Battery Sizing Guide

By Kayla Liu April 8th 2024

Battery types and capacity relate to the overall inverter or charger performance. To size a proper battery, you need to identify the loads that you will be utilizing, as well as an estimated duration (hours/day) you will be using the load. Oversizing should be considered due to efficiency losses.

Follow the steps below to size a bank specific to your applications. In the steps. a 120V 0.4A fan running for 12 hours per day serves as an example.

Step 1: Determine Your Load Watts (Amps x Volts)

Every electronic device is labeled with watts (W) directly, or with voltage (V) and amperage (A). You can multiply the voltage and amperage to calculate the watts.

Watts (W) = Volts (V) x Amps (A)

Example: Fan Watts = 120V * 0.4A = 48 Watts

Step 2: Estimate Load Run-Time in Watt-Hours (Wh)

Estimate the run-hours per day you will be using the load and multiply this by your Watts per load.

Load Run-Time = Load Watts x Load Run Hours

Example: Fan Run-Time = 48W x 12 hours = 576 Wh

Step 3: Determine Theoretical Battery Capacity in Amp-Hour (Ah)

Divide your load run-time by the battery voltage to get the theoretical battery capacity (Ah).

Theoretical Battery Capacity (Ah) = Load Run-Time (Wh)/Battery Rated Voltage (V)

Use the 576Wh fan as an example. The required battery capacity should be 48 Ah (= 576Wh/12V).

Step 4: Oversize Battery Capacity

The Theoretical Battery Capacity (Ah) in Step 3 represents the minimum battery capacity to run your load for your intended time. Note that this assumes 100% use of a battery, which is not recommended. Assuming 50% depth of discharge, you need to multiply this Theoretical Battery Capacity by 2 and 1.25 to account for some efficiency losses.

Recommended Battery Capacity = Theoretical Battery Capacity x Oversize Ratio x Efficiency Loss Ratio

Use the fan as an example. Recommended battery capacity = 48Ah x 2 x 1.25 = 120Ah

Therefore, a 120Ah battery bank, or close, will be able to support a 12-hour run time for a 48V fan while also prolonging battery life for the best system size possible.
In scenarios where multiple batteries are connected in series, parallel, or series-parallel, the system capacity varies specific to the battery connection type. The table blow lists system capacity involving three batteries.