A solar battery bank stores the energy your panels make so you can use it at night and on cloudy days. If it is too small, you run out of power. If it is too big, you waste money. This guide shows how to size it step by step, with a formula, examples and a calculator you can export to PDF.
The formula
- Daily Wh: your total daily energy use.
- Days of autonomy: how many days without sun the batteries should cover. Many homes use 1 to 3.
- Depth of discharge (DoD): the share of the battery you can safely use. Lithium iron phosphate (LiFePO4) is often about 80% to 90%, while lead-acid is usually limited to about 50%.
- Inverter efficiency: typically 90% to 95%.
- Battery bank voltage: 12 V, 24 V or 48 V, set by your inverter.
Worked example
You use 5 kWh (5,000 Wh) per day and want 1 day of autonomy. Your inverter is 93% efficient, and the bank is 48 V.
- LiFePO4 (90% usable): 5,000 / (0.9 x 0.93) = about 5,970 Wh. At 48 V, that is about 125 Ah.
- Lead-acid (50% usable): 5,000 / (0.5 x 0.93) = about 10,750 Wh. At 48 V, that is about 225 Ah.
The lead-acid bank needs nearly twice the capacity for the same usable energy, which is one reason many people choose lithium despite its higher price per battery.
Try the solar battery bank calculator
Enter your daily or monthly energy use, days of autonomy, battery type and voltage. The calculator gives the battery energy needed, the Ah at your bank voltage, and how many batteries to connect in series and parallel. You can export the result as a PDF.
Solar Battery Bank Calculator
How big should your battery bank be?
Your numbers
Choosing a battery type fills in a typical usable depth of discharge. You can change it. Always match the bank voltage to your inverter.
Result
Checks
Bank size at different days of autonomy
Battery Wh = Daily Wh x Days / (DoD x Inverter efficiency). Ah = Wh / bank voltage. Batteries in series = bank V / battery V, in parallel = bank Ah / battery Ah, both rounded up. Estimates only. Have your final design reviewed by a qualified electrician or installer.
Choosing the system voltage
Higher voltage means lower current, which allows thinner wire and less loss. As a common rule of thumb:
- 12 V: small systems, roughly up to 1,000 to 1,500 W.
- 24 V: mid-size systems, roughly up to 3,000 W.
- 48 V: larger home systems above about 3,000 W.
Always follow your inverter's required battery voltage.
Series and parallel
Connecting batteries in series adds voltage and keeps the Ah the same. Connecting them in parallel adds Ah and keeps the voltage the same. For example:
- 4 x 12 V 100 Ah in series = 48 V, 100 Ah (4.8 kWh).
- 4 x 12 V 100 Ah in parallel = 12 V, 400 Ah (4.8 kWh).
The total energy is the same. Use identical batteries of the same age and type, and follow the manufacturer's limits on how many can be connected.
Lithium vs lead-acid
| LiFePO4 | Lead-acid | |
|---|---|---|
| Usable capacity | About 80% to 90% | About 50% |
| Cycle life | Often several thousand cycles | Usually a few hundred to about 1,500 |
| Weight | Lighter | Heavier |
| Upfront cost | Higher | Lower |
| Maintenance | Very low | Some types need watering |
Tips
- Check the maximum charge and discharge current of your battery against your array and inverter.
- Leave some spare capacity, because batteries lose capacity as they age.
- Protect the bank with correctly sized fuses or breakers close to the battery.
- Cold weather reduces battery performance, especially for lead-acid, and lithium should not be charged below freezing unless it has built-in heating.
Disclaimer: This article is for general information and education only. Numbers are estimates and examples, not a certified design. Have your final solar design reviewed by a qualified electrician or installer and follow your local electrical codes.
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