Battery Backup Time Calculator
Estimate how long a battery bank can support a load based on stored energy, usable capacity, and system efficiency.
Introduction
The battery backup time calculator estimates how long a battery bank can support a connected load before the usable stored energy is exhausted. This calculation is common in UPS design, solar battery systems, telecom backup, CCTV power planning, emergency lighting, and off-grid energy projects. Instead of looking only at ampere-hour capacity, the calculator converts storage into watt-hours so it can be compared directly with load power in watts.
This page also applies system efficiency and usable battery capacity to give a more realistic runtime estimate. That matters because many real systems should not discharge the battery fully, and inverter or conversion losses can noticeably reduce actual backup time.
It is especially useful when comparing battery options for home backup, UPS systems, telecom cabinets, and small solar storage setups where runtime planning is an important part of the design decision.
Formula
Stored energy (Wh) = V x Ah
Usable energy (Wh) = Stored energy x efficiency x usable capacity
Backup time (hours) = Usable energy / Load power
Variable Definitions
V = battery bank voltage in volts
Ah = battery capacity in ampere-hours
Load power = connected load in watts
Efficiency = system efficiency as a decimal percentage
Usable capacity = portion of the battery intended for discharge
Units
Battery voltage is entered in volts, capacity in ampere-hours, and load in watts. The calculator converts the battery energy to watt-hours and returns the estimated runtime in hours plus a simpler hours-and-minutes format for quick planning.
Worked Example 1
Consider a 24 V battery bank rated at 200 Ah feeding a 350 W load. Assume 85 percent efficiency and 80 percent
usable capacity.
Stored energy = 24 x 200 = 4800 Wh
Usable energy = 4800 x 0.85 x 0.80 = 3264 Wh
Backup time = 3264 / 350 = 9.33 hours
This is about 9 hours and 20 minutes of estimated runtime.
Worked Example 2
A 12 V, 100 Ah battery supplying a 120 W load at 90 percent efficiency and 70 percent usable capacity gives:
Stored energy = 12 x 100 = 1200 Wh
Usable energy = 1200 x 0.90 x 0.70 = 756 Wh
Backup time = 756 / 120 = 6.30 hours
This is a practical estimate for a small backup or solar storage system.
Practical Notes
Real battery runtime depends on temperature, battery chemistry, battery age, discharge current, inverter losses, and cable losses. Lead-acid systems often provide less usable capacity at high discharge rates, while lithium systems usually hold performance more consistently. Use this calculator as a planning tool, then verify the final design with battery data sheets and the actual load profile.