kVA vs kW: what an inverter's rating really means
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Inverters are sold by a headline number, usually in kVA. Appliances are labelled in watts. Mixing the two up is one of the most common reasons a new system trips the first time someone switches on the kettle.
The short version
- kW (kilowatts) is real power: what your appliances actually use. 1 kW = 1,000 W.
- kVA (kilovolt-amperes) is “apparent power”: voltage × current, before accounting for how efficiently the load uses it.
- The link between them is the power factor: kW = kVA × power factor.
Many inverters are rated with a power factor of 0.8. On those, a 5 kVA inverter delivers about 4 kW of real power, and a 3.5 kVA unit about 2.8 kW. Some newer inverters are rated at a power factor of 1, so their kVA and kW figures are the same. The only way to know is the spec sheet.
What to look for on the spec sheet
Ignore the model name and find these lines:
- Rated output power, in W or kW. This is the number that matters for running appliances. If only kVA is given, look for the power factor and multiply.
- Surge or peak power, and for how long. Fridges, freezers, pumps and air conditioners draw several times their running power for a moment when they start. Inverters typically allow about twice their rating for a few seconds, but this varies.
- Battery voltage (12 V, 24 V or 48 V). Bigger inverters use higher voltages to keep currents, and cable sizes, manageable.
- Waveform. Pure sine wave inverters suit every appliance. Modified sine wave units are cheaper but can make motors run hot and some electronics misbehave.
How much headroom to leave
An inverter running at 100% of its rating for hours on end runs hot and wears out faster. A common rule is to load it to no more than about 80% of its continuous rating. So for 2 kW of appliances running together, look for at least 2.5 kW of real output, which on a 0.8 power factor inverter means at least a 3.1 kVA label.
Then check starting surges. If the biggest motor in the house is a 1 kW pump that starts at around 3 kW, the inverter needs to cope with the running load plus that surge for a second or two.
A worked example
Say you want to run 10 LED bulbs (100 W), a fridge (150 W), two fans (150 W), a TV (60 W) and a laptop (60 W) at the same time: 520 W.
- With 25% headroom: 520 × 1.25 = 650 W continuous.
- Fridge starting surge: about 3 × 150 = 450 W, so the brief peak is 520 + 300 extra = 820 W.
- A 1 kW inverter covers this comfortably. In kVA at a 0.8 power factor, that is about 1.25 kVA.
Add a 1.5 HP air conditioner (around 1,300 W running) and the picture changes: the running load is now about 1.8 kW, and the compressor’s start can push the peak towards 4.5 kW. You are now looking at a 3 kW-plus inverter, which is why air conditioning is the single biggest driver of system size.
Size your own inverter in the calculator, with your appliances and starting surges included.
Common mistakes
- Treating kVA as kW. A “5 kVA” inverter is not a 5 kW inverter unless its power factor is 1.
- Adding up appliances you never use together. The inverter only has to carry what runs at the same moment. Plan when the iron, kettle and microwave run, and you can buy a smaller inverter.
- Forgetting the battery. A big inverter on a small battery will run your load, but not for long. Inverter size sets how much you can run; battery size sets for how long.