Battery Backup Calculator
Battery size = your load × backup hours ÷ (voltage × depth of discharge × system efficiency). For a typical home — about 400W of load with 2 hours of backup — you need roughly a 12V 110Ah battery. Add your own appliances below and get a practical size in minutes.
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- What do you use at home?
What do you use at home?
Pick your appliances — we'll fill in typical power for each. You can adjust everything later.
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Total load
0 W
Daily energy
0 Wh
How long will a battery run my load?
Estimated backup time
≈ 2.8 hours
150Ah 12V battery at 400W load, using typical discharge and efficiency assumptions.
Based on an 80% depth of discharge and typical inverter (90%) and battery (85%) efficiencies — the same assumptions used across this calculator. Real results vary with battery age and temperature.
Worked example
3 ceiling fans, 5 LED bulbs, 1 TV, 1 router and 4 CCTV cameras add up to about 385W of load and 3.1kWh of daily energy. For 2 hours of backup, that needs a 12V 110Ah battery — which also leaves the depth-of-discharge and efficiency margins that keep the battery healthy for years.
Common mistakes
- Buying by the shopkeeper's "hours" claim without adding up your own load — backup time depends on what you run.
- Forgetting that fridges and water pumps draw extra power at startup, so they need headroom, not just running watts.
- Deeply discharging a battery past its safe limit regularly — the recommendation already protects the battery by limiting depth of discharge.
- Sizing the battery from watts alone instead of energy (watts × hours), which is what the amp-hour rating actually stores.
Frequently asked questions
How long will a 150Ah battery run my home?
It depends entirely on your load. A 150Ah 12V battery stores about 1.5kWh of usable energy (after depth-of-discharge and efficiency limits). At a 400W load that's roughly 3 hours; at 800W, about 1.5 hours. Use the calculator above with your actual appliances to get your number.
12V, 24V or 48V — which battery system do I need?
Small home setups (roughly up to 130Ah at 12V) work best at 12V — it's the most common, cheapest and easiest to maintain in Bangladesh. As the required capacity grows, 24V or 48V systems become more practical because they carry the same power with less current and thinner cables. This calculator picks the voltage automatically — you don't need to choose.
Does adding solar change the battery size I need?
No — the battery is sized by your backup requirement (load × hours), and solar panels are sized separately to cover your daily energy and to recharge the battery. Both appear in the result because a complete home system needs both.
Tubular, flat plate or lithium — does the type matter?
The type affects cost, lifespan and maintenance, not the amp-hour size you should buy. A tubular lead-acid battery handles frequent, deep backup use best for most Bangladeshi homes. If you know what you're buying, the Advanced settings section lets you adjust depth of discharge and efficiency — but the default recommendation is a safe starting point.