Q: What's the difference between VA and watts? How do I convert VA to watts? The inverter is rated in VA, but my appliances are rated in watts.

A: The short answer is that some devices, those with a power factor (PF) of less than 1, consume more VA than watts. The formula is VA = Watts / PF (or Watts x PF = VA). Some examples:

A 100-watt laptop charger with a power factor of 0.7 would require 100 / 0.7, or 143 VA.

Incandescent lights and toasters have a PF of nearly 1, so their VA and watt rating are equal.

Motors can have PFs ranging from .2 to .8, depending on how heavily they are loaded. So their VA rating is around 1.25 (heavily loaded) to 5.0 (lightly loaded) times the watt rating.

Inverter manufacturers are sometimes accused of trying to inflate their output by listing the VA, a larger number, instead of watts. But as we have seen above, the relationship between those numbers can vary. VA is the controlling specification for transformers and power delivery, including inverters. The manufacturers can only try to estimate how many watts you'll be able to consume as a convenience. Here are some guidelines for estimating purposes, but measurements are the best way to go.

Resistive loads, like incandescent lights and heating devices have PFs of essentially 1.0, and you can treat watts as VA.

Power Factor Corrected (PFC) devices, which include some chargers and power supplies for electronics, have PFs of .85 to .95. Unless you are running your inverter near the limits, these should not be a concern. 

Computers with an Energy Star rating of 4, and many other devices, comply with the "80 Plus" initiative, and have PFs of 80 or better.

LED lights that are run on AC have PFs in the 90 range, except that dimmers can interfere with that and cause the VA to run very high. This does not apply to DC-powered lights.

Arc welders have PFs of 35-60, depending on type.

A little background on the science behind it all: VA stands for Volts x Amps, which we learned in high school science class equals watts. And that is true for DC circuits, but only sometimes true for AC power. The minimally technical explanation is that, with alternating current, power is available as a sine wave, smoothly transitioning from positive to negative on each wire. Resistive loads like incandescent lights or toasters use the power as it comes, and for them, watts are equal to VA. Some loads, especially motors, create a load pattern that can be out of sync with the power supply. So even though the total power consumed is one amount, the load at a point in time (the "valleys" in the sine wave) can be higher. Motors can even "give back" a little power during each AC cycle, the "power" curve below the line in the example below. Note also the current being out of sync with the voltage, which is the cause of the increased VA.

Comparing VA to watts - chart

In the end, measuring your devices is a great idea, not only for sizing your inverter, but for evaluating your power usage and utilizing the most efficient appliances. Meters like the Kill-A-Watt, under $20 at Amazon, will display poth wattage and VA. Or, if you have a true RMS (and it must be RMS) clamp meter, like the Fluke 323 or many others, you can pick up the amps and compute from there.