Buyer guide
How long will a site battery run?
Runtime is usable energy divided by average load. Each half of that sentence hides assumptions, and the honest answer is a range until the load is measured.
Start with energy, not the model name
A model name or a maximum power rating cannot tell you how long a system will run. Runtime comes from two things:
- usable energy: how many kWh the system can actually deliver to your loads;
- average load: how many kW the site draws, averaged over the period.
Divide the first by the second. Everything else in this guide is about getting those two numbers right.
From nominal capacity to usable energy
The capacity on a data sheet is the starting point, not the answer. The Ampd 200 brochure lists a rated nominal DC capacity of 215 kWh; the Ampd 400 lists 430 kWh. That is energy at the battery. The supplied brochures do not state usable AC energy.
Between the battery and your loads, energy is set aside or lost in several ways.
| Step | What happens |
|---|---|
| Usable charge range | The system does not charge to the very top or discharge to the very bottom |
| Conversion | Turning DC into AC loses a few per cent |
| Auxiliary consumption | Cooling and controls use energy, more in hot or cold weather |
| Reserve | You may choose to keep some energy back for the unexpected |
A worked example
Take an illustrative 20 kW average load that must run for eight hours.
Energy delivered: 20 kW × 8 h = 160 kWh.
Now work back to the nominal capacity that would be needed. Assume, for illustration, that 80% of nominal capacity is usable and that the discharge path is 92% efficient:
160 ÷ (0.80 × 0.92) = 217.4 kWh of nominal DC capacity.
So a 160 kWh job needs roughly 217 kWh on the data sheet, before allowing for auxiliary consumption, heat or cold, ageing and engineering margin. Both percentages are assumptions for the example, not manufacturer figures.
Try your own figures below. Every assumption is editable.
Runtime explainer
A screening calculation with every assumption on show. Change any of them and see how far the answer moves.
Screening result
Not included: the system’s own consumption for cooling and controls, temperature effects, ageing, any reserve kept back, and engineering margin. Peaks must also stay within the system’s output ratings. Actual runtime is established in a technical assessment. Capacities from the Ampd 200 IEC brochure, Rev 1.3, dated 29 April 2026 and the Ampd 400 IEC brochure, Rev 1.3, dated 29 April 2026.
An average hides the peaks
An average load is the right figure for energy. It says nothing about whether the system can deliver the peaks.
A site averaging 20 kW may start a motor that draws many times that for a few seconds. The peaks have to sit within the system’s output ratings: the nominal output for anything sustained, and the time-limited overload ratings for brief surges. The Ampd 200, for example, is rated at 193 kW / 214 kVA nominal, and 292 kW / 297 kVA for up to 10 seconds.
A battery can have enough energy for the day and still trip on a peak it cannot supply. Power and energy are checked separately.
Conditions that move the answer
- Temperature. Output can be reduced at high ambient temperature, and cooling uses more energy. The Ampd 200 brochure gives an ambient operating range of −30 to +55 °C (derating from 45 °C); the Ampd 400 gives −20 to +50 °C (derating from 45 °C).
- Ageing. Capacity declines gradually with use. The brochures list a rated battery cell life of 9,000 cycles, which is a cell rating and is stated not to equal warranted cycles.
- The real duty. A site’s load changes by phase, season and shift. A winter cabin compound and a summer one are different jobs.
Runtime is half the question
A battery that runs for a shift then has to be recharged. If the charging source cannot return the day’s energy before the next shift, the battery starts lower each morning until it runs out.
So the complete question is: how long will it run, and can it recover in the time available? Our guide to charging a construction site battery covers the second half.
What turns an estimate into a firm figure
A screening calculation gives a range. These make it firm:
- A measured load profile, or failing that a list of equipment and working hours.
- The usable AC energy of the exact configuration proposed.
- Expected site temperatures.
- The charging source, its capacity and the hours it is available.
- Any reserve you want kept back.
You do not need all five to begin. A generator rating and your working hours are enough for a first assessment. Get a site power proposal and the remaining figures are established in the technical review.
Source: Ampd 200 IEC brochure, Rev 1.3, dated 29 April 2026; Ampd 400 IEC brochure, Rev 1.3, dated 29 April 2026. Manufacturer-published figures for the IEC variant, not independently tested. Specifications can change, so the current data sheet for your territory is confirmed in a project proposal.
Product figures on this page: Ampd 200 Brochure IEC Rev 1.3, dated 29 April 2026; Ampd 400 Brochure IEC Rev 1.3, dated 29 April 2026. Manufacturer-published, not independently tested.
Questions buyers ask
How do I estimate how long a battery will run my site?
Divide the usable AC energy of the system by your average load. If you only have nominal DC capacity, first reduce it for the usable charge range and conversion losses. The result is a screening figure: it excludes the system's own consumption, temperature effects, ageing and any reserve.
Why is usable energy lower than the capacity on the data sheet?
Data sheets usually state nominal DC capacity at the battery. A system keeps part of that in reserve at the top and bottom of its charge range, loses some in conversion to AC, and uses energy for cooling and controls. What reaches your loads is lower.
Does a bigger battery always run longer?
At the same load, yes. But runtime is only half the question. A larger battery also takes more energy to recharge, so the charging source and the time available for charging decide whether the arrangement works day after day.
Can you guarantee a runtime figure?
Not from a general calculation. A runtime figure is only firm when it is based on the actual load profile, the configuration supplied and the site conditions. That is established in a technical assessment and stated in a proposal.