Battery Size Calculator

Size a battery so it lasts the runtime you need, with margin.

How the Math Works

The Battery Size Calculator uses a straightforward electrical engineering formula to determine the required battery capacity. The calculation multiplies the load current (in amperes) by the desired runtime (in hours), then divides by the usable fraction of the battery to account for efficiency losses. For example, if a device draws 2 amps and needs 5 hours of operation from a battery with 80% usable capacity, the required capacity would be (2 x 5) / 0.8 = 12.5 amp-hours. This mathematical approach ensures you select a battery that won't be overworked or prematurely depleted.

Practical Applications

To use this calculator practically, first determine your device's current draw, which is typically listed in the device's specifications or can be measured with a multimeter. Next, decide how many hours of operation you need, considering any backup requirements. Finally, research the usable fraction of your intended battery type - common values range from 0.5 for lead-acid (due to discharge limitations) to 0.9 for lithium-ion. Input these three values into the calculator to find the minimum battery capacity needed for your specific application.

Day-to-Day Use

This calculator helps you make informed decisions about battery-powered devices in everyday situations. Whether you're choosing a portable power bank for your phone and laptop, selecting a backup battery for critical equipment, or sizing a solar power system for your home, this tool prevents you from buying batteries that are too small or wasting money on excessively large ones. It's particularly valuable for campers, RV owners, and anyone relying on electronic devices during power outages, ensuring you maintain power when you need it most.

Worked example

2 A for 5 hours at 80% usable needs a 12.5 Ah battery.

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