Battery Backup Runtime Calculator
Find out how long your battery bank will power your home during an outage
How to Use This Calculator
- Enter your total battery bank capacity in amp-hours (Ah). Add multiple batteries together.
- Select the battery bank voltage (a single 51.2V LiFePO4 server-rack battery is common for home backup).
- Enter the total wattage of the loads you want to keep running (fridge, lights, router, etc.).
- Pick the depth of discharge (100% for LiFePO4, 50–80% for lead-acid to protect battery life).
- Set the inverter efficiency (95% is typical for modern pure-sine inverters).
- Read your backup runtime in hours and minutes.
Runtime Formula
Usable Energy (Wh) = Ah × Voltage × DoD × Inverter Efficiency
Runtime (hours) = Usable Energy (Wh) ÷ Load (Watts)
Example: 200Ah × 51.2V × 1.0 × 0.95 ÷ 1000W = 9.73 hours
| Backup Load | 51.2V / 100Ah LiFePO4 | 51.2V / 200Ah LiFePO4 | 48V / 400Ah Lead-Acid (80% DoD) |
|---|---|---|---|
| 300 W (fridge + lights) | 16.2 hrs | 32.4 hrs | 40.9 hrs |
| 1,000 W | 4.9 hrs | 9.7 hrs | 12.3 hrs |
| 2,000 W | 2.4 hrs | 4.9 hrs | 6.1 hrs |
| 3,000 W | 1.6 hrs | 3.2 hrs | 4.1 hrs |
| 5,000 W | 1.0 hrs | 1.9 hrs | 2.5 hrs |
Batteries & Inverters for Backup Power
Frequently Asked Questions
Can I run my whole house on battery backup?
You can back up essentials indefinitely with a properly sized LiFePO4 bank. Whole-house backup with heavy loads (AC, electric heat) typically requires 20–40 kWh of storage. Prioritize critical loads to extend runtime.
Why is LiFePO4 better for backup than lead-acid?
Lithium iron phosphate allows 100% depth of discharge, lasts 4,000–6,000 cycles, is lighter, and delivers consistent voltage. Lead-acid should be limited to 50% DoD to avoid damage.
How do I calculate total load watts?
Add the running wattage of each appliance you want backed up. Check nameplate ratings — a fridge uses ~200–400W running (but 1,200W surge), LED lights ~10W each, a router ~20W, and a TV ~150W.