UPS Runtime Calculator
Estimate how long a UPS will support your load from battery capacity, string voltage, inverter efficiency and depth of discharge.
Inputs
Rated capacity of one string, at the manufacturer's discharge rate.
Block voltage x number of blocks in series (e.g. 20 x 12 V = 240 V).
VRLA is typically 80%. Discharging deeper shortens battery life.
Results
- Estimated runtime
- 219minutes
- Usable energy
- 18.2kWh
- Nominal stored energy
- 24.0kWh
- Discharge rate
- 0.21C
About 3.6 hours.
After depth-of-discharge and inverter losses.
Below 1C, where published capacity figures are broadly reliable.
The calculation
Runtime is stored energy divided by the rate you consume it. Getting there takes three steps:
Nominal energy (kWh) = (Ah × string voltage) ÷ 1000Usable energy = nominal × depth of discharge × efficiencyRuntime (hours) = usable energy ÷ load in kW
A 100 Ah string at 240 V stores 24 kWh. Discharging to 80% and losing 5% in the inverter leaves 18.2 kWh. Against a 5 kW load that is 3.6 hours — or, more realistically, rather less. The reasons why matter.
Why real runtime is shorter than the arithmetic
The Peukert effect
Lead-acid batteries deliver less total energy the faster you discharge them. A battery rated 100 Ah at a 20-hour discharge rate might deliver only 60-70 Ah when drained in 15 minutes. This is the Peukert effect, and it is not a defect — it is inherent to the chemistry.
It matters enormously in data centers, where UPS strings are sized for minutes rather than hours. The discharge rate shown above as a "C rate" is the indicator: at 1C you are draining the entire nominal capacity in an hour. Above that, treat the published Ah figure as optimistic and size from the manufacturer's discharge tables instead, which give watts-per-cell against runtime.
Ageing
VRLA batteries are considered end-of-life at 80% of rated capacity, and they reach that point after roughly three to five years of service. Design for the end of life, not the day of installation, or the runtime you tested at commissioning will not be there when you need it.
Temperature
Battery capacity is quoted at 25°C. Capacity falls in the cold — around 20% less at 0°C — and while heat temporarily increases capacity, it destroys service life: every 10°C above 25°C roughly halves it. A battery room running warm is quietly consuming the asset.
Depth of discharge
| Chemistry | Typical usable DoD | Note |
|---|---|---|
| VRLA / AGM | 80% | Deeper discharge sharply reduces cycle life |
| Flooded lead-acid | 80% | Longer life, needs a ventilated battery room |
| Lithium-ion (LFP) | 90-95% | Higher usable fraction, longer life, BMS-managed |
How much runtime do you actually need?
More is not automatically better. The question is what the runtime is for:
- With a generator: you need enough to cover generator start and load transfer, typically 30-90 seconds, plus margin for a failed first start attempt. Five to ten minutes is a common design point.
- Without a generator: you need enough to shut down cleanly. That means the time for orderly application shutdown and storage flush, which is usually 10-20 minutes, not hours.
- Cooling is the hidden constraint. UPS protects IT load, but chillers usually run on utility or generator power. A room with 30 minutes of battery and no cooling can exceed safe intake temperatures within a few minutes at high density.
Buying an hour of battery for a site with a generator is largely wasted capital, and it adds weight the floor loading calculator may care about. Size for the transfer, not for the outage.
Verify, do not assume
Every figure here is a planning estimate. Load-bank test the actual installation at commissioning and re-test annually — battery failures are silent, and a string that has quietly lost a cell will show full float voltage right up until the moment it is asked to deliver.