Buyer guide
kW, kVA and kWh: what each one means on site
Three units appear on every battery and generator data sheet. They answer different questions, and mixing them up is the most common reason a site ends up with the wrong equipment.
Three units, three different questions
| Unit | What it measures | The question it answers |
|---|---|---|
| kW | Real power | How hard is the equipment working right now? |
| kVA | Apparent power | How much does the supply have to carry? |
| kWh | Energy | How much is used over a period? |
A site power specification needs all three. A battery system can have ample power and too little energy, or plenty of energy and too little power. Either one fails on site.
kW: the load at a moment
A kilowatt is a rate. A hoist motor, a row of cabin heaters and a concrete pump each draw a certain number of kW while they run. Add up what runs together and you have the load the supply must meet at that moment.
Two figures matter: the peak load, which the supply must be able to deliver, and the average load, which decides how much energy is used over the day. On most construction sites the average is far below the peak.
kVA: what the supply has to carry
Motors and transformers draw current that does no useful work but still flows through the cables, the generator and the inverter. Apparent power, in kVA, includes it. The ratio of kW to kVA is the power factor.
As an illustration, an 80 kW load with a power factor of 0.8 needs 100 kVA from its supply. The same 80 kW at a power factor of 0.95 needs about 84 kVA.
This is why generators are sold by kVA, and why a battery system states both figures. A supply has to stay inside its kW limit and its kVA limit at the same time. If your site is motor-heavy, the kVA limit is often reached first.
kWh: power over time
Energy is power multiplied by time. An illustrative 20 kW load running for eight hours uses 20 × 8 = 160 kWh.
Short, heavy loads use less energy than people expect. An illustrative lift drawing 150 kW for 30 seconds uses about 1.25 kWh. A crane can be a very large power problem and a fairly small energy problem, which is exactly the shape of load a battery handles well.
Nominal and overload: two kinds of power rating
A data sheet usually carries more than one output figure. Read the label next to each.
- Nominal output is the rating for normal, sustained operation under the manufacturer’s stated conditions. Compare this with your sustained load.
- Overload ratings are higher outputs available for a stated short time. They exist for motor starting and brief peaks.
The Ampd 200 is a clear example. Its nominal output is 193 kW / 214 kVA. It can supply 233 kW / 248 kVA for up to 60 seconds, and 292 kW / 297 kVA for up to 10 seconds. Those are three different ratings, and only the first is sustained.
Ampd 200
- Nominal output193 kW / 214 kVA
- Overload, up to 60 seconds233 kW / 248 kVA
- Overload, up to 10 seconds292 kW / 297 kVA
Ampd 400
- Nominal output386 kW / 428 kVA
- Overload, up to 60 seconds466 kW / 496 kVA
- Overload, up to 10 seconds585 kW / 595 kVA
Capacity on a data sheet is not energy at your loads
The same care applies to kWh. The Ampd 200 brochure lists a rated nominal DC capacity of 215 kWh, and the Ampd 400 lists 430 kWh. That is energy at the battery.
The AC energy delivered to your loads is lower. A system keeps some charge in reserve at the top and bottom, loses a little in conversion, and uses energy for its own cooling and controls. The supplied brochures do not state usable AC energy, so it is confirmed for the configuration in a project proposal. Our runtime guide walks through the arithmetic.
What a generator rating does and does not tell you
Many buyers know one number: the size of the generator on site. That is a useful start. It gives an upper bound on the peak the site was expected to draw.
It says little about the average. Generators are sized for the largest starting load, then spend much of the day lightly loaded. A site with a 250 kVA generator may average a small fraction of that. The gap between peak and average is where a battery earns its place, and it is the first thing a technical review will try to establish.
Which figures to bring to a proposal
You do not need all of these to start. “Not sure” is an acceptable answer to any of them.
- Largest load, in kW or kVA, and what causes it.
- Average load across a working day, if it has been measured.
- Hours of operation, and any periods a generator must not run.
- Current generator rating in kVA.
- Grid connection capacity, if there is one.
- Site voltage, phase and frequency.
With those, a likely arrangement can be identified and the remaining questions are specific. Get a site power proposal and bring whatever you have.
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
What is the difference between kW and kWh?
kW is power: the rate at which equipment uses electricity at a given moment. kWh is energy: power delivered over time. A 20 kW load running for eight hours uses 160 kWh. Power tells you whether a system can supply the load; energy tells you for how long.
Why are generators rated in kVA and loads in kW?
kVA is apparent power, the total the supply has to carry, including the part that does no useful work. Motor-heavy loads have a power factor below one, so they need more kVA than their kW figure suggests. A supply has to be within both its kW and its kVA limit.
Can I size a battery system on its maximum output figure?
No. The highest figure on a data sheet is usually an overload rating that applies for a stated number of seconds. Compare your sustained load with the nominal output, and treat overload ratings as cover for brief peaks such as motor starting, subject to an engineering check.
Does a battery's kWh capacity tell me how long it will run?
Only roughly. The capacity on a data sheet is normally nominal DC capacity at the battery. The AC energy that reaches your loads is lower, because of the usable charge range, conversion losses and the system's own consumption. Runtime also depends on your average load.