Powerwall: Three-phase house

[Jack182182]

Hi

Apologies if this has been answered elsewhere but I struggled to find it if so.

I have recently purchased a newly constructed property with a three-phase power supply. I believe it was built this way because it also has an air source heat pump, car charger, and no mains gas (so everything is powered through electrics).

We had planned on installing a Powerwall during the build phase so contacted a contractor to come and review the site and give us a quote.

When he came he informed us that as the house is three-phase, and the powerwall is not, we would only be able to connect it to one of the phases and therefore it wouldn’t be able to power everything in the house. This is probably fine as we’d connect to the phase that made the most sense and any excess could be sold to the grid. A tariff here in the UK allows you to purchase / sell energy at the same price so I think that’d work fine.

However, subsequently the same contract has told us that the Powerwall would also be limited to storing energy equivalent to 1/3 of the energy produced from the solar panels. This has a much larger impact as it implies that 2/3 would be wasted. I hadn’t seen this anywhere online so wanted to check - is this correct? These guys have been a little flakey so I’m not sure I trust everything they say!

Thanks and apologies again if this has been covered elsewhere.

 

[miimura]

I am not in UK, so am not authoritative on these issues, but this is my recollection of the issues you may face.

1. The Powerwall currently available is only one phase and therefore can only support one of your phases during a power cut.

2. If you can put your solar on that same phase, then you will be able to survive extended power cuts. If you have to use 3-phase solar in order to get enough export permission, then your backup duration will be limited because the solar must cut out with the grid power.

3. The Powerwall energy storage is independent of the solar. However, you have to check the metering situation. For example, when the solar is producing on 3 phases and you absorb all the power on one phase, does the meter net out zero? Does Tesla properly measure 3-phase solar inverters? Do they measure one phase and calculate the total as 3X?

 

[slcasner]

It's curious that the records returned by the /api/devices/vitals endpoint include some members hinting that 3-phase support is planned:

ISLANDL1L2PhaseDelta float64
ISLANDL1L3PhaseDelta float64
ISLANDL2L3PhaseDelta float64

For our USA split single phase installations (240V), L1 and L2 are populated.

 

[Jack182182]

Thanks for your replies. Really appreciate it.

My understanding (although very basic!) was that energy generated by the solar installation could all flow to the Powerwall (and therefore the full benefit of the solar would flow into the battery) but that when the battery was discharging it could only provide power to one phase. I was fine with this.

However, they are now saying that only 1/3 of the solar can be used by the Powerwall, which feels odd to me.

I’m fine with a situation where the Powerwall can only provide power to one phase in the house, as the tariff would allow me to sell excess power stored in the Powerwall and buy power for the other phases all at the same rate (so net nil impact, less any losses in transmission). However, it seems to be a much more significant issue if the Powerwall can only absorb energy from 1/3 of the solar installation, as that means 2/3 will likely be wasted a lot of the time.

It feels wrong to me but I know very little about this stuff.

A 3-phase option would be much better but as I understand it the Powerwall3 doesn’t have this? Ideally we’d want the Powerwall though as it enables access to what looks like a pretty good tariff in the UK.

 

[power.saver]

Thanks for your replies. Really appreciate it.

My understanding (although very basic!) was that energy generated by the solar installation could all flow to the Powerwall (and therefore the full benefit of the solar would flow into the battery) but that when the battery was discharging it could only provide power to one phase. I was fine with this.

However, they are now saying that only 1/3 of the solar can be used by the Powerwall, which feels odd to me.

I’m fine with a situation where the Powerwall can only provide power to one phase in the house, as the tariff would allow me to sell excess power stored in the Powerwall and buy power for the other phases all at the same rate (so net nil impact, less any losses in transmission). However, it seems to be a much more significant issue if the Powerwall can only absorb energy from 1/3 of the solar installation, as that means 2/3 will likely be wasted a lot of the time.

It feels wrong to me but I know very little about this stuff.

A 3-phase option would be much better but as I understand it the Powerwall3 doesn’t have this? Ideally we’d want the Powerwall though as it enables access to what looks like a pretty good tariff in the UK.

If you have a 3-phase inverter, it generates power across all 3 phases and can be used by all 3 phases. If the PW is installed on only 1 phase of the 3-phase system, then it can only capture 1/3 of the generated power. But the other 2/3 is available for use on the other phases, so it is not lost. What it really means is that it will take 3 times as long to charge up the PW than would be possible if the inverter was single phase and on the same phase as the PW.

 

[Jack182182]

Thanks. That does makes sense.

I suppose the key disadvantage is that it will take longer to charge then as it can only use 1/3 of the energy generated? So if I have 6kW installation the Powerwall would only get power from 2kW and therefore would take 6 hours or so to charge instead of 2? I assume any excess power generated during the day from the other 2/3 could be sold back to the grid?

For instance, the tariff here in the UK is something like 15p per kWh for both buying and selling. So in theory, any excess power generated that cannot be used, or stored in the Powerwall, could be sold and if I then needed power on another phase when solar wasn’t generating I could buy at the same rate (and the Powerwall could also potentially sell back to the grid).

The tariff itself looks very strong compared to “normal” tariffs in the market, which is why I wanted to explore the Powerwall option. It feels like whilst it may not be ideal due to the 3-phase supply to the house, apart from the slower charging it shouldn’t be materially detrimental economically?

 

[power.saver]

Thanks. That does makes sense.

I suppose the key disadvantage is that it will take longer to charge then as it can only use 1/3 of the energy generated? So if I have 6kW installation the Powerwall would only get power from 2kW and therefore would take 6 hours or so to charge instead of 2? I assume any excess power generated during the day from the other 2/3 could be sold back to the grid?

For instance, the tariff here in the UK is something like 15p per kWh for both buying and selling. So in theory, any excess power generated that cannot be used, or stored in the Powerwall, could be sold and if I then needed power on another phase when solar wasn’t generating I could buy at the same rate (and the Powerwall could also potentially sell back to the grid).

The tariff itself looks very strong compared to “normal” tariffs in the market, which is why I wanted to explore the Powerwall option. It feels like whilst it may not be ideal due to the 3-phase supply to the house, apart from the slower charging it shouldn’t be materially detrimental economically?

I don't think it is ideal, for the following reasons:

1. You are creating a system with 1 PW and 2kW solar. This is very low solar, and will take a long time to charge the PW. That means load shifting will be very limited (unless you also have some form of grid charging to supplement the solar).
2. The PW will work as a backup, but only for 1/3 of your house, and only as a battery. Since the solar inverter is installed across 3 phases, if the grid goes down, the inverter will shut off, and you won't have any recharging from solar during the outage. Once the PW is depleted, you won't have power until the grid is restored.

I would suggest researching other battery systems to see if any 3-phase solutions are available. That way you could maximize using the 6kW solar power and possibly run the entire house during an outage.

 

[BGbreeder]

Tesla has discussed the possibility of a three phase version in the past, but I haven't seen any movement towards a product, other than the MegaPacks. I have a lower power phase application that could benefit, so I try to keep an eye on it.

I would explore with your installer what they mean by "wasted". Powerwalls have an upper limit of 5kW for a charging rate. Thus if the house load plus 5kW is less than the solar output off grid, you won't be able to charge your Powerwall at all. On-grid, you are probably correct that you can push the solar out on three phase and buy it back for your loads on the phase backed up by your Powerwall. However, there are some customers in the UK who have had their local utility restrict how much power could be placed back on the grid at their site, so please do check first. It may be that your site is one of those and your installer is correct that the unused solar will not be exportable.

All the best,

BG

 

[Jack182182]

Thanks - I’ll double check with them.

Protection from outages isn’t my number 1 priority, it’s more about getting an efficient tariff and reducing costs. Unfortunately in the UK you need a PW to access the particular tariff, otherwise a 3-phase battery would seem much more sensible. Unless things have changed it seemed like the Tesla UK tariff was significantly better than alternatives.

I expect that the PW will be able to charge from grid (I believe this is standard isn’t it?), as well as solar, and would learn to do this when appropriate?

Re the point around the PW not being charged at all, how would this work? I expect, although waiting for confirmation, that the phases are split relatively evenly across where load is expected in the house (e.g. Kitchen/downstairs, heat pump, upstairs/car charging - or something like this). In theory the total house load could be high, with the heat pump working, however I was planning on connecting the PW to the phase powering the kitchen/downstairs.

Thanks again for the responses - feels quite a complicated one to think about!

 

[BGbreeder]

@Jack182182 Yes, welcome to the club, we have jackets! Yes, it is complicated, and yes, it can be difficult to know what questions to ask to get the information one needs, as one doesn't know what one doesn't know. I would double check for accuracy any information that you rely on for your decision making, as there is so much mis/disinformation going around about solar/powerwalls/tariffs/local wiring/grid.

So, I don't think that it is you in particular, I think it is that these issues are genuinely complex, and are often very local in their details and marginal cost/marginal benefit decisions. The complexity doesn't help widespread adoption.

It is a best electrical practice to distribute loads evenly across all three phases, and ideally with three phase devices, rather than single phase devices for all of the major loads (heat pump, hot water, etc.) In the US, where grid outages are more frequent, many folks have rearranged their loads to be on a sub-panel for Powerwall backup, foregoing other loads. In the UK, given how infrequent outages are, I doubt that it makes much sense, bar those hardy souls living in places like the Pennines.

My understanding is that in the UK Powerwalls do charge from the grid, as well as solar.

All the best,

BG

 

[jgleigh]

And most residential users in the US are on split-phase power, so we don't run into true three-phase power that often.

 

[BGbreeder]

And most residential users in the US are on split-phase power, so we don't run into true three-phase power that often.

And to make it even worse there are two kinds of three phase available in the US, three phase "star" or "Wye", aka 4-wire, 208/120V, where three phase are 120V to ground common in office buildings, and three phase "delta", aka three wire, 240/120V, where two phases are 120V to ground. More here if you are curious; Three-Phase System Theory Explained | Star and Delta Connection | Electrical Academia The UK version is nominally 230/400V (+/-10%) to standardize with the EU, though in practice many locales are at 240V.

All the best,

BG

 

[Jack182182]

Wow - thanks.

This feels so difficult. On top of all this trying to find a contractor is difficult too, never mind finding one that’s trustworthy and competent. It’s also difficult to verify their advice given all the complexities. I think I’ve tried 4 or 5 locally now without much success.

I realise a lot of you are from the US but if anyone sees this and could recommend a good contractor to speak to in the UK (midlands) then that would be great. I think it really is about discussing the exact installation as it feels like some small differences could have a big impact and as you said above - I don’t know what I don’t know.

 

[miimura]

Be sure to check some of the Powerwall threads in the UK sub-forum. There are quite a number of users there, so you should be able to find a recommendation for a good installer.

 

[Vines]

And to make it even worse there are two kinds of three phase available in the US, three phase "star" or "Wye", aka 4-wire, 208/120V, where three phase are 120V to ground common in office buildings, and three phase "delta", aka three wire, 240/120V, where two phases are 120V to ground. More here if you are curious; Three-Phase System Theory Explained | Star and Delta Connection | Electrical Academia The UK version is nominally 230/400V (+/-10%) to standardize with the EU, though in practice many locales are at 240V.

All the best,

BG

Those 2 you described are most common but this article describes the high leg delta configuration we see sometimes. It is interesting to understand how both 120/240 split-phase power along with three-phase power can be derived from the same conductors.
High Leg Delta - Wiring 240V, 208V & 120V, 1 & 3-Phase Panel

 

[BGbreeder]

@Vines I admit to not being an electrician, but isn't high leg delta, just "delta" with a neutral? When two legs are 120 to ground, the third "wild" leg is automatically 208 to ground by the math, right?

All the best,

BG

 

[miimura]

@Vines I admit to not being an electrician, but isn't high leg delta, just "delta" with a neutral? When two legs are 120 to ground, the third "wild" leg is automatically 208 to ground by the math, right?

All the best,

BG

Usually people refer to the Delta voltages phase to phase, which is 240. However, you're right, the high leg is only 208V to the Neutral and Ground.

 

[Vines]

@Vines I admit to not being an electrician, but isn't high leg delta, just "delta" with a neutral? When two legs are 120 to ground, the third "wild" leg is automatically 208 to ground by the math, right?

All the best,

BG

It is a delta configuration, but different enough I thought I'd mention it since we were talking theory.

I am also not an electrician...yet. I am working on it.

 

[JohnWD]

Hi, I am in UK with 3-phase, 8.4kWp PV, 2 Powerwalls (started with 1 and added 2nd later). System in 3 years and on Octopus Go tariff (also have Tesla EV).
I would be happy to explain how this all works on the phone but too much to type here and I think various comments on here may only have confused rather than clarified.
please PM me if you would like to discuss. (I’m not selling anything btw!)

 

[Mark Purcell]

I have a 3 phase solar inverter and a PW2.

The PW2 charges and discharges over one phase, but it can monitor all three. I have a single phase critical circuits sub-board that it backups during an outage.

When my three phase solar is exporting the powerwall charges at the full rate upto 5 kW, not 1/3 as suggested above. It does this by monitoring all phases and balancing export on phase 2 & 3 by increasing import on phase 1. So the instant net power draw from the grid is zero across all three phases.

During an outage a single phase solar inverter can recharge the battery, a three phase solar inverter cannot recharge the battery during an outage, but that wasn't important to me so I has happy with that limitation.