Backup time in hours is roughly battery volts x amp-hours x usable share x UPS efficiency ÷ load in watts. A 12 V 185 Ah tubular battery holds 2.22 kWh. Using half of it through an 80% efficient UPS, it runs a 298 W load for about 3 hours by that simple formula.
Lead-acid batteries give less than that when the load is heavy, so the calculator also applies Peukert's law. For the same example it gives 2 hours 20 minutes, which is closer to what people see at home. Switch to What battery do I need? to size a battery for your load-shedding hours.
Backup time for common battery setups
At 80% UPS efficiency, using 50% of a tubular battery and 90% of a lithium one. Tubular figures include the Peukert correction for a C20-rated battery.
| Load | 12 V 185 Ah tubular | 2 × 12 V 185 Ah tubular | 25.6 V 100 Ah lithium | 51.2 V 100 Ah lithium |
|---|---|---|---|---|
| 100 W | 8 hr 40 min | 17 hr 46 min | 18 hr 26 min | 36 hr 52 min |
| 200 W | 3 hr 46 min | 8 hr 40 min | 9 hr 13 min | 18 hr 26 min |
| 300 W | 2 hr 19 min | 5 hr 20 min | 6 hr 9 min | 12 hr 17 min |
| 500 W | 1 hr 15 min | 2 hr 53 min | 3 hr 41 min | 7 hr 22 min |
| 800 W | 43 min | 1 hr 39 min | 2 hr 18 min | 4 hr 36 min |
| 1,000 W | 33 min | 1 hr 15 min | 1 hr 51 min | 3 hr 41 min |
Two tubular batteries on a 24 V UPS last more than twice as long as one on a 12 V UPS, because each battery carries half the current and lead-acid loses less capacity at lower currents.
Worked example: fans, lights, TV and router
The calculator opens with 2 ceiling fans (80 W each), 4 LED bulbs (12 W each), a Wi-Fi router (10 W) and an LED TV (80 W): 298 W in all. The battery is one 12 V 185 Ah tubular on a 12 V UPS.
- Battery energy: 12 x 185 = 2,220 Wh.
- Power the UPS takes from the battery: 298 ÷ 0.80 = 372.5 W, so the current is 372.5 ÷ 12 = 31.0 A.
- Simple estimate: 2,220 x 50% ÷ 372.5 = 2.98 hours.
- Peukert correction: the battery is rated over 20 hours, which means 9.25 A. At 31 A it gives only 20 x (185 ÷ (31.0 x 20))^1.2 = 4.68 hours to empty. Half of that is 2 hours 20 minutes.
With two of these batteries on a 24 V UPS, the same load runs for about 5 hours 22 minutes. For 4 hours from a 12 V UPS you need about 289 Ah, so two 185 Ah batteries in parallel. A single 25.6 V 100 Ah lithium battery on a 24 V UPS runs the same load for about 6 hours 11 minutes.
What the settings mean
Depth of discharge
This is how much of the battery you use before the UPS cuts off. Tubular lead-acid batteries last the most years when drained to about half; sellers quote cycle life at up to 80%. Lithium (LiFePO4) batteries are normally used to 80% to 90%, which is why a smaller lithium battery can match a bigger tubular one.
UPS efficiency
The UPS turns 12 V or 24 V DC into 220 V AC and loses some energy doing it, more at very light loads. Most home units manage 70% to 90%.
C20 or C10
A battery's Ah is measured over a set time. A 185 Ah battery at the 20-hour rate (C20) gives 9.25 A for 20 hours. Drawn faster, it gives fewer amp-hours; Peukert's law with an exponent of 1.2 estimates by how much. Check your battery's label for its rating hours.
System voltage
Small home UPS units run on one 12 V battery. Bigger units need two 12 V batteries in series for 24 V, and solar inverters often use 48 V. Batteries in series add their volts; batteries in parallel add their amp-hours.
Figures are estimates for a healthy battery at room temperature. An old battery can give half its original backup. Charging the battery back up also takes units from your bill, more than the UPS delivered; the electricity usage calculator can add those losses. For backup from solar panels, use the solar system calculator.
UPS backup questions
How do I calculate UPS backup time?
Multiply battery volts by amp-hours by the number of batteries to get watt-hours. Multiply by the share you will use (50% for tubular, 90% for lithium) and by the UPS efficiency, then divide by the load in watts. For lead-acid at heavy loads, reduce the answer further; the calculator does this with Peukert's law.
How long will a 150 Ah battery run a fan?
A 12 V 150 Ah tubular battery holds 1,800 Wh. One 80 W ceiling fan through an 80% efficient UPS draws 100 W from the battery. Using half the battery, that is about 9 hours by the simple formula, and a little less with the Peukert correction. A DC inverter fan at 35 W lasts more than twice as long.
Which battery size do I need for 4 hours of backup?
It depends on the load. For 300 W on a 12 V UPS you need about 290 Ah of tubular battery, which in practice means two 185 Ah batteries. Enter your appliances, choose What battery do I need? and set 4 hours.
Is lithium better than tubular for a UPS?
Lithium (LiFePO4) lets you use about 90% of its capacity, keeps its capacity at high loads, lasts more cycles and charges faster. It costs more up front. A tubular battery is cheaper but should only be drained to about half, so it needs nearly twice the rated capacity for the same backup.
Can I run a fridge or AC on a UPS?
A small inverter fridge can run on a UPS rated for its starting surge, which can be several times its running watts. An AC needs a large inverter and a big battery bank: a 1 ton inverter AC at 700 W would drain a 12 V 185 Ah tubular battery in well under an hour.
Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.