Calculator KB-01
UPS and inverter runtime calculator
Calculate how long a UPS, inverter, or power station will run from a battery under a given load.
Result
| Load | Time |
|---|
In brief: Calculate UPS or inverter runtime from a battery: capacity, battery type, depth of discharge, efficiency and the Peukert effect. For AGM, GEL, LiFePO4 and Li-ion. Example: Runtime: 1 h 48 min.
Calculation example
| Battery type | Lead-acid AGM |
|---|---|
| Battery voltage | 12 V |
| Battery capacity | 100 Ah |
| Load power | 200 W |
| Inverter efficiency | 85 % |
| UPS self-consumption | 15 W |
| Depth of discharge | 50 % |
| Battery condition | 100 % |
| Runtime | 1 h 48 min |
|---|---|
| Energy delivered | 451 Wh |
| Power drawn from battery | 250 W |
| Battery current | 20.9 A |
| Discharge current as a fraction of capacity | 0.21 C |
How it is calculated
First, account for losses: the power drawn from the battery is the load divided by the inverter efficiency, plus the device's own power consumption. The battery current is then:
t = H · (C / (I · H))k · DoD
Here, C is the capacity adjusted for the battery's condition, H is the discharge time for which that capacity is specified (20 hours for lead-acid batteries, 5 hours for lithium batteries), k is the Peukert exponent (1.2 for AGM, 1.15 for GEL, 1.03 for LiFePO4, 1.05 for Li-ion), and DoD is the depth of discharge. The Peukert effect accounts for the fact that at high current, a battery delivers less than its rated capacity; if the current is below the rated value, no extra capacity is credited.
What to look for
- Batteries in series: voltage adds up, capacity stays the same. In parallel: capacity adds up, voltage stays the same. Two 12 V 100 Ah batteries in series make a 24 V 100 Ah battery bank.
- Lead-acid batteries last noticeably longer if you don't discharge them below 50%. LiFePO4 batteries can comfortably deliver 80–90%.
- An old battery no longer delivers its rated capacity. If it's several years old, set “Battery condition” to 60–80%.
- The result is an estimate. In cold weather, a lead-acid battery delivers noticeably less, and compressor-based equipment needs extra starting current headroom.
Frequently asked questions
How long will a UPS run on a 100 Ah battery?
At 12 V, with a 200 W load, 85% inverter efficiency, and discharge down to 50%, a lead-acid AGM battery will last about 1 h 48 min. A LiFePO4 battery of the same capacity, discharged down to 90%, will last more than twice as long.
Why is the actual runtime shorter than simply dividing capacity by current?
The inverter’s efficiency and own power consumption get in the way, as does the Peukert effect: at high current, a battery delivers less than its rated capacity. Lead-acid batteries also shouldn’t be discharged below 50%.
Which battery should I choose for a boiler or router?
Measure the power consumption first. For occasional outages, AGM or GEL will do; for frequent ones, LiFePO4: it can handle thousands of cycles, compared with hundreds for lead-acid. A boiler needs an inverter with a pure sine wave.
