Calculate battery backup time, capacity, voltage, or power from three inputs, and find recommended Ah capacity from load and duration.

Nominal voltage of the battery bank (12, 24, 48, etc.).

Amp-hour rating printed on the battery.

Total wattage the batteries must supply.

How long the load must stay powered.

Related Calculators

Battery Backup Formula

The basic battery backup relationship is based on watt-hours. Battery energy in watt-hours equals capacity in amp-hours multiplied by voltage.

BT = (C * V) / P
  • BT = backup time in hours
  • C = battery capacity in amp-hours (Ah)
  • V = battery voltage in volts (V)
  • P = power consumption in watts (W)

The basic calculator can also solve for the missing value by rearranging the same formula:

C = (P * BT) / V
V = (P * BT) / C
P = (C * V) / BT

The backup recommendation mode uses the same capacity formula:

Recommended Capacity = (Load * Duration) / System Voltage
  • Load = average load in watts
  • Duration = required backup time in hours
  • System Voltage = battery system voltage in volts
  • Recommended Capacity = required battery capacity in Ah

The calculator converts supported units before applying the formula. Kilowatts are converted to watts, horsepower is converted to watts, milliamp-hours are converted to amp-hours, millivolts are converted to volts, and minutes are converted to hours.

Common Battery Backup Values

These tables help you interpret the result and choose reasonable inputs.

System voltage Common use Notes
5 V USB power banks, small electronics Often rated in mAh, but usable output depends on conversion losses.
12 V Small UPS systems, vehicles, LED lighting, routers A common low-voltage backup system size.
24 V Larger DC backup systems, solar battery banks Uses lower current than 12 V for the same watt load.
48 V Inverters, telecom power, larger battery systems Common when power demand is higher.
Unit Conversion Example
kW to W 1 kW = 1000 W 0.5 kW = 500 W
HP to W 1 HP ≈ 745.7 W 2 HP ≈ 1491.4 W
mAh to Ah 1000 mAh = 1 Ah 20000 mAh = 20 Ah
minutes to hours minutes ÷ 60 = hours 90 min = 1.5 h

Battery Backup Examples

Example 1: Find backup time

You have a 12 V, 100 Ah battery powering a 150 W load.

BT = (100 * 12) / 150
BT = 8 hours

The ideal backup time is 8 hours.

Example 2: Find required battery capacity

You need to run a 300 W load for 4 hours on a 24 V battery system.

C = (300 * 4) / 24
C = 50 Ah

The required ideal battery capacity is 50 Ah.

Battery Backup FAQ

Why is actual backup time often lower than the calculator result?

The formula gives an ideal estimate. Real systems lose energy through inverters, wiring, battery heating, and voltage conversion. Batteries may also not deliver their full rated capacity at high loads or low temperatures. If you need a dependable backup time, choose a battery capacity higher than the calculated minimum.

Should you use peak load or average load?

Use average load for estimating runtime. Use peak load to make sure the inverter, UPS, or battery system can handle startup surges. Devices with motors, compressors, pumps, or power supplies can briefly draw much more than their normal running wattage.

What does amp-hour rating mean for battery backup?

Amp-hours describe how much current a battery can supply over time at its rated voltage. To compare battery backup for different voltages, convert amp-hours to watt-hours using Wh = Ah * V. A 100 Ah battery at 12 V stores about 1200 Wh, while a 100 Ah battery at 24 V stores about 2400 Wh.