Inverter Battery Backup Time: How to Calculate It (Formula + Free Calculator)
How long will your inverter battery last? Backup time formula (V × Ah × efficiency ÷ load), 150 Ah and 200 Ah examples, and a free calculator.
ON THIS PAGE
- Free inverter backup time calculator
- The backup time formula explained
- Step 1: battery energy in watt-hours
- Step 2: subtract the inverter's losses
- Step 3: divide by your load
- Worked examples
- Why the real backup is shorter: depth of discharge
- VA vs watts: is your inverter big enough?
- How to get more backup time
- Frequently asked questions
The power goes out, the inverter beeps, and the question is always the same: how long will it last? You can answer it in 30 seconds with one formula — no guessing, no salesman.
Free inverter backup time calculator
Enter your battery and your load. The calculator shows the rule-of-thumb time (full drain) and a realistic time that keeps some charge in reserve so the battery lives longer.
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The backup time formula explained
Step 1: battery energy in watt-hours
A battery label gives volts and amp-hours (Ah). Multiply them to get watt-hours (Wh) — the real energy in the tank:
- 12 V × 150 Ah = 1,800 Wh
- 12 V × 200 Ah = 2,400 Wh
- Two 12 V 150 Ah batteries (24 V system) = 3,600 Wh
Step 2: subtract the inverter's losses
An inverter turns 12 V DC into 230 V AC (India, UK, Europe) or 120 V AC (USA, Canada). That conversion wastes some energy as heat. Most home inverters are 80–90 % efficient; a cheap or old one can be lower. Use 80 % if you don't know.
Step 3: divide by your load
Add up the watts of everything that will run during the power cut. That total is your load.
| Load during a power cut | Watts |
|---|---|
| 4 LED bulbs (9 W) | 36 W |
| 2 ceiling fans (75 W) | 150 W |
| LED TV 43" | 80 W |
| Wi-Fi router | 10 W |
| Total | 276 W |
Backup = 1,800 × 0.8 ÷ 276 ≈ 5.2 hours (full drain).
Worked examples
| Battery | Energy | Load | Backup (80 % efficiency, full drain) |
|---|---|---|---|
| 12 V 100 Ah | 1,200 Wh | 200 W | 4.8 h |
| 12 V 150 Ah | 1,800 Wh | 200 W | 7.2 h |
| 12 V 150 Ah | 1,800 Wh | 300 W | 4.8 h |
| 12 V 150 Ah | 1,800 Wh | 500 W | 2.9 h |
| 12 V 200 Ah | 2,400 Wh | 300 W | 6.4 h |
| 24 V (2 × 150 Ah) | 3,600 Wh | 500 W | 5.8 h |
| 48 V 100 Ah lithium | 4,800 Wh | 800 W | 4.8 h |
Notice the pattern: double the load → half the backup. Switching off one 75 W fan can buy you a lot of extra time.
Why the real backup is shorter: depth of discharge
The simple formula assumes you can use every last watt-hour. In real life, you shouldn't:
- Tubular lead-acid (the tall inverter batteries): plan to use about 80 %.
- Flat-plate lead-acid (car-type): about 50 % if you want it to last years.
- Lithium (LiFePO4): about 90 %, and it handles thousands of cycles.
Lead-acid capacity also drops at high current (the Peukert effect): a 150 Ah battery rated at a 20-hour rate gives less than 150 Ah if you pull 40 A from it. And an old battery has less capacity than its label says. So the realistic number in the calculator above is the one to plan with.
VA vs watts: is your inverter big enough?
Inverters are sold in VA (volt-amperes), not watts. For home loads, watts ≈ VA × 0.8 (the power factor).
- 900 VA inverter → about 720 W maximum
- 1,100 VA inverter → about 880 W maximum
- 1,500 VA inverter → about 1,200 W maximum
The battery decides how long it lasts; the inverter VA decides how much it can run at once. Plug in a 2,000 W water heater on a 900 VA inverter and it will trip on overload — no matter how big the battery is.
How to get more backup time
- Cut the load: LED bulbs, BLDC fans (about half the watts), turn the TV brightness down.
- Use a pure sine wave inverter with high efficiency (90 %+).
- Keep the battery healthy: top up distilled water in lead-acid batteries, keep terminals clean, and avoid deep discharges.
- Add capacity: a second battery doubles the Wh (in series for a 24 V inverter, or in parallel if the inverter is 12 V).
- Add solar: a hybrid or solar inverter refills the battery during the day.
Enter your battery (V, Ah) and your load in watts — see how many hours your inverter or UPS will really last.
Frequently asked questions
How long will a 150 Ah battery last?
A 12 V 150 Ah battery stores 1,800 Wh. With an 80 % efficient inverter it runs a 300 W load for about 4.8 hours, a 200 W load for about 7.2 hours and a 500 W load for about 2.9 hours — a little less in practice, especially with an older battery.
How do I calculate inverter backup time?
Multiply the battery voltage by its amp-hours, multiply by the inverter efficiency (about 0.8), and divide by the total watts of everything that is switched on.
How long will a 200 Ah battery last at 500 W?
12 V × 200 Ah = 2,400 Wh. 2,400 × 0.8 ÷ 500 ≈ 3.8 hours with a full drain, or about 3 hours if you keep 20 % in reserve.
What size inverter do I need?
Add up the watts of everything you want to run at the same time and divide by 0.8. For 600 W of load you need at least a 750 VA inverter; pick the next size up (for example 900 VA) for some headroom.
Can I run a fridge on an inverter?
Yes, with a pure sine wave inverter of at least 800–1,000 VA. A fridge compressor pulls a big surge for a moment when it starts, so the inverter needs headroom even though the fridge's average power is only 40–60 W.
