Watts, VA and Power Factor Explained

Watts tell you how much real work the power does. VA tells you how much the inverter has to supply to get that work done. The two can be the same, or VA can be higher. Power factor is the link.

The two kinds of power

Real power is measured in watts. It's the part that heats a kettle element or turns a fan. Apparent power is measured in volt-amperes, written VA. It's the total the supply has to deliver, whether or not all of it ends up as work.

A simple heating element is a purely resistive load, and there the two figures match¹. But where a load has a reactive part, "the apparent power is greater than the real power as voltage and current are no longer in phase"¹. Motors and transformers are the usual reactive loads.

Think of a mug of beer with foam on top. The beer is the real power and the foam is the extra. You carry the whole mug, though, so the foam counts when you check the mug's size.

What power factor is

The relationship between real and apparent power "is described by the power factor"¹. More exactly, power factor "is defined as the ratio of real power to apparent power"².

So watts equal VA multiplied by power factor. A power factor of one means the two match. Anything lower means the watts are fewer than the VA. You'll often find the figure on a spec sheet, or you may need to ask the maker.

Why some labels use VA

VA is the unit used for apparent power, so it suits equipment that has to carry the whole current. Transformers are a known example: "VA ratings are also often used for transformers"¹. Some inverter makers rate in VA for the same reason, and some use watts. Many give both.

That's why comparing two inverters is tricky. A figure of one thousand in VA is not the same as one thousand in watts, unless the power factor is one. Always check the unit before you compare.

What to look at when you shop

• Which unit does the headline number use?
• Does the spec sheet give a power factor, or both a watt and a VA rating?
• Is the rating continuous, or a short surge only?
• What does each of your devices list on its own label, in watts or VA?
• If a device lists amps instead, ask the maker, or check its manual, for the watt figure.

Be wary of a listing with a big bare number and no unit. The unit changes what the number means.

Adding up your load

Use the watt figures on your devices' labels for a first sum. If a device lists only VA, take the cautious route and treat that as the draw, unless its manual says otherwise. Compare the total with the inverter's continuous rating in watts. If the inverter lists only VA, ask for its watt figure.

Items with motors, such as pumps, often need extra for starting. The manual for the device usually says how much. The page on waveform and your equipment has related checks, and reading an inverter label for where these figures sit.

Sizing a permanent system for a house, with its wiring and protection, is for a licensed electrician and the maker's instructions. Don't do the wiring yourself.