You probably do this all the time, but you don’t even know it.

I’m talking about AC power calculations with P = V x I.

Technically, it works. But when you think about it, it shouldn’t. AC has constantly changing voltage and current over time – unlike DC.

So how on earth does it work?

Well, the short version is: RMS, or root mean square!

The long version involves a (simplified) explanation of AC power involving Watts, Volt-Amperes, and Power Factor.

But not to worry: At the end of the day, you can safely use P = V x I with AC, and you’ll still end up with a result that’s close enough for horseshoes!

Keep in mind that while P = Vrms x Irms is apparent power, you can still calculate real power using P = Vrms x Irms x cos ϕ, where ϕ is the phase angle between the current and voltage – and cos ϕ is just the power factor.

The usual problem with this approach is that very few gizmos will have a label or documentation specifying the power factor!

So, you can guess, or you can just do P = Vrms x Irms, get apparent power, and you’ll be close enough.

Finally, note that if you have two apparent power values for 2 different gizmos, you cannot just add those two apparent powers together – unless they have the exact same power factor.

Isn’t this FUN?! It just gets hairier and hairier…

But the bottom line is, do P = V x I and you’ll be close enough. Besides, that’s what you were doing anyway! 😉

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