Battery Backup Runtime Calculator
Estimate how many hours a battery backup system will run your devices during a power outage. Essential for sizing UPS systems, solar storage, and off-grid setups.
About this calculator
Battery backup runtime depends on the usable energy stored in the battery and the power demanded by connected devices. The formula is: Runtime (hours) = (Battery Ah × Battery Voltage × Depth of Discharge% / 100 × Inverter Efficiency% / 100) / Load Watts. Battery capacity in ampere-hours (Ah) multiplied by voltage gives watt-hours (Wh) of stored energy. Depth of discharge (DoD) reflects how much of that energy you can safely use — for example, lead-acid batteries are typically limited to 50% DoD to preserve battery life, while lithium batteries can often reach 80–90%. Inverter efficiency (typically 85–95%) accounts for energy lost as heat during DC-to-AC conversion. Dividing usable watt-hours by the load gives runtime in hours.
How to use
Scenario: A 100 Ah, 12 V lead-acid battery powers a 200 W load. DoD is set to 50% and inverter efficiency is 90%. 1. Enter Battery Capacity = 100 Ah. 2. Enter Battery Voltage = 12 V. 3. Enter Total Load = 200 W. 4. Enter Depth of Discharge = 50. 5. Enter Inverter Efficiency = 90. 6. Runtime = (100 × 12 × 50/100 × 90/100) / 200 = (100 × 12 × 0.5 × 0.9) / 200 = 540 / 200 = 2.7 hours. Your devices will run for approximately 2 hours and 42 minutes.
Frequently asked questions
What depth of discharge should I use for my battery backup calculation?
Depth of discharge (DoD) is the percentage of battery capacity you draw down before recharging. For flooded lead-acid batteries, a DoD of 50% is recommended to extend cycle life significantly. AGM lead-acid batteries can typically handle 60–70% DoD. Lithium iron phosphate (LiFePO4) batteries are designed for 80–90% DoD without significant degradation. Using a higher DoD than recommended shortens battery lifespan, so always check your battery manufacturer's specifications before setting this value.
How does inverter efficiency affect battery backup runtime?
An inverter converts DC battery power to AC power for household devices, but this conversion is never 100% efficient — some energy is lost as heat. A typical inverter operates at 85–95% efficiency depending on load level and design quality. If your inverter is 90% efficient, only 90% of the battery's usable energy actually reaches your devices. This directly reduces runtime: a battery that theoretically holds 600 Wh delivers only 540 Wh of usable AC power at 90% efficiency. Always use a realistic efficiency figure for accurate planning.
Why does my actual battery backup runtime differ from the calculated estimate?
Several real-world factors cause actual runtime to deviate from theoretical calculations. Battery age and sulfation reduce effective capacity below the rated Ah figure. Peukert's effect means batteries deliver less total energy when discharged quickly under heavy loads. Ambient temperature also affects battery performance — cold temperatures notably reduce lead-acid capacity. Additionally, not all devices draw a constant wattage; some have startup surges that stress the system. The calculator provides a useful planning estimate, but building in a 20–30% safety margin is advisable for critical applications.