What size inverter do you need?

The commonest sizing mistake is to look at a monthly bill and buy an inverter to match it. Units per month tell you how much energy you use. An inverter is sized by power — how much you draw at one moment. They are different questions and they have different answers.

Step one: list what runs at the same time

Go room by room and write down what could realistically be on together on a summer evening. Rough figures for a typical house:

Add up a plausible worst case. Five fans, ten lights, a fridge, a television and one inverter air conditioner comes to about 2,100 W.

Step two: allow for the startup surge

This is the step people skip, and it is the step that trips systems.

Anything with a motor — a fridge, a pump, a compressor — draws several times its running figure for a fraction of a second when it starts. A 750 W pump can pull two to three kilowatts for that instant. If the inverter cannot supply it, the system trips, and it will keep tripping until somebody works out why.

So size for the running load plus headroom for the largest motor starting. In the example above, 2,100 W of steady load with a fridge and a pump in the mix wants an inverter comfortably above 3 kW.

Step three: convert kVA to kilowatts

Inverters are often sold in kVA, and the number on the box is not the number you get. Multiply by the power factor, which for these units is usually around 0.8:

• 5 kVA × 0.8 = 4 kW usable
• 7 kVA × 0.8 = 5.6 kW usable
• 8 kVA × 0.8 = 6.4 kW usable

A "7 kVA" inverter is a 5.6 kW inverter. Comparing a kVA figure against a kW figure makes one unit look a quarter larger than it is, and that is how people end up undersized.

Worked example

A house with five fans, twelve lights, a fridge, a television and one inverter air conditioner:

• Running load: about 2,200 W
• Largest motor start: the fridge compressor, allow about 600 W extra
• Design figure: about 2,800 W, plus room to grow

A 5 kVA unit gives 4 kW usable and carries this comfortably. Add a second air conditioner and you are at roughly 3,800 W of running load, which wants 7 kVA or more.

Do not oversize either

An oversized inverter costs more, and running far below its rating is less efficient. Buying double what you need is not caution, it is waste. Size for your real load plus honest room for growth — one more air conditioner, not five.

A note on batteries

The inverter and the battery solve different problems. The inverter must supply the power you draw at once; the battery must hold the energy you want to use overnight. A large battery behind a small inverter still trips. A large inverter with a small battery runs out after an hour. Both have to be right, which is why we work them out together from your actual load.

Our load calculator does this properly from the appliances you own, and there is more on the units we fit on the inverters page.