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.
Runtime Chart by Battery Size
Quick reference showing estimated backup runtime at common load levels for popular battery bank sizes. All values assume 51.2V LiFePO4 batteries at 100% DoD with 95% inverter efficiency.
| Battery Bank | Usable kWh | 500W Load | 1,000W Load | 2,000W Load | 3,000W Load |
|---|---|---|---|---|---|
| 100Ah (5.1 kWh) | 4.9 kWh | 9.7 hrs | 4.9 hrs | 2.4 hrs | 1.6 hrs |
| 200Ah (10.2 kWh) | 9.7 kWh | 19.5 hrs | 9.7 hrs | 4.9 hrs | 3.2 hrs |
| 300Ah (15.4 kWh) | 14.6 kWh | 29.2 hrs | 14.6 hrs | 7.3 hrs | 4.9 hrs |
| 400Ah (20.5 kWh) | 19.5 kWh | 38.9 hrs | 19.5 hrs | 9.7 hrs | 6.5 hrs |
| Tesla Powerwall (13.5 kWh) | 12.8 kWh | 25.7 hrs | 12.8 hrs | 6.4 hrs | 4.3 hrs |
| 600Ah (30.7 kWh) | 29.2 kWh | 58.4 hrs | 29.2 hrs | 14.6 hrs | 9.7 hrs |
Recommended Battery Products
Recommended Inverters for Battery Backup
Pair your battery bank with a compatible hybrid or off-grid inverter. The inverter must match your battery voltage (typically 48V) and provide sufficient AC output for your loads:
Use our Battery Series/Parallel Calculator to plan your battery bank configuration, or the Battery Sizing Calculator for full system sizing.
Backup Load Planning Guide
Typical wattage for common household loads during an outage:
| Appliance | Running Watts | Surge Watts | Daily Usage |
|---|---|---|---|
| Refrigerator | 200-400W | 1,200W | 2-4 kWh |
| LED Lights (10) | 100W | 100W | 0.5-1 kWh |
| WiFi Router | 20W | 20W | 0.5 kWh |
| TV (55") | 150W | 150W | 0.5-1 kWh |
| Well Pump (1/2 HP) | 500W | 2,000W | 1-2 kWh |
| Furnace Blower | 300-800W | 1,500W | 3-8 kWh |
| Window AC (1 ton) | 1,200W | 3,500W | 8-12 kWh |
Surge watts are important — your inverter must handle the momentary startup load, which can be 3-5x the running wattage for motors and compressors.
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