Understanding units

This is not directly related to emonCMS, but I know that there are folk on here that understand the intricacies of electricity way better than me …

My new(ish) SMETS2 smart meter can display Total Active Import & Export Energy (kWh), along with other things like Reactive Energy Import & Export (kvarh) etc. - which I don’t understand.

But my Tesla Powerwall app shows some very different data - see below.

How should I interpret all of this? Are the two sets of data supposed to be showing the same thing? FWIW I am very surprised at the SMETS2 export values, as I had thought that the Powerwall would have all but stopped exports.


I’m quite sure there will be a Wikipedia article about it, which will probably go into far too much detail for you. :wink:

It might help to refer to this Learn→Electricity Monitoring→AC Power Theory

If you have a load that is pure resistance - like an electric kettle, voltage and current waves coincide exactly. Multiply voltage and current at any instant, and the power is always positive. The kettle gets hot. If you measure voltage and current with a muiltimeter and multiply the readings, the number you get will be the power. I was taught this was real power, the modern name is active power. The units of power are watts (W) or kilowatts (kW).

If you have a load that is a pure reactance - a suppression capacitor comes close, so does the voltage coil in your old ‘Ferraris’ rotating-disc electricity meter, the two waves are exactly 90° shifted relative to one another, and if you multiply voltage and current, half the time it’s positive and half the time it’s negative. There’s no nett power flowing, nothing gets hot. If you measure voltage and current with a multimeter and multiply the readings, you get a number, but it won’t be the power (that’s zero). I was taught this was imaginary power, the modern name is reactive power. The units of reactive power are volt-amperes reactive (VAr) or kilovolt-amperes reactive (kVAr).

If you have a load that’s a combination of the two, then the waves will be shifted somewhere in between, the positive parts and the negative parts won’t exactly balance, and the power that flows will be part way in between too. If you measure voltage and current with a multimeter and multiply the readings, you get a number, it won’t be the real power, and it won’t be the reactive power either. This is the apparent power. The units of apparent power are volt-amperes (VA) or kilovolt-amperes (kVA).

So, for most real loads, you have apparent power in VA, this is the largest number always.
You have real power, which is normally what you pay for, and it can be equal to but never larger than apparent power, and you have reactive power, normally this is small.

Power companies don’t like any reactive component in the power they supply, because the reactive component of current still flows even though they don’t get paid for carrying it and it costs them money in cable, switchgear and transformer ratings.


If only my A level electronics teacher at college, explained it as simply 25 odd years ago!

We’ve had a lot of sunshine in the UK and the excess solar has to go somewhere when the battery is full.

Thank you @Robert.Wall - very helpful info.

The SMETS2 meter shows the following values today. I have been recording the first two lines each month (kWh), so have been doing the right thing. I now also understand how the Reactive figures are arrived at.

Quite how Tesla calculate their values remains a mystery, but they are also in kWh so presume they are Active Power too.

@stuart - agreed, but the Tesla export values are way off from the SMETS2 export values. Strange. Also, for the past 8 weeks or so one half of my 4 kWh peak inverter has been u/s, so my production has actually been pretty poor. Hopefully a replacement will be fitted next week. (As an aside, my SMA inverter was 5.5 years old and was covered by a 5 year warranty. Despite asking for some leniency, none was given so have gone to another make for the new one (SolarEdge)).

An obvious question - is the Tesla Powerwall measuring export at the same place that the meter does - i.e. there are no other connections between there and the meter?

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Yes - I believe so, but you have sowed sufficient doubt that I will go away and check the wiring. :grinning: