UPS and inverter backup time calculator
Enter your battery's Ah and voltage, the battery type and your load. You'll get a realistic backup time and a sensible inverter size, whether you're planning for load-shedding or a home UPS.
By Zubair Abid. Updated .
Quick answer
Backup time (h) = Ah × V × number of batteries × usable fraction × inverter efficiency ÷ load (W). A single 150 Ah, 12 V lead-acid battery running a 300 W load gives about 2.5 hours, because only half the battery should be used and the inverter loses around 15%.
Battery bank
What it powers
How is UPS backup time calculated?
Backup (h) = Ah × V × Batteries × DoD × Efficiency ÷ Load (W)DoD = depth of discharge: the share of the battery you can safely use.
Ah × V gives the battery's energy in watt-hours. A 150 Ah, 12 V battery stores 1,800 Wh. Depth of discharge is where most online calculators go wrong: lead-acid and tubular batteries last far longer if you only use about half their capacity, so this tool counts 50%. Lithium and LiFePO4 batteries can safely use about 90%.
Inverter efficiency covers the energy lost turning 12 V DC into 230 V or 120 V AC. Home inverters are usually 80 to 90% efficient; the default here is 85%.
Worked example: fans, lights and Wi-Fi
Two ceiling fans (150 W), four LED lights (40 W), a router (12 W) and a TV (70 W) add up to about 272 W. On one 150 Ah, 12 V tubular battery:
150 × 12 × 0.5 × 0.85 ÷ 272 ≈ 2.8 hours
Switch to a 150 Ah, 12.8 V LiFePO4 battery and the same load runs for about 5.4 hours, because you can use 90% of it instead of 50%.
What size inverter do I need?
Inverters are rated in VA, not watts. For most homes, divide your load in watts by a power factor of 0.8, then add 25% headroom so the inverter isn't running flat out:
Inverter VA ≈ Load (W) ÷ 0.8 × 1.25
A 300 W load needs roughly a 500 VA unit. Fridges, water pumps and air conditioners need extra headroom for their start-up surge. Air conditioners are usually impractical on a battery inverter unless it is designed for them.
Battery types compared
| Type | Usable share used here | Typical life | Notes |
|---|---|---|---|
| Flooded lead-acid / tubular | 50% | Hundreds of cycles to a few years | Cheapest; needs water top-ups |
| AGM / gel | 50% | Similar to lead-acid | Sealed, no maintenance |
| LiFePO4 (lithium iron phosphate) | 90% | Thousands of cycles | Costs more up front, lighter, much longer life |
Real backup also drops at high loads: lead-acid batteries deliver less than their rated Ah when discharged quickly, and every battery loses capacity as it ages. If your battery is more than two years old, expect 20 to 30% less than the result.