Battery Backup Time Calculator for Home
A 200 Ah, 12 V tubular battery runs a 300 W load for about 3 h 24 min through a UPS while using only the recommended 50% of its capacity. Backup time equals battery Ah × volts × usable depth of discharge × inverter efficiency ÷ load watts.
How this result was calculated
200 Ah × 12 V × 50% usable × 85% inverter efficiency = 1,020 Wh usable. 1,020 Wh ÷ 300 W = 3.4 hours, about 3 h 24 min.
Key facts
- Backup time (hours) = battery Ah × battery volts × usable depth of discharge × inverter efficiency ÷ load in watts.
- Tubular and lead-acid batteries should be used to about 50% of their capacity; lithium (LiFePO4) batteries to about 90%.
- A typical home UPS converts battery power to AC at about 85% efficiency, so part of the stored energy is lost as heat.
- A 200 Ah, 12 V tubular battery holds about 1,020 Wh of usable energy for a UPS, which gives about 3 h 24 min at 300 W or about 7 h 39 min at 100 W on a 150 Ah battery.
Formula
t = (Ah × V × DoD × η) ÷ P
- t = backup time in hours
- Ah = battery capacity in amp hours
- V = battery bank voltage
- DoD = usable depth of discharge
- η = inverter efficiency
- P = load in watts
Inverter battery backup calculator: the same formula for every inverter
A home UPS, an inverter and a hybrid solar inverter all do the same job on backup: they turn DC battery power into AC for your fans, lights and TV. So the inverter battery backup calculation is the same as the UPS one. What changes is the efficiency figure: a basic home UPS is about 85% efficient, while a good hybrid inverter is closer to 90%. Change the inverter efficiency under Advanced to match your unit.
The battery bank voltage matters as much as the Ah. Two 200 Ah batteries in series on a 24 V inverter store twice the energy of one, but the Ah stays 200; the volts double. Enter the bank voltage, not the voltage of a single battery, and the Ah of a single battery, not the sum.
Loadshedding UPS calculator: backup hours during loadshedding
To plan for loadshedding, compare the backup hours with the length of your longest outage. With the default 200 Ah battery and a 300 W load you get about 3 h 24 min, enough for a typical outage of one to two hours but not for a long evening cut. If the backup is short, the cheapest fix is a smaller load: switch the TV off and keep only fans and lights, and the backup roughly doubles for every halving of the watts.
Backup is only half the story. The battery must also recharge before the next cut. A tubular battery that has given a full backup needs several hours of grid power to refill, so on days with several outages the second and third backups are shorter than the first. Lithium batteries recharge faster and can use more of their capacity, which is why they suit frequent loadshedding better.
Worked examples
150 Ah battery with fans and lights
150 Ah × 12 V × 50% × 85% = 765 Wh. At 200 W the backup is 3 h 50 min.
Two 200 Ah batteries on a 24 V UPS
200 Ah × 24 V × 50% × 85% = 2,040 Wh. At 600 W the backup is 3 h 24 min.
100 Ah lithium battery
100 Ah × 12.8 V × 90% × 85% = 979 Wh. At 300 W the backup is 3 h 16 min.
One fan and a few LED lights on a 150 Ah battery
A ceiling fan with a few LED bulbs is about 100 W. 150 Ah × 12 V × 50% × 85% = 765 Wh, so the backup is about 7 h 39 min.
Backup time of a 12 V tubular battery at common loads
| Battery | Load | Backup time |
|---|---|---|
| 100 Ah | 100 W | 5 h 6 min |
| 100 Ah | 300 W | 1 h 42 min |
| 150 Ah | 200 W | 3 h 50 min |
| 150 Ah | 400 W | 1 h 55 min |
| 200 Ah | 300 W | 3 h 24 min |
| 200 Ah | 500 W | 2 h 2 min |
Common mistakes
- Using the full Ah rating. Draining a tubular battery below about 50% shortens its life, so only that part counts as usable.
- Forgetting the inverter loss. A UPS turns about 85% of battery energy into AC power.
- Adding Ah of batteries in series. Two batteries in series double the volts, not the Ah.
- Expecting the label Ah at high loads. Lead-acid batteries are rated over about twenty hours; drained in a few hours they deliver noticeably less, so keep a safety margin.
Safety: Keep batteries in a ventilated place, use correctly rated cables and fuses, and have a licensed electrician connect a UPS to house wiring.
Frequently asked questions
How long will my UPS battery last?
On a 200 Ah, 12 V tubular battery with a 300 W load, about 3 h 24 min. Halve the load and the backup roughly doubles; an old or hot battery gives less.
What is the backup time of a 150 Ah battery?
A 150 Ah, 12 V tubular battery holds about 765 Wh of usable energy through a UPS. That is about 3 h 50 min at 200 W (fans and lights) or about 7 h 39 min at 100 W.
How many hours of backup does a tubular battery give?
It depends on the load. A tubular battery should only be used to about 50% of its rating, so a 200 Ah battery gives about 3 h 24 min at 300 W, and a 150 Ah battery about 3 h 50 min at 200 W.
What is the backup time of a lithium battery?
Longer than a tubular battery of the same Ah, because lithium (LiFePO4) can safely use about 90% of its capacity against about 50% for tubular. A 100 Ah, 12.8 V lithium battery gives about 3 h 16 min at 300 W.
How many hours of battery backup for a fan and lights?
One ceiling fan with a few LED lights is about 100 W. On a 150 Ah tubular battery that is about 7 h 39 min; two fans with lights, about 200 W, give about 3 h 50 min.
What is the battery backup formula in hours?
Backup hours = battery Ah × battery volts × usable depth of discharge × inverter efficiency ÷ load in watts. For a tubular battery use about 50% usable and about 85% inverter efficiency; for lithium use about 90% usable.
Why is my real backup different from the calculator?
The calculator shows backup while using only the recommended part of the battery. Many UPS units keep running until the battery is nearly empty, which gives longer backup but shortens battery life. Old batteries, heat and a higher real load give shorter backup.
Is backup time the same for AC and DC loads?
No. DC loads run straight from the battery without inverter loss. Set inverter efficiency to one for a DC load.
Sources
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