harming it. A higher figure means more usable energy.
How many days of backup do I need?
It depends on how long outages last in your area and what you want to run.
Does the battery need a power rating too?
Yes. Capacity is in kWh, but power in kW decides which appliances can run at once.
Does battery size depend on my solar array?
The array must be able to recharge the battery as well as run your home.
Do batteries lose capacity?
Yes, over time and with heat. Allow some spare capacity.
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Last updated: November 2026