This free UPS battery backup calculator shows how long your battery will run your fans, lights and appliances. Add up the appliances you want to run during load shedding, enter your battery details, and see how many hours of backup you will get. You can also flip the calculation to find what battery capacity you need for a given number of hours, for lead-acid, tubular or lithium batteries.
1. Your load
| Appliance | Watts each | Quantity |
|---|
2. Battery
How to use the UPS Battery Backup Calculator
- Add your appliances, or enter the total load in watts directly.
- Choose the battery type, battery voltage and capacity (Ah).
- Enter the number of batteries and the inverter efficiency.
- Enter the backup hours you want to find the battery size needed, then press Calculate backup.
Real runtime is often a little lower than the UPS battery backup calculator shows for old batteries or hot rooms. For how the different UPS types work, see Wikipedia’s uninterruptible power supply article.
Battery backup formula
Example
A 12 V 200 Ah tubular battery holds 2,400 Wh. Using 50% of it through an 85% efficient UPS gives 1,020 Wh. Running 3 fans, 5 LEDs, a router and a TV (390 W) gives about 2.6 hours.
Typical appliance wattage
| Appliance | Watts |
|---|---|
| Ceiling fan (standard AC) | 70–90 W |
| DC inverter fan | 30–40 W |
| LED bulb | 9–15 W |
| Wi-Fi router | 10–20 W |
| LED TV (40″) | 60–90 W |
| Laptop | 45–90 W |
| Inverter refrigerator (average) | 80–150 W |
| 1-ton inverter AC (average) | 700–1,100 W |
Switching ordinary ceiling fans to DC inverter fans roughly doubles backup time, which is often cheaper than adding another battery.
Frequently asked questions
How long will a 200Ah battery last?
A 12 V 200 Ah lead-acid battery at 50% usable capacity and 85% efficiency gives about 1,020 Wh, so around 3.4 hours at 300 W or 5 hours at 200 W. A 200 Ah lithium battery gives nearly twice that.
What size UPS do I need?
Add up the watts you will run together and divide by 0.8 to get VA. For 600 W, choose at least a 750–1000 VA UPS, with some headroom for motor start-up surges from fans and pumps.
Why use only 50% of a lead-acid battery?
Deep discharges quickly damage lead-acid plates. Limiting to 50% can double or triple battery life. Lithium iron phosphate (LiFePO4) batteries tolerate 80–90% discharge.
Can I run an AC on a UPS?
Only with a large inverter (3 kW or more) and a big battery bank. A 1-ton inverter AC can draw 700–1,100 W, so an hour of AC needs roughly 1.5–2.5 kWh of battery.
What is the battery backup time formula?
Backup hours = battery voltage × Ah × usable depth of discharge × inverter efficiency ÷ load in watts. For a 12 V 200Ah lead-acid battery at 50% depth, 85% efficiency and a 300 W load: 12 × 200 × 0.5 × 0.85 ÷ 300 = 3.4 hours.
How does an inverter battery backup calculator treat lithium batteries?
Lithium (LiFePO4) batteries can use about 80–90% of their capacity, against about 50% for lead-acid, so the same Ah rating gives roughly 1.6 to 1.8 times the backup.
Why does my backup run out sooner than calculated?
Batteries lose capacity with age and heat, lead-acid batteries deliver less at high loads, and motors in fans and fridges draw extra power at start-up. An older lead-acid battery may give only 60–70% of its original backup.
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Results are estimates for general information. Check critical decisions with a qualified professional.