Can anyone tell me, if on a 3 phase supply, phase L1 is net importing 100W and Phase L2 is net exporting 100W, does it record kWH usage based on the 100W import, or does it record zero? (assuming modern digital meter)
No.
What your meter does will depend both on the meter itself and probably more importantly, on how your electricity supplier has asked for it to be programmed. I think the only way you will find out is by experimenting and observing what it records.
In the UK, there is a recommendation that the meters should operate in the same way as a Ferraris (mechanical) meter. As a 3-phase one of those has 3 discs on a common shaft, that would record zero. But there’s no guarantee that a modern meter will do that.
As another data point, here in Aus they will be configured to read 0 in the case you describe. It’s not uncommon to have a 3-phase supply to a house with only a single-phase PV inverter. In that case there’s no financial advantage to be had by moving your big loads around to be on the same phase as the inverter (* or even off the same phase as the inverter in the case of high feed in tariffs). It’s no doubt more grid friendly to try to keep the phases balanced, but I’m not sure installers pay much attention to that.
(*) If you get paid 50c/kWH for exports and only pay 20c/kWH for imports and the meter didn’t add net power up across phases, you could game the system by putting your inverter on one phase, and all your loads on the other two phases. Then, thanks to that 5:2 ratio, you could end up being paid to consume energy.
Following-up on the 3 Phase issue:
Alarm bells rang out when the Energy Company wanted to increase our Monthly Direct Debit.
We had over 8KW of Solar on the roof & coming into Summer & generating a big surplus of energy during daytime - typically over 6KW even overcast - but the Energy Company’s supply meter was still registering energy imported from the Grid! The house is single phase supplied. Only the Solar PV on all 3 phases.
The meter is arranged to sum INDIVIDUAL phases = (imported - exported energy)
BUT any export is actually set to ZERO. Then show the total of these individual phases.
This means that IF for example:
On Phase 1 - export 1KW - PV
import 3KW - our house
On Phase 2 - export 1KW - PV
import 0KW
On Phase 3 - export 1KW - PV
import 0KW
The meter will ACTUALLY indicate 2KW imported, even though a NETT 2KW were EXPORTED
COOP Energy & their meter services provider Lowri Beck described this in writing as a standard meter, incompatible with Solar PV although its what they installed against.
Apparently what you have to ask for is a VECTOR SUMMATION METER. (how would a non-techie know this)
This has taken me almost a year to resolve, with vendors refusing to come to site at first, but eventually admitted liability. I’ve received compensation, but this fell short of actual loss.
What do you mean by that? Presumably, the house is supplied on a single phase, but outbuildings/workshops are supplied by the other two phases (and zero consumption there)?
Or, you have no connection other than the PV to the second & third phases?
The original supply was single phase, supplying everything on site.
3 Phase supply upgrade was required by the Power Distribution Company to balance its feed-in.
We lost the benefit of having Solar at all, no significant savings to be made with the ‘standard’ meter provided.
Yes that’s right the other 2 phases have no consumption.