SOlar and New Zealand Three phase metering
Problems for three phase properties with batteries
Learn why three phase properties have challenges when installing batteries or running an EV charger from solar.
Problems for three phase properties with batteries?
Most three phase hybrid inverters with batteries 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 battery will discharge 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., 27c) while only getting paid the export rate (e.g., 8c) for every kWh that is transferred out 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., 27c – 8c = 19c. 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 batteries discharge to the grid to support load on the other phases – in fact, this is better for those countries because the batteries act in tandem as a single unit. It does not matter on which phase they are installed. They simply discharge together to support the load on whichever phase demands it. However, in New Zealand this behaviour is financially detrimental to installations where the installation is three phase. Because of this we don’t recommend installing most three phase hybrid inverters with batteries.
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 do only do this during a power cut—i.e., when in backup mode. There are one or two that can also provide an unbalanced function when the grid is connected.
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.
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 and can supply up to 7.3, 9.7, or 12.2 kW of peak output on each phase. Sigenergy has three phase models ranging from 10 - 30 kW.
The other solution for three phase properties with unbalanced load is to install the battery on one phase only and move critical loads to that phase. Solar can be installed on the other phases, but the hybrid inverter with battery is installed as a separate system on one phase with its own monitoring and therefore won’t discharge to the grid when load on other phases would otherwise demand it. This is done with solutions like the Franklin Whole Home, Tesla Powerwall, or Enphase IQ batteries and it also solves the financial penalty as described above.
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 an 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.
