I'll give you partial credit, you should not do the square and square root, like a normal rms calc, however, you must use integration.

If you can't/don't want integration & just use two signal readings, they must be RMS measurements, not avg measurement.

So a meter that is going to simply multiply 2 values (Volt & amps), must use RMS to be accurate.

Thank you for the partial credit.

Did you read what I wrote and look at the links? First, if you just 'solve' your equation you'll find that what comes out is just the mean of the function or samples. The integral of instantaneous power over a time period is energy, not power. Energy divided by a time period is.....average power. It's like magic.

As for multiplying Vrms and Irms to get power, please get it through your head that it isn't 'accurate', it doesn't work at all unless the load is purely resistive.

Of course the best way is to instantly read and mult I & V to form watts in real time & get an overall reading. I suspect chapo meters are not doing this...they might just take the avg of each & multiply those, giving poor results.

I don't know the basis for your assumption that they don't do this. There really is no other way to get a meaningful result, although there are other factors that could affect accuracy and I wouldnt' be surprised if a $10 wattmeter was less than highly accurate. The missing information is the sample rate, which I don't see in any of their spec sheets. One just lists "SAMPLE RATE: sample/s".

How do these RC meters calculate? You seem to propose they can just take average current*avg voltage & give an accurate watts. They need to use RMS waveform values to at least have some hope.. For non reactive loads, they must include a power factor.

*Use rms voltage and/or rms current to calculate average power, resulting in meaningful power values.* and using RMS is exactly what I put in my **first **post, then that seemed to be immediately questioned!

Again, please reread--I said nothing about using average volts or amps. They way a wattmeter works is 1) take samples of

*instantaneous* voltage and current 2) calculate the

*instantaneous* power at each sample time point and then 3) average the calculated power results over a set time period, this is displayed as active or real power in W, Watts.

To calculate RMS current and voltage over the same time period, the MCU would calculate the square of each sample, then the mean of those squares, then the square root of that mean. RMS = Root of the Mean of the Squares. The meter can then display Watts, RMS Volts, RMS Amps and, by multiplying RMS Volts and RMS Amps it gets what is called the

*apparent *power, typically known as VA or Volt-Amperes. It can also display PF, Power Factor which is W/VA, and if you wish

*reactive* power, VAR, which is VA-W. That's how they work.

The important point in all that is that the power calculation is not derived from RMS measurements, period--and it cannot be derived from them alone. A bare-bones watts-only meter would not bother calculating or measuring RMS at any point. However, once you have all those sample points and an MCU with nothing else to do, it doesn't add much cost to add on all those extra readings.