As power.saver says, reactive power is something that only applies to AC devices. Although I remember Watts = Volts x Amps from school, that only applies to DC or purely resistive AC loads. I had to research what reactive power is. Professional electricians may criticise the exact terminology I have used but I think it is essentially correct. Here is a fairly good explanation:
https://www.electrical4u.com/electrical-power-factor/
Unfortunately it involves a certain amount of fairly basic maths and equations, high school physics stuff. Scroll down to the Power Factor Triangle if equations are not your thing. if you haver heard of RMS or root mean square, this is what it's about.
Reactive power does no useful work but still has to be generated, so costs money. Until recently, domestic devices drew very little reactive power and was noit metered by traditional domestic electricity meters. So the small cost of generating it is just incorporated into your electricity bill.
Examples of purely resistive devices are old fashioned incandescent light bulbs, cooker elements and electric fires - the glowing element of an electric fire is just low resistance wire that gets hot when a current is passed through it. With these devices, by definition, the power factor is 1 or 100%.
However, AC motors do draw reactive power. My old fridge has a power factor of about 0.6, whereas with my new freezer it's 0.85. Modern electronic devices like TV's, computers etc draw an element of reactive power. More and more of these devices are finding their way into the domestic market, so reactive power supply is becoming a more significant factor.
Industry tends to use a lot of high power electric motors so potentially use a lot of reactive power. So industrial meters do measure reactive power, separately and ther customers are charged for it, with penalties for excessive reactive power demand. It is possible to reduce the reactive power demand by good design or fitting things like capacitor banks and other methods. There is quite an indusrty supplying reactive power reduction devices, They are not cheap, but the cost of reactive power surcharges make it worthwhile. The electricity supply industry needs to encourage reduction in reactive loads - by doing so it reduces the amount of power that has to be generated and hence the need for more power stations, so it's really a good thing.
Unfortunately, this seems to have crept into the domestic market via smart meters and probably accounts for why some people have noticed a rise in their bills - particularly if the have old deep freezes etc. My Tesla Powerwall seems to draw an inordinate amount of reactive power compared to real power, and this occurs when it is making small grid demands in order to keep in synchronisation with the mains frequency. It does this continuously, while supplying the house demand, otherwise presumably, clock time would go out of synch and there would probably be a big flash and hot smell when switching over! However, it also puts a similar amount of both real and reactive power back into the grid, the net consumption being near to zero. Again, unfortunately my smart meter only records import, not export. I am being paid for solar export aty the assumed 50% of energy generated by my panels. I understand that new solar installations use real export metering, presumably by smart meters set to record export ans well as import.
Here is an example of the data output from my Powerwall Gateway:
........"last_communication_time":"2020-01 11T19:00:00.340646564Z","instant_power":7,"instant_reactive_power":73,"instant_apparent_power":73.33484846919642,.........
You can see that instant_reactive_power is 10 times bigger than instant_power - that's real power that traditional meters record. If you add together the squares of real and reactive power and take the square root of the result you get instant_apparent_power.
Thes evalues are in watts and jump around all over the place, occasionally increasing to 500watts reactive power or more, but only for split seconds. Sometimes they are negative.
Tesla Powerwalls are not the only home batteries having this problem. My friendly EDF manager found a similar problem with his experimental home battery setup, which is probably why I've had good cooperation is ferreting out the answer to this. The powers that be seem to have kept very quiet about the whole issue, and it is quite a new phenomenon so the effects of metering reactive power may not have been fully appreciated, or they were not thought to be noticeable. An increase of a few watts from metering reactive power draw from freezers etc on the normal household load would bne very difficult to detect. It is only because with our home batteries we should be drawing zero power from the grid that it has become apparent.
I am hoping that the information I have been given about Tesla being able to adjust the reactive load is correct, otherwise I can only put up with the additional cost, about £100 per year on the ON-Peak tariff I changed to a couple of months ago.
