Solar Battery Bank Size Calculator

Enter the total energy consumption per day, the number of backup days required, maximum depth of discharge, and voltage into the calculator to determine the solar battery bank size.

Solar Battery Bank Size Formula

A solar battery bank is sized by converting the energy you need during an outage or low-solar period into amp-hours at a specific system voltage, then adjusting for the maximum allowable depth of discharge. This calculator gives a strong baseline capacity target for battery-based solar systems, backup systems, cabins, RVs, and off-grid installations.

BB = \frac{P \cdot D \cdot 100}{V \cdot DI}

BB = required battery bank size in amp-hours (Ah)
P = energy consumption per day in watt-hours (Wh/day)
D = number of backup days required
V = system voltage in volts (V)
DI = maximum depth of discharge entered as a percentage

You can also think about the process in two steps: first calculate the total energy that must be stored, then convert that stored energy into battery capacity.

E_{req} = P \cdot D

BB = \frac{E_{req}}{V \cdot (DI/100)}

The result is the nominal battery bank size needed at the chosen voltage. If you allow a smaller portion of the battery to be used, the required bank size increases.

How to Use the Calculator

Estimate your total daily energy usage in watt-hours.
Choose how many days of backup you want the battery bank to support.
Enter the maximum depth of discharge your battery chemistry can safely handle.
Enter the battery bank voltage for the system.
Use the result to choose a real battery configuration with at least that much total capacity.

If your loads are listed in kilowatt-hours, convert them to watt-hours before sizing the bank.

Wh = kWh \cdot 1000

How Each Input Changes the Result

Input	What It Means	Effect on Battery Bank Size
Energy consumption per day	Total energy your loads use in one day	Higher daily usage increases required capacity directly
Backup days	How long the system must run without meaningful recharging	More days require a larger bank
Depth of discharge	How much of the battery you plan to use before recharging	Lower allowable discharge means a larger nominal bank is needed
Voltage	The nominal battery bank voltage	Higher voltage reduces the amp-hour value needed for the same energy storage

Example

Assume a system uses 6 kWh per day, requires 2 days of backup, has an 80% maximum depth of discharge, and operates at 48 V.

Wh = 6 \cdot 1000 = 6000

BB = \frac{6000 \cdot 2 \cdot 100}{48 \cdot 80} = 312.5

In this case, the system needs about 312.5 Ah at 48 V. In practice, most users round up and choose the next larger available bank size rather than sizing exactly to the minimum result.

Converting Battery Capacity to Stored Energy

Loads are usually discussed in watt-hours or kilowatt-hours, while batteries are often sold in amp-hours. These conversions help compare the calculator output to actual battery products.

E = Ah \cdot V

kWh = \frac{Ah \cdot V}{1000}

That means two battery banks with the same amp-hour rating can store very different amounts of energy if their voltages are different.

Practical Sizing Notes

Daily load is the main driver. Underestimating energy use is one of the fastest ways to undersize a battery bank.
Autonomy matters. A system designed for one day of backup will be much smaller than one designed for multiple cloudy days.
Depth of discharge affects usable storage. A battery that is only lightly discharged each cycle needs more total nameplate capacity to deliver the same usable energy.
Voltage changes the amp-hour number. Higher-voltage systems deliver the same energy with fewer amp-hours.
The calculator is a baseline sizing tool. Real systems often add extra margin for inverter losses, wiring losses, battery aging, temperature effects, and future load growth.

Common Input Mistakes

Using watts instead of watt-hours for daily energy consumption
Entering monthly or weekly usage as if it were daily usage
Comparing amp-hours across different voltages without converting to energy
Choosing a depth of discharge that is deeper than the battery manufacturer recommends
Ignoring surge loads, inverter efficiency, or seasonal changes in power demand

Choosing an Actual Battery Bank

After calculating the required amp-hours, choose a battery configuration that matches your system voltage and meets or exceeds the target capacity. The final bank should also be checked against discharge rate limits, charging rate limits, inverter requirements, and the battery manufacturer’s recommended operating range. For most installations, it is better to treat the calculator result as a minimum design point rather than an exact final specification.