Battery Backup Time Calculator
Find out how long a battery bank will power your equipment during an outage, given voltage, capacity, and inverter efficiency. Ideal for sizing UPS systems, off-grid setups, and emergency power installations.
About this calculator
Battery backup time depends on how much energy is stored in the battery, how efficiently the inverter converts DC to AC, and how fast the connected load consumes that energy. The formula used is: backup time (minutes) = round(((V_batt × C_Ah × η_inv × 0.8) / P_load) × 60 × 100) / 100, where V_batt is battery voltage, C_Ah is capacity in amp-hours, η_inv is inverter efficiency as a decimal, 0.8 is the safe depth-of-discharge limit (drawing a lead-acid battery below 20% remaining charge dramatically shortens its lifespan), and P_load is the total load in watts. Multiplying V_batt × C_Ah gives stored energy in watt-hours; dividing by load gives hours of runtime; multiplying by 60 converts to minutes. Lithium batteries can tolerate deeper discharge (down to 0.2 remaining), so the 0.8 factor remains appropriate for either chemistry.
How to use
Suppose you have a 48 V battery bank with 100 Ah capacity, an inverter at 90% efficiency, and a total load of 500 W. Step 1 — stored energy: 48 × 100 = 4,800 Wh. Step 2 — apply depth-of-discharge: 4,800 × 0.8 = 3,840 Wh usable. Step 3 — apply inverter efficiency: 3,840 × 0.90 = 3,456 Wh delivered. Step 4 — divide by load: 3,456 / 500 = 6.912 hours. Step 5 — convert to minutes: 6.912 × 60 = 414.72 minutes ≈ 414.72 min (about 6 h 55 min). Enter these values in the calculator to confirm the result instantly.
Frequently asked questions
How long will a 100 Ah battery last on a 500 W load?
At 12 V, a 100 Ah battery stores 1,200 Wh. Applying 80% depth of discharge and 90% inverter efficiency gives about 864 Wh of usable AC energy, which powers a 500 W load for roughly 1.7 hours (103 minutes). At 48 V, the same 100 Ah bank stores 4,800 Wh and lasts nearly 6.9 hours under the same conditions. Battery voltage therefore has a dramatic effect on backup duration — always match your battery bank voltage to your load requirements when designing a system.
What is depth of discharge and why does it matter for battery life?
Depth of discharge (DoD) is the percentage of a battery's total capacity that has been used before recharging. The calculator limits usable capacity to 80% (DoD = 0.8) because discharging lead-acid batteries below 20% remaining capacity causes sulfation, which permanently reduces capacity and shortens cycle life. Lithium iron phosphate (LiFePO4) batteries can safely discharge to 80–90% DoD, but the 80% limit remains a conservative and safe default for most backup applications. Consistently staying within the recommended DoD can extend battery lifespan from a few hundred cycles to several thousand.
How does inverter efficiency affect battery backup time?
An inverter converts DC battery power to AC power for your appliances, but no inverter is perfectly efficient. A 90% efficient inverter wastes 10% of the battery energy as heat during conversion. This means a 4,800 Wh battery bank effectively delivers only 4,320 Wh of AC energy. At lower efficiencies (some budget inverters reach only 80–85%), the loss is more significant. Always check the inverter's efficiency specification at your expected load level — inverter efficiency curves peak at 70–90% of rated load and drop at very light loads, so an oversized inverter running a small load may underperform its rated efficiency.