Battery Backup Time Calculator

Step-by-Step Guide

1. 1Enter Battery Voltage: Input the nominal battery voltage — commonly 12V, 24V, or 48V.
2. 2Enter Battery Capacity: Input the battery's rated capacity in amp-hours (Ah).
3. 3Enter Load Power: Input the total power in watts (W) drawn by everything running on this battery.
4. 4Enter Efficiency and Depth of Discharge: Input system efficiency percentage and how much of the battery's capacity is safe to use.
5. 5Read the Backup Time Result: View backup time in hours and a formatted hours-and-minutes figure, plus current draw.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Backup time in hours and formatted hours/minutes
• ✓Total and effective (efficiency-adjusted) energy in Wh
• ✓Current draw calculation
• ✓Depth of discharge adjustment
• ✓Full step-by-step derivation
• ✓Six battery presets + eight load presets
• ✓Calculation history
• ✓Export calculation as a text file
• ✓No signup required

Pro Tips

• 💡Set depth of discharge to 50% for lead-acid batteries rather than 100%, even though the calculator allows it — this reflects realistic safe usage and produces a more accurate, conservative backup time estimate.
• 💡Add up every device you plan to run simultaneously into a single total load power figure rather than calculating each device separately — backup time is determined by the combined draw, not any single device.
• 💡Re-run the calculation periodically with your battery's actual measured capacity, not just its original rated capacity — capacity degrades with age and cycle count, and using stale figures overstates real backup time.
• 💡Use the built-in load presets as a sanity check when estimating an unfamiliar appliance's power draw — actual nameplate wattage can differ from assumptions, especially for compressor-based appliances like refrigerators.
• 💡For critical backup applications, calculate backup time at your realistic worst-case load (not average load) to ensure the battery covers the scenario that actually matters during an outage.

Common Mistakes to Avoid

• ✕Assuming 100% depth of discharge is always safe. Lead-acid batteries discharged repeatedly below 50% suffer significantly shortened cycle life — the calculator lets you set any DoD, but the assumption you choose should match your battery chemistry's actual safe operating range.
• ✕Ignoring system efficiency entirely and using the theoretical Wh figure directly. Real inverters and battery systems always have some loss — skipping this adjustment overstates backup time, sometimes significantly.
• ✕Using nameplate or peak power instead of realistic average power for variable-draw appliances. A refrigerator's compressor cycles on and off, so its average power draw is meaningfully lower than its peak startup wattage.
• ✕Forgetting that backup time scales inversely, not linearly in an intuitive direction, with load. Adding a second appliance that doubles total load power doesn't just 'use more battery' — it specifically halves the remaining backup time.
• ✕Comparing backup time calculations across different battery ages without accounting for capacity fade. A three-year-old lead-acid battery may have lost 20-30% of its original rated Ah capacity, which this calculator doesn't automatically detect.