Short answer: a single 150Ah inverter battery runs a 250-watt load (two fans, three LED bulbs, a TV and a Wi-Fi router) for about 5 hours. The same battery lasts about 8 hours on two ceiling fans alone, and about 3 hours on a 400-watt load. Use the calculator below for your own battery and appliances.
Inverter battery backup time calculator
Enter your battery and what you will run. The result updates as you type.
| Appliance | Typical watts | How many |
|---|---|---|
| you enter |
Assumes 80% usable capacity for lead-acid (90% for lithium) and 85% inverter efficiency. Real backup falls as a battery ages. Check the wattage label on your own appliances for a closer result.
The backup time formula
Backup time depends on two things: how much energy the battery can deliver, and how fast your appliances use it.
Backup time (hours) = (Battery Ah × Battery voltage × Usable share × Inverter efficiency) ÷ Total load in watts
- Battery Ah is the capacity printed on the battery, such as 150Ah.
- Battery voltage is 12V for a standard inverter battery.
- Usable share is the part of the capacity you can draw before the inverter cuts off. About 80% is a fair figure for a tubular lead-acid battery, and about 90% for lithium.
- Inverter efficiency covers the energy lost when the inverter converts battery power to mains power. About 85% is typical for a home inverter.
- Total load is the wattage of everything running at once.
Worked example
Take one 150Ah tubular battery running two ceiling fans (75W each), three LED bulbs (9W each), a TV (60W) and a router (10W).
- Usable energy: 150 × 12 × 0.80 × 0.85 = 1,224 watt-hours.
- Total load: 150 + 27 + 60 + 10 = 247 watts.
- Backup time: 1,224 ÷ 247 = 4.96 hours, or about 4 hours 57 minutes.
Many guides skip the usable share and the inverter efficiency and simply divide Ah × 12 by the load. That gives 7.3 hours for this example, which is more than a real battery will deliver.
Backup time by battery size and load
The table shows hours of backup for one 12V tubular battery, using the same 80% usable share and 85% inverter efficiency.
| Battery | 100 W | 200 W | 300 W | 400 W | 500 W | 600 W | 800 W |
|---|---|---|---|---|---|---|---|
| 100Ah | 8.2 | 4.1 | 2.7 | 2.0 | 1.6 | 1.4 | 1.0 |
| 120Ah | 9.8 | 4.9 | 3.3 | 2.4 | 2.0 | 1.6 | 1.2 |
| 150Ah | 12.2 | 6.1 | 4.1 | 3.1 | 2.4 | 2.0 | 1.5 |
| 180Ah | 14.7 | 7.3 | 4.9 | 3.7 | 2.9 | 2.4 | 1.8 |
| 200Ah | 16.3 | 8.2 | 5.4 | 4.1 | 3.3 | 2.7 | 2.0 |
| 220Ah | 18.0 | 9.0 | 6.0 | 4.5 | 3.6 | 3.0 | 2.2 |
For two batteries, double the hours.
How many watts your appliances use
These are typical figures. The label on your own appliance is more accurate.
| Appliance | Typical watts |
|---|---|
| Ceiling fan (regular) | 75 |
| Ceiling fan (BLDC) | 30 |
| LED bulb | 9 |
| LED tube light | 20 |
| LED TV, 32 to 43 inch | 60 |
| Wi-Fi router | 10 |
| Laptop | 60 |
| Desktop computer | 150 |
| Air cooler | 180 |
| Refrigerator (while running) | 150 |
Why your real backup can be lower
- Battery age. A lead-acid battery loses capacity every year. A four-year-old battery may hold much less than its label says.
- Heavy loads. Lead-acid batteries are rated for a slow, 20-hour discharge. Drain one in two or three hours and it delivers less than its rated capacity.
- Incomplete charging. Frequent power cuts or low mains voltage can leave the battery partly charged when the next cut starts.
- Starting surges. Refrigerators, coolers and pumps draw several times their running wattage for a moment at start-up. The inverter has to handle that surge.
- Heat and low water. High temperatures and a low electrolyte level both reduce capacity and battery life.
How to get more backup from the same battery
- Switch to BLDC fans. Two regular fans draw about 150W; two BLDC fans draw about 60W. On a 150Ah battery that is roughly 8 hours against 20 hours.
- Use LED lighting only. One old 40W tube light uses as much as four LED bulbs.
- Keep heavy appliances off the inverter. Irons, geysers, mixers and microwaves empty a battery in minutes.
- Maintain the battery. Top up distilled water on schedule and keep the terminals clean.
- Add a second battery if your inverter supports 24V. This doubles the backup.
Frequently asked questions
How long will a 150Ah battery run 2 fans?
About 8 hours for two regular 75W ceiling fans, and about 20 hours for two 30W BLDC fans.
How long will a 200Ah battery last?
About 8 hours on a 200W load, 5.4 hours on 300W and 4 hours on 400W.
Does a lithium battery give more backup than a tubular battery of the same Ah?
Yes, by roughly 20%. A lithium battery has a slightly higher voltage (12.8V against 12V) and lets you use more of its capacity, so a 150Ah lithium battery delivers about 1,470 watt-hours against about 1,220 for a tubular one.
Can I run an air conditioner on an inverter battery?
Only with a large inverter and several batteries. A 1.5 ton air conditioner draws well over 1,000 watts, which would empty a single 150Ah battery in about an hour.
How many batteries does a 3 kVA inverter need?
Most 3 kVA home inverters run on a 48V battery bank, which is four 12V batteries, though some use 36V or 24V. The inverter’s label or manual states the voltage.
