Battery Energy Storage Sizing Calculator

Sizing a battery for a given load and duration means working backward through two loss mechanisms: round-trip efficiency (energy lost to internal resistance and power-conversion losses on the way in and out of the battery) and depth of discharge, or DoD (the usable fraction of nameplate capacity a battery is designed to be cycled through, since most chemistries lose calendar life if routinely discharged further). The energy the load actually needs is load power times duration; dividing by efficiency gives the usable energy the battery must supply, and dividing that by DoD gives the nameplate capacity that has to be installed.

This calculator also reports the resulting C-rate — the load power divided by the nameplate energy capacity, in units of 1/hour — which is a quick check on whether the sizing is power-limited or energy-limited. A very low C-rate (well under 1C) usually means the duration requirement, not the instantaneous power requirement, is what's driving the battery size; a C-rate approaching or exceeding a cell's rated maximum discharge rate means power delivery, not energy capacity, may become the binding constraint, and the battery may need to be oversized on energy just to meet the power demand.

Use this for a first-pass backup power or peak-shaving sizing estimate — a UPS, a microgrid battery, or a home/commercial energy storage system. It doesn't model temperature effects on capacity and efficiency, cycle-life degradation over the system's service life, inverter/converter sizing, or the battery's maximum continuous or peak discharge current rating, all of which a detailed system design needs to check separately.