SOlar and New Zealand Three phase metering
Problems for three phase properties with solar
Learn why three phase properties have challenges when installing solar or running an EV charger from solar.
Problems for three phase properties with solar?
Most three phase hybrid inverters discharge evenly across the phases (as is required by the NZ standard 4777.2), however if the load is not balanced (as it won’t be in a residential house) then the inverter will send some of its power to the grid to support load that requires it on another phase. This is problematic for New Zealand installations because of our independent three phase metering. The problem with this is that the customer gets charged the import rate (e.g., 30c) while only getting paid the export rate (e.g., 10c) for every kWh that is exported to the grid, and then imported from the grid between the phases. This costs the customer the difference between the import and export rate for electricity I.e., 30c – 10c = 20c. As the load varies between the phases this export/import behaviour causes no financial penalty to installations in countries where the grid metering is aggregated across the phases. But in New Zealand our meters are independently billed across the phases. In other countries it does not matter if a solar inverter sends excess generation to the grid and therefore ‘supports’ load on other phases. This also means a solar inverter installed overseas can be installed on any phase - or even only one phase. However, in New Zealand import is independent of export and this is financially detrimental to installations where the installation is three phase.
Please note, that this problem also occurs whether or not the hybrid inverter has a battery connected.
There are however, a small number of three phase hybrid inverters coming onto the market that can charge and discharge asymmetrically across the phases. These inverters provide unbalanced output. I.e., they can match their charge and discharge to the imbalance of loads across the phases. However, most of these inverters can only do this during a power cut—i.e., when in backup mode, but there are now some that can also provide an unbalanced function when the grid is connected. We recommend that one of these inverters is used for three phase solar installations.
Getting enough power for large single phase loads
Another problem is that a three phase hybrid inverter can only output up to one third of its power on a single phase. This is a limiting factor for running larger single phase motors e.g., water pumps. This means that for example even a large 10kW inverter could only supply 3.3kW to run a single phase pump. This may not be enough to start the pump if there are other loads running at the same time. The solution to this is to install a larger inverter.
Solving the three phase problem
These two problems are solved with the new range of three phase hybrid inverters from Sigenergy and Sungrow. Sungrow has three models available—SG15T, SG20T, and SG25T. These provide unbalanced output while grid connected. Sigenergy has three phase models ranging from 10 - 30 kW.
Another (not as good) solution for three phase properties with unbalanced load is to install the solar on one phase only and move daytime loads to that phase. The main problem with this is that the other phases won’t benefit from any solar generation. A battery can be added to this solar, but again it only benefits one phase. This is done with solutions like the Franklin Whole Home, Tesla Powerwall, or Enphase IQ batteries. The other problem is that not all load can be moved to one phase (due to the potential to trip the supply fuse on the loaded phase), and comprehensive consumption monitoring of all phases is not possible.
Problems for three phase properties with EV charging from excess solar
This problem with our per-phase metering in New Zealand also applies to EV charger installations. If you have solar panels and want to charge from excess solar and your supply is three phase, then we don’t recommend connecting a solar aware EV charger to three phases. This may seem counter-intuitive; however, the reason is because a three phase EV charger looks at the net surplus across the three phases before deciding if there is enough surplus solar to initiate a charging session. This combined surplus must be at least 4.2kW as a car with a three phase onboard charger requires 1.4kW per phase to begin charging. In this case the EV charger thinks that excess solar that is spare on one phase can be ‘used’ by another phase. The problem in New Zealand is that our three phase meters read the import and export power on each phase individually and do not net this across the phases. In this case you could be charged for imported power on one or more phases, while your three phase EV charger thinks it is charging from net excess solar—in reality it is not—and this can cost you money. Because of the above, and if you intend to charge an EV from excess solar and your supply is three phase, then we recommend connecting a single phase EV charger to one of your three phases. This is not a problem when charging from excess solar on a single phase site.
