How long will a 150Ah battery last during a power cut?
The honest math: a 150Ah 12V battery stores about 1.1–1.2 kWh of usable energy. Worked examples for common Indian households, and the four factors that change runtime.
Quick answer
- A 150Ah 12V battery holds ~1.8 kWh nominal, but usable energy is about 1.1–1.2 kWh after depth-of-discharge and inverter losses.
- Typical result: 3 fans + 4 LED lights + a TV for roughly 6–8 hours; a single 0.75 W LED-heavy home can stretch past 10 hours.
- Adding a fridge or air conditioner cuts runtime dramatically — motors and compressors dominate energy use.
The math in one paragraph
Energy = battery volts × amp-hours × usable depth of discharge × system efficiency. For a standard 12 V, 150 Ah tubular battery: 12 × 150 = 1,800 Wh nominal. With a practical 80% usable depth of discharge and ~85% inverter efficiency, usable energy is about 1,800 × 0.80 × 0.85 ≈ 1,220 Wh — roughly 1.2 units of electricity. Runtime is that number divided by the watts of everything running at once.
Worked examples with typical Indian appliances
A common backup load: 3 ceiling fans (75 W each running, ~225 W), 4 LED bulbs (9 W each, 36 W), one LED TV (~60 W) and a Wi-Fi router (~15 W) — about 335 W together. 1,220 ÷ 335 ≈ 3.6 hours flat out, or 6–8 hours if you run fans on medium speed and lights only where needed, which halves the average draw.
Contrast that with a fridge (~150–250 W with compressor cycling) added: the same battery drops to roughly 3–4 hours. An air conditioner (1,000–1,500 W) would drain it in well under an hour, which is why ACs are usually excluded from inverter backup in India and why “inverter AC” labels refer to compressor technology, not backup.
Four factors that change the answer
First, battery age and health: capacity falls over years of cycling, and a sulphated tubular battery may hold only half its rating. Second, discharge rate: drawing 500 W continuously achieves less usable energy than drawing 200 W, because high current increases effective losses (the Peukert effect). Third, actual depth of discharge: regularly draining below recommended limits shortens battery life even if it buys runtime tonight. Fourth, temperature: capacity drops measurably in hot lofts and cold winters.
If your cuts are long or your load list bigger, the fix is sizing, not squeezing: either a larger battery bank (24 V or 48 V systems with two or more batteries) or lithium (LiFePO4), which offers more usable depth of discharge per rated Ah at a higher price.
How to check this for your own home
List the appliances you actually need during a cut, read nameplate watts (do not trust generic lists), and multiply by the hours you need them. Compare the total against the ~1.2 kWh usable figure for 150Ah — or enter your list in the backup planner on this site, which shows VA and Ah requirements with assumptions stated.
One practical test beats all math: during your next cut, run your intended backup load and time how long the battery actually lasts. Repeat once a year; the trend tells you when the battery is ageing out.
Frequently asked questions
How many hours does a 150Ah battery last on 2 fans and 2 lights?
Two fans (~150 W) and two 9 W LEDs (~18 W) draw about 170 W together. With ~1,200 Wh usable, expect roughly 6–7 hours at full speed, more if fans run slow — real results vary with battery age and condition.
Can a 150Ah battery run a refrigerator?
Yes, but expect only about 3–4 hours because compressor cycling averages 150–250 W. Modern inverter-technology fridges cycle more gently and stretch this somewhat; older fridges can halve it.
Is a bigger Ah battery always better?
More Ah means more runtime but also more charging time and cost. Size from your load list: if your cuts are short and your load small, 100–150 Ah is enough; if you run many hours daily, consider a higher-capacity bank or lithium.