Inverter Battery Backup Time & Capacity Calculator
Calculate exact inverter battery backup hours or solve for required battery Ampere-Hours (Ah) based on your running electrical load, chemistry discharge depth (DoD), and DC system voltage.
Battery Backup Performance Results
Autonomy EngineBattery Backup Formulas & Peukert Law
Battery backup duration follows the fundamental energy equation: Backup Time (Hours) = (Battery Ah × System Voltage × Efficiency × DoD) / Running Load Watts. Sizing accounts for continuous discharge current and battery cycle life protection.
| Battery Capacity | 150W Load (Basic Fans + Lights) | 300W Load (TV + Fridge + Fan) | 500W Load (Workstation + Lighting) | 800W Load (Heavy Emergency Load) |
|---|---|---|---|---|
| 100 Ah (12V) | ~5.4 Hours | ~2.7 Hours | ~1.6 Hours | ~1.0 Hours |
| 150 Ah (12V) | ~8.1 Hours | ~4.1 Hours | ~2.4 Hours | ~1.5 Hours |
| 200 Ah (12V) | ~10.8 Hours | ~5.4 Hours | ~3.2 Hours | ~2.0 Hours |
| 2x 150 Ah (24V) | ~16.3 Hours | ~8.1 Hours | ~4.8 Hours | ~3.0 Hours |
Frequently Asked Questions about Inverter Battery Backup
How do you calculate inverter battery backup time?
The standard engineering formula is: Backup Time (Hours) = [Battery Ah × Battery Voltage (V) × Inverter Efficiency (typically 85% or 0.85) × Depth of Discharge (DoD)] / Running Load (Watts). For a 150Ah 12V tubular battery running a 300W load at 80% DoD and 85% efficiency: (150 × 12 × 0.85 × 0.80) / 300 = 4.08 hours of backup.
Why should I never discharge my battery to 100%?
Discharging a lead-acid battery beyond its rated Depth of Discharge (DoD) causes severe sulfation on lead plates, permanently destroying battery chemistry within months. Tubular batteries should not exceed 80% DoD, while flat-plate batteries should stop at 70%. Lithium LiFePO4 batteries have integrated BMS protection allowing safe discharges up to 90-95%.
How do I choose between 12V, 24V, and 48V battery systems?
System voltage determines continuous DC current draw: Current (Amps) = Watts / Volts. For loads under 1000W, a 12V single-battery system is economical. For 1000W to 2500W loads, a 24V (2 batteries in series) system cuts current in half, preventing overheating. Above 2500W, a 48V (4 batteries in series) system is required for safe conductor sizing.
Does running a ceiling fan on high speed drain the battery faster?
Yes. A conventional ceiling fan consumes 75W at maximum speed but only 20-30W at speed 1 or 2. Upgrading to a BLDC (Brushless DC) ceiling fan reduces energy consumption to 28-32W at top speed, more than doubling battery backup time during power outages.