If you've been comparing inverters recently, you've probably noticed that some manufacturers list their output in watts (W) and others in volt-amperes (VA). It's an easy thing to overlook, but choosing the wrong unit can mean choosing an inverter that can't actually power your equipment. So, let's clear it up.
What's the difference between watts and volt-amperes?
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Watts (W) Real power The useful work done by an electrical device, the power that actually drives motors, heats elements, or runs circuits. |
Volt-amperes (VA) Apparent power A combination of real power and reactive power. It represents the total electrical demand placed on a supply, not just the useful portion. |
Think of it like a pint of beer: the liquid is the real, useful part (watts), while the foam on top is the reactive component. Apparent power (VA) measures the whole glass, liquid and foam together. You're paying for the full pint, but only the liquid does the work.
*Diagram showing the difference between watts and volt-amperes using a pint of beer.
So — is a 12V 1000VA inverter the same as a 12V 1000W inverter?
Not necessarily. The relationship between VA and W depends on something called the power factor, a value between 0 and 1 that reflects how much of the apparent power is actually doing useful work:
Watts = Volt-amperes × Power factor
e.g. 1000 VA × 0.8 power factor = 800 W of real power
Many electrical loads, particularly motors, transformers, and some electronics, have a power factor below 1. This is because devices like motors and transformers store energy in magnetic fields and release it back into the circuit rather than converting it into useful output.
In technical terms, watts measure the real power your devices actually use, while VA measures the total power the inverter has to supply, including those inefficiencies and reactive loads. Your inverter still has to handle all of it, even the portion that never does any real work. That means a 1000VA inverter may only deliver 800W (or less) of usable power, depending on what's connected to it.
In practice, a power factor of 0.8 is the most common rating you'll see across inverters, which is why the example above is a useful rule of thumb. Understanding this distinction is essential when sizing an inverter for critical applications.
Why does it matter in practice?
Undersizing an inverter, even by a small margin, can lead to overloads, equipment damage, or unexpected shutdowns at exactly the wrong moment. For critical-use systems like marine, off-grid, or backup power, that's not a risk worth taking.
This is why accurate power auditing matters. Rather than leaving you to navigate VA vs W on your own, the team at Merlin Power works through your specific power requirements with you, accounting for real loads, power factors, and system design, so you can buy with confidence.
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Whether you're managing a commercial fleet, fitting out a campervan, or upgrading your vessel's power system, our experts are on hand to help with a comprehensive power audit, so when you buy from us, you buy power you can rely on.