How to Calculate Inverter & UPS Battery Backup Time
Calculating your battery bank's expected run time ensures you select the correct battery capacity (Ah) and inverter rating for power outages, off-grid cabins, or solar backup installations.
The Battery Backup Formula
The standard formula for estimating battery backup duration is:
Understanding Depth of Discharge (DoD) by Battery Type
Draining a battery completely damages its internal plates and drastically reduces cycle life. Different battery chemistries have distinct safe Depth of Discharge limits:
- Lead-Acid / Tubular Batteries (50% DoD): Discharging beyond 50% causes sulfation and cuts lifespan in half. For a 150Ah 12V battery, only 75Ah (~900Wh) is usable.
- AGM & Gel Sealed Batteries (70% DoD): Better recovery from deep discharge cycles and maintenance-free. Provides ~70% usable capacity.
- Lithium Iron Phosphate / LiFePO4 (90% DoD): Modern lithium solar batteries safely discharge 85% to 90% of rated capacity with 3,000+ to 6,000+ cycle life.
Typical Home Appliance Power Consumption Guide
| Appliance | Typical Wattage | Approx. Backup on 150Ah 12V Battery |
|---|---|---|
| Wi-Fi Router | 10W | ~76 Hours |
| 2x LED Bulbs | 20W | ~38 Hours |
| Ceiling Fan | 75W | ~10 Hours |
| Laptop Computer | 65W | ~11.7 Hours |
| 50" Smart TV | 120W | ~6.3 Hours |
| Inverter Refrigerator | 150W | ~5.1 Hours |
Frequently Asked Questions
What size battery do I need for a 500W load for 4 hours?
Total required energy = 500W × 4 hrs = 2,000 Wh. Accounting for 85% inverter efficiency and 50% lead-acid DoD: Required capacity = 2,000 / (12V × 0.50 × 0.85) = ~392 Ah at 12V (e.g., two 200Ah 12V batteries in parallel).
Does inverter efficiency affect battery backup time?
Yes. DC-to-AC conversion incurs power loss (usually 10% to 15% as heat). A typical inverter has an efficiency rating between 85% and 92%.
What is Peukert's Law?
Peukert's Law states that the faster a lead-acid battery discharges (higher current draw), the lower its effective usable capacity becomes due to internal resistance and chemical diffusion limitations.
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