Quote Originally Posted by dhouston View Post
You need to take several thousand instantaneous readings of both current and voltage per second
Not really. In most cases, you can do it slowly. Consider a 60Hz sine wave at first. You could statiscally get a sample at every degree if you sampled one time in each cycle for 360 consecutive cycles, as long as your measurement interval was not synced to the 60Hz. You could obtain those 360 samples in 6 seconds. So, you could measure voltage, then current, then wait for 15msec and do it again. Plenty of time for calculations. Of course, you don't get cycle-by-cycle results, but I don't think a KILOWATT does either. If the update rate is too fast, humans can't read it.

And you don't need to grab every degree, either. If the crest factor is not too high, you can get really good results by sampling every 10 degrees or so. If, by chance, the input was a pure sine (I agree that isn't too likely, but it is a starting point), and you sampled 5 degrees before peak and 5 degrees after peak, sin 85 = .996 That is a really small error. You might say that the waveform isn't changing very fast at the peak, and you would be right, the rate of change (COS) is greatest at zero-crossing. The sampling error would be highest there. But, the energy at zero crossing is low, so it doesn't contribute much to the RMS values.