UPS Battery Sizing Calculator Formula

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

Use the UPS Battery Sizing Calculator

Formulas Used

Required Battery Capacity

capacity_ah = actual_draw_watts * runtime_hours / battery_voltage / (depth_of_discharge / 100)

Battery Energy

capacity_kwh = actual_draw_watts * runtime_hours / (depth_of_discharge / 100) / 1000

Discharge Current

battery_current = actual_draw_watts / battery_voltage

Actual Battery Draw

actual_load_with_loss = actual_draw_watts

Variables

VariableDescriptionDefault
load_wattsTotal Load(W)2000
runtime_minutesDesired Runtime(min)30
battery_voltageBattery Bank Voltage(V)48
inverter_efficiencyInverter Efficiency(%)90
depth_of_dischargeMax Depth of Discharge(%)80
runtime_hoursDerived value= runtime_minutes / 60calculated
actual_draw_wattsDerived value= load_watts / (inverter_efficiency / 100)calculated

How It Works

UPS Battery Sizing

Proper battery sizing ensures the UPS provides the required runtime at the expected load.

Formula

Capacity (Ah) = (Load / Efficiency) x Runtime (hours) / Battery Voltage / DOD

Where DOD is the maximum depth of discharge (80% for lead-acid to extend life, 90% for lithium).

Sizing Considerations

  • Battery capacity degrades over time (plan for end-of-life capacity)
  • Temperature affects capacity (reduce by 1% per degree C below 25 C)
  • Higher discharge rates reduce effective capacity (Peukert effect for lead-acid)

    • Worked Example

    2000 W load, 30 min runtime, 48 V battery, 90% inverter efficiency, 80% DOD.

    load_watts = 2000runtime_minutes = 30battery_voltage = 48inverter_efficiency = 90depth_of_discharge = 80
    1. 01Runtime in hours: 30 / 60 = 0.5 hours
    2. 02Actual draw: 2000 / 0.90 = 2222 W
    3. 03Capacity: 2222 x 0.5 / 48 / 0.80 = 28.9 Ah
    4. 04Energy: 2222 x 0.5 / 0.80 / 1000 = 1.39 kWh
    5. 05Discharge current: 2222 / 48 = 46.3 A

    Ready to run the numbers?

    Open UPS Battery Sizing Calculator