[Amps] rms Volts, Amps and Watts

[Steve Thompson]

jeff, wa1hco wrote:

> Power meters measure RMS current and RMS voltage. But Watts RMS
> only equal V rms x I rms when the power factor is 1. For lower
> power factors you pay for Vrms x I rms which is greater than
> Wrms.

Sorry to be pedantic and boring, but I'm taking my prize 
hobbyhorse out for a ride...

rms voltage x rms current does not give rms power. It gives mean 
or average power. rms power can be calculated but has no useful 
value.

If you look at power as Vrms^2/R - the r in rms stands for 
(square)root, rms voltage is (square)root of the mean squared 
voltage. When you calculate the V^2 value, you're squaring a 
square root - the two cancel and disappear in a puff of algebra. 
Vrms^2 is Vms, or the mean (average) of V^2. Divide that by R and 
you have the mean or average power.

OK, rms power is so widely used that most everyone assumes that 
mean power is what's intended, but it's incorrect terminology. I 
find it a bit worrying that the ETSI spec for digital mobile radio 
sets (EN 301166) is awash with references to rms power. I'd always 
assumed that such specs. were drawn up by committees of people who 
knew what they were doing. It seems not.

Steve