In short. Runtime depends on battery energy, load, inverter efficiency, depth of discharge and battery condition. Estimate the energy needed for the autonomy, apply allowances for ageing and temperature, and confirm the result against the manufacturer’s discharge tables and by a site test.
The estimate
Usable energy = Rated energy × Depth of discharge × Ageing factor × Temperature factor
Worked example (illustrative)
A string of batteries has a nominal 40 kWh. Usable depth of discharge is 0.8, ageing factor 0.85 (end of life) and temperature factor 1.0. Usable = 40 × 0.8 × 0.85 = 27.2 kWh. Inverter efficiency 0.95. The load is 100 kW.
Runtime = 27.2 × 0.95 ÷ 100 = 0.258 h, about 15.5 minutes. At a load of 60 kW it would be about 26 minutes, though it will not scale linearly in practice, because lead-acid batteries lose capacity faster at higher discharge rates. The manufacturer’s constant-power discharge table at the actual load is the reliable source.
What the autonomy covers
- Time for the generator to start and take load, plus a failed first start.
- Time to shut systems down in order if no generator exists.
- Time for staff to respond.
See autonomy engineering.
Factors to apply
| Factor | Typical consideration |
|---|---|
| Ageing | Design to end-of-life capacity, often 80 per cent of rated |
| Temperature | Cold cuts capacity; hot cuts life |
| Discharge rate | High rates reduce available energy |
| End voltage | Higher cutoff, less energy |
| Design margin | Extra for uncertainty and growth |
Chemistry notes
- Valve-regulated lead-acid: lower cost; limited life, temperature sensitive.
- Lithium-ion: lighter and longer life; needs a battery management system and fire-safety design.
- Nickel-cadmium: long life and wide temperature range, higher cost.
Testing on site
- Fully charge the batteries and let them stabilise.
- Apply the real load or a load bank at the design power.
- Discharge to a safe end point, recording time, voltage and temperature.
- Compare with the expected value and the design margin.
- Recharge and record recharge time.
Do this with care and with a plan if the load must stay protected during the test. See battery monitoring.
Battery energy needed
Battery energy (kWh) = load kW ÷ inverter efficiency × runtime (h) ÷ usable depth of discharge, corrected for ageing and temperature
Worked example (illustrative)
Load 200 kW, inverter efficiency 95 percent, 10 minutes runtime: DC energy = 200 ÷ 0.95 × (10 ÷ 60) = 35.1 kWh. For a lead-acid VRLA system discharged at a high rate, usable fraction is lower than nameplate, so rely on the maker’s discharge table for 10 minutes. With an ageing factor of 1.25 and temperature factor of 1.05, required nameplate approximately 35.1 × 1.31 ≈ 46 kWh equivalent at the short rate, which the maker’s table converts into cells or blocks.
| Question | Answer drives |
|---|---|
| How long until the generator is on the load? | Minimum runtime |
| What if the generator fails to start? | Extended runtime or orderly shutdown time |
| Temperature of the battery room? | Capacity and life |
| Chemistry? | VRLA, flooded, lithium-ion: different life and discharge behaviour |
Typical autonomy choices
5 to 15 minutes for sites with a reliable generator, 30 minutes or more where generator start is uncertain or for orderly shutdowns of processes. Ageing means a battery that gives 15 minutes new might give 10 near end of life.
Battery voltage is not capacity. Test by discharge. See battery autonomy and runtime engineering.
Common mistakes
- Using the catalogue minutes for a new battery.
- Ignoring recharge time before the next outage.
- Scaling runtime linearly with load.
What to send us
Send the load, target autonomy and ambient conditions. Send your application details. See also UPS capacity.
Questions engineers ask
How do I extend runtime?
Add battery strings or cabinets, reduce the load, or add a generator.
Why is runtime not proportional to load?
Batteries deliver less total energy at high discharge rates.
How often should I test capacity?
On a schedule set by the manufacturer and risk, often annually, with more frequent monitoring.
What is end-of-life capacity?
The point, often 80 per cent of rated capacity, at which a battery should be replaced.
Figures are illustrative. Use manufacturer discharge data for design.