Minimum size inverter for 3 phase solar?

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BossByss

4 posts

Wannabe Geek

#318858 27-Feb-2025 09:01

I'm in the final stages of settling on a solar installation quote. One of the installers has advised us to go with a 10kW inverter (with 12kW pv) so we can get the best export rates.

We are high users of power (everything electric including water, heating, car) but work/study from home so planning to shift load as much as possible. Still, there'll be a decent amount of export especially in summer, and more credits to cover winter wouldn't be a bad thing, until we get a battery in a few years...

Another installer (and my preferred quote) has advised us to go for a larger, 12kW, inverter (with 13.5kW pv) because we are on 3 phase power, and he feels that a smaller inverter will be too limiting in what we will have available per phase especially with high start up power pullers such as the water pump. I can totally see his point as well, especially since the inverter size is the "bottleneck" of any installation.

Any experiences with 3 phase/size inverter/loads tripping? I'll gladly hear them!

tripper1000

1617 posts

Uber Geek

#3348348 27-Feb-2025 12:00

10kw = 3.3 kw per phase.

12kw = 4 kw per phase.

There is 700 watts or 3 amps difference, so I don't buy the "high start up power pullers" justification. If you are seriously concerned about that, then change the pump to the 3 phase water pump since for the same horse-power they have much lower per-phase start current, are more efficient and are self-starting which makes them simpler and less failure prone. You can go down a 3 phase rabbit-hole - while your chasing diminishing gains, don't forget to also switch your hob, oven, hot-water and heat-pumps over to 3 phase for maximum efficiency and optimum phase balancing.

In both installations, the inverter is the "bottle neck". However in reality no solar system in N.Z. produces it's theoretical output, except for maybe 10 minutes on the day of the summer solstice. 70% of the time, clouds in the sky are the bottle-neck.

timmmay

20587 posts

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#3348365 27-Feb-2025 13:26

My reading suggests that even in Australia where they get a lot more sun inverters tend to be undersized up to 33%, in NZ we get less sun so I'd say keep the inverter lower. This is just based on some quick reading, so please do your own research to find out more about this topic.

We're getting 9.2kw solar with 6kw inverter (6.6kw peak) put in soon. Because we have the panels split north / west we don't expect the inverter to limit and restrict our output often.

SomeoneSomewhere

1804 posts

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#3348367 27-Feb-2025 13:37

Unless it's being set up with a battery for standalone emergency operation, startup currents are not a concern. They're short enough that the energy cost is almost non-existent.

Definitely go for most panels possible, and don't worry too much about inverter sizing. Energy that hits your roof instead of a panel is lost energy. Low inverter capacity will only limit output during full summer sunlight when you're almost certainly exporting significantly anyway.

With three phase, NZ doesn't net meter between phases, so you want an inverter that can be configured to output unbalanced power between phases. Otherwise, you can be in a situation where you're generating 6kW (2kW/phase), but because two phases are loaded at 1kW and one at 4kW, you get charged for 2kW of import and credited for 2kW of export simultaneously

richms

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#3348368 27-Feb-2025 13:44

Inverters will briefly shut down when a large load starts up causing stupid amounts of distortion on the line and the voltage to drop below their limits. They will come back on quickly as the sine wave settles down. This will cause minimal reduction in your output and I doubt that there would be difference between how a 10 and a 12 would handle this, more likely a difference between brands and I have seen no tests on how quickly they recover after a large inductive load is connected.

If the pump is running that often causing problems then perhaps it needs looking at.

Richard rich.ms

Valok

40 posts

Geek

#3348371 27-Feb-2025 13:46

your solar will most likely be setup on one of the phases

SomeoneSomewhere

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#3348372 27-Feb-2025 13:47

Valok:

your solar will most likely be setup on one of the phases

No, they're expressly talking about 10-12kW three phase inverters.

SteveXNZ

59 posts

Master Geek

Lifetime subscriber

#3354270 15-Mar-2025 22:40

Sorry I’m a bit late to this conversation, but happy to share my experiences.

I have an AlphaESS Smile T10-HV 3-phase 10kW inverter fed by 16kW of panels on two MPPT strings. Battery storage is 2 x8kWh 3-phase units. I have an all-electric house with water pumps, heat pumps, spa pool and a 7kW EVSE, so the per-phase loads can be substantial. I’ve never had any issues with high start-up currents for any of the pumps. Transients seem to be easily handled by the inverter.

Everything works as designed, and I endorse a previous comment about it being essential that your inverter has an “unbalanced power” mode so you can minimise your grid draw while simultaneously exporting.

But if I had the choice to select or upgrade to a 12kW inverter, I’d take it, and it’s all to do with batteries. You say you’ll “get a battery in a few years” – so it’s on your plan. My only caution there is to get some assurance from your installer that future batteries will be compatible with your inverter.

But here’s the reasoning for when batteries are on the cards.

As you work with your sparky to balance your loads over the phases you’ll find that 4kW is just a little easier than 3.3kW. This is particularly relevant for the select few backup circuits which remain energised on grid outage, but they’re actually fed from the inverter and not the grid to get the transition time to under 20msec, so are continually subject to the per-phase power limit. Naturally you’ll want your fridges on backup, but that presented a problem for me. On the same circuit as a fridge was the toaster, jug and coffee machine… That caused a soft trip on that circuit which took a few minutes to recover, so reluctantly that circuit is back on the grid side.

The other consideration is the rate at which your inverter can charge the batteries. I’m on Electric Kiwi’s Movemaster plan which gives me a free Hour of Power. Needless to say, everything’s switched on over that period, and I can pump 10kWh into the batteries for free at 6am to get me through the morning peak. But 12kWh would be better.

Whichever way you go, monitoring and management of your inverter and grid power flows per phase and in real time is essential. Most have some sort of cloud system accessible by a web or mobile interface, but you’ll likely find it very limiting, and useless if the backend servers or network are down. Best option is a local Modbus interface accessed by a Home Assistant integration or similar.

All the best for your selection and implementation. Once you go solar and select a retailer with variable tariffs you become much more attuned to your consumption and export, and will seek to optimise accordingly.

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BossByss

4 posts

Wannabe Geek

#3355751 20-Mar-2025 21:47

Thanks everyone for your input! Yes, I'm looking at a 3 phase install and the aim is to balance the loads as evenly as possible across the 3 phases given the lack of net metering - both in design and with the chosen inverter (Fronius) being able to handle unbalanced loads. Even after "dismissing" the potential issue with the water pump start-up power draw, I've decided to go for the larger size as this allows for more load per phase - pretty useful with the 3 phases.

And thanks @SteveXNZ

That's a very helpful response. I read it AFTER I'd locked in a quote but still had some doubts: this set my mind at ease. Like you, we have a power-hungry home (electric heating, cooking, hot water, 2 EVs) and when I ran a mid-sunny-day scenario and all the loads I'd want to run to try and make the most of self-consumption, I realised the extra load per phase with the 12kW inverter would definitely be welcome.

I've got an install date set now! Do you have any tips for distributing the loads well? I'm hoping to get the phase loads balanced nicely, and have download half-hourly usage data to see how the phases are currently balanced (badly), and making a note of our "power-hungry" devices, their likely time-of-use, and which breaker they're on. Anything else to help the electrician get it right?

SteveXNZ

59 posts

Master Geek

Lifetime subscriber

#3355797 20-Mar-2025 22:56

BossByss:

Do you have any tips for distributing the loads well? I'm hoping to get the phase loads balanced nicely, and have download half-hourly usage data to see how the phases are currently balanced (badly), and making a note of our "power-hungry" devices, their likely time-of-use, and which breaker they're on. Anything else to help the electrician get it right?

Not a lot to add, but phase balancing has to be a "good enough" exercise for your heavy hitters - hot water, HVAC, oven, hob & EVSE. Seasonal as well as daily variations will change the distribution, and nothing else will consistently balance an EVSE pulling 7kW. So expect significant phase differentials at times, which is why the inverter unbalanced mode is important. Also be aware of the maximum you can pull on one phase - typically 63A/15kW. You don't want to exceed that or you'll start tripping circuit breakers.