Inverter Battery Backup Calculator
By Santaji GadepatilinUpdated October 27, 2026
How long an inverter runs during a power cut depends on the battery's capacity and the load connected. See how to calculate battery backup time and how float and boost charging keeps batteries healthy. This free inverter battery backup calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the inverter battery backup calculator
- Enter the battery capacity in Ah and voltage (most home inverter batteries are 12 V).
- Enter the number of batteries (a 24 V inverter uses two 12 V batteries in series).
- Enter the connected load in watts — add up the fans, lights and other devices you will run.
- Adjust efficiency and usable depth of discharge if you know them.
The calculator shows the backup time and usable energy. To size a battery for a target time, press × on Battery capacity and enter the hours you want.
Inverter battery backup formula
Backup (h) = Ah × V × n × η × DoD ÷ Load (W)
Backup time chart (12 V battery, 85% efficiency, 80% DoD)
| Load | 100 Ah | 150 Ah | 200 Ah | 2 × 150 Ah (24 V) |
|---|---|---|---|---|
| 100 W | 8.2 h | 12.2 h | 16.3 h | 24.5 h |
| 200 W | 4.1 h | 6.1 h | 8.2 h | 12.2 h |
| 300 W | 2.7 h | 4.1 h | 5.4 h | 8.2 h |
| 400 W | 2.0 h | 3.1 h | 4.1 h | 6.1 h |
| 500 W | 1.6 h | 2.4 h | 3.3 h | 4.9 h |
| 750 W | 1.1 h | 1.6 h | 2.2 h | 3.3 h |
| 1000 W | 0.8 h | 1.2 h | 1.6 h | 2.4 h |
Typical home loads
| Appliance | Power (approx.) |
|---|---|
| LED bulb | 9–12 W |
| Ceiling fan (regular) | 60–75 W |
| Ceiling fan (BLDC) | 28–35 W |
| Wi-Fi router | 10–15 W |
| LED TV (43–55 in) | 60–120 W |
| Laptop | 45–65 W |
| Refrigerator (average running) | 100–200 W |
| Desktop computer | 150–300 W |
A home with 4 fans, 6 LED lights, a router and a TV draws about 300–400 W.
Choosing the inverter (VA) rating
Inverters are rated in VA. Divide your peak load in watts by about 0.7–0.8: a 600 W peak load needs at least an 800–900 VA inverter. Do not confuse inverter VA (how much power at once) with battery Ah (how long it lasts).
Battery types
| Battery | Typical life | Usable DoD | Maintenance |
|---|---|---|---|
| Flat plate (lead-acid) | 2–4 years | 50% | Water top-up |
| Tall tubular (lead-acid) | 5–8 years | 70–80% | Water top-up |
| Lithium (LFP) | 8–12+ years | 80–100% | None |
Getting more backup
- Switch to BLDC fans and LED lights — they can halve the load.
- Keep batteries topped up with distilled water and in a ventilated, shaded spot.
- Replace batteries as a set; mixing old and new reduces backup.
Frequently asked questions
How long will a 150Ah battery run a 300 W load?
About 4.1 hours with a 12 V battery, 85% inverter efficiency and 80% usable capacity.
How do I calculate inverter battery backup?
Backup hours = battery Ah × voltage × number of batteries × inverter efficiency × usable depth of discharge ÷ load in watts.
What size battery do I need for 4 hours of backup?
Press × on Battery capacity, enter your load and set the backup time to 4 hours. For 300 W on 12 V, about 147 Ah.
Why is real backup shorter than calculated?
Lead-acid batteries deliver less capacity at high currents, age over time and lose capacity in heat. Older batteries may give 50–70% of their rated backup.