Power ratings

That is the correct formula for a resistive load. It is an upper bound on more complicated loads.

For instance, loads that have an inductive or capacitive component use some of that current to store energy in magnetic or electric fields, each half cycle, but return it to the power line, before storing it the other way, the next half cycle. So the rms current may include components that do not represent consumed power, but just borrowed power.

Yes.

Kind of.

Probably.

Why do you want to know?

P is virtually NEVER Vrms * Irms.

P is virtually NEVER Vrms * Irms.

I would like to know the typical max current draw from a PC monitor. If I new the energy starrating was 3W and I could use P = Vrms * Irms, then I would know the typical max. Of course, this assumes that most monitors are energy star compliant.

Thanks, David

Vrms * Irms is close enough for most purposes. Add 50% as a safety factor.

"David"

Eg: I am using a fairly new, 17 inch CRT monitor made in China.

On the back is printed " 100 - 240VAC 60/50Hz 1.5A "

However, when tested, the current draw is only 0.36 A at 240 VAC.

At 100 VAC it rises to 0.72 A.

These two figures calculate to 86VA and 72 VA respectively.

However, when measured, the actual POWER consumption only 56 watts !!

The current draw from the AC supply is in the form of short pulses at

At 240 VAC input, the peak value of these pulses is 1 amp.

At 100 VAC input, the peak value increases to 1.5 amps.

However, I reckon that " 1.5A " on the back alludes to the fuse fitted inside.

Go figure......

....... Phil