Battery Capacity Calculator Formula

Understand the math behind the battery capacity calculator. Each variable explained with a worked example.

Use the Battery Capacity Calculator

Formulas Used

Load Current Draw

draw_current = load_current

Usable Capacity Needed

usable_ah = load_current * hours / (efficiency / 100)

Total Battery Capacity

total_ah = load_current * hours / (efficiency / 100) / (dod / 100)

Battery Energy

energy_kwh = load_current * hours / (efficiency / 100) / (dod / 100) * voltage / 1000

Variables

VariableDescriptionDefault
load_powerLoad Power(W)500
voltageBattery Voltage(V)48
hoursRequired Backup Time(hours)4
dodMax Depth of Discharge(%)80
efficiencySystem Efficiency(%)90
load_currentDerived value= load_power / voltagecalculated

How It Works

Battery Capacity Sizing

Battery capacity must account for the load, duration, efficiency losses, and depth-of-discharge limits.

Formula

Required Ah = (P / V) x Hours / Efficiency / DoD

where P is load power in watts, V is battery voltage, Hours is the backup duration, Efficiency accounts for inverter and wiring losses, and DoD is the maximum allowable depth of discharge (to protect battery life).

Worked Example

500 W load on a 48V battery system for 4 hours, 80% DoD, 90% efficiency.

load_power = 500voltage = 48hours = 4dod = 80efficiency = 90
  1. 01Load current = 500 / 48 = 10.42 A
  2. 02Usable capacity = 10.42 x 4 / 0.90 = 46.3 Ah
  3. 03Total capacity = 46.3 / 0.80 = 57.9 Ah
  4. 04Battery energy = 57.9 x 48 / 1000 = 2.78 kWh

Ready to run the numbers?

Open Battery Capacity Calculator