New solar install advice for very power hungry household!

Heedtheadvice

You don't actually need (from your perspective ) three phase

Some worst case conditions:

If it is assumed your 16,000kWh is averaged over 180 days that is 88kWh per day. Let's just round that up to 100.

If it all came from a 5 hour overnight grid import that is 20kW import per hour. That equates ( at 4 Amps per kW ) a 100A grid draw.

That is typically just about the max current per phase for a domestic environment. That will be why you have 2 phases supplied and for grid balancing for the DNO. If you have a neighbour on one of those phases, who might have a big draw too or you increased yours, ( maybe heat pump, EV etc) that makes balancing g more difficult or a refusal to allow it.

If you do go for 3 phases, for a reasonable cost that helps the DNO and you would probably be more likely to get a future expansion.

It is not a nicety or theoretical desire to have phase current balance it is a necessity so the idea of your property being 3 phase balanced with batteries for all three is brilliant. I would even suggest, if your finances allow to go for at least 3 phases at x 13 to 16 kWh batteries for the times the sun don't shine!

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/81923336#Comment_81923336

Totally agree. If it wasn't available cheap, I don't think it would be worth it.

My main thought for 3phase, is to allow (or improve the chances) of getting permission for far more PV capacity. Typically, you'd probably get 11.04kW approval, that's what my sister got, without even asking. [Edit - The OP mentions ~15kWp of PV.]

The counter argument of course, is that 2phase may allow more than enough, especially as we have one lucky forum member with ~12kW approval on single phase (not that I'm jealous …. 😉). And the application of batts, especially DC side, allows for far more PV, without the risk of clipping, if well managed.

So may not be needed, v's very cheap on this occassion.

There are other concerns too, such as potentially having less supplier options. My sister's home is also a business, and small holding, so a commercial leccy contract, which is different again.

One for some homework I suspect before leaping in.

Edit - not sure about this, but I think I once heard that a 3phase heatpump is more efficient than a single phase, but that could be wrong. Again a thought depending on future changes at the property.

Screwdriva

I believe the home appliances would need to be changed (or kept one on each phase) and the consumer unit would also need replacing. It's not a straightforward switch.

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/81923635#Comment_81923635

Yep, in the case of my sister, the vast majority of circuits are single phase, spreading load across the 3phases. Only the circuits (so far) for EV charger and HP are 3phase. Oops … and the PV 8kW inverter with 10.3kWp.

Throwback to HP's, AI says this (I asked using a negative (are 3 phase heat pumps less efficient) as I get the impression that AI likes to agree with you):

No, 3-phase heat pumps are generally more efficient, not less, compared to single-phase units

. They offer superior energy efficiency, especially for larger residential or commercial applications, due to better power distribution and reduced energy loss.

QrizB

Typically, you'd probably get 11.04kW approval, that's what my sister got, without even asking.

3.68kW per phase is "notification only" under G98, AIUI. No need to ask.

I once heard that a 3phase heatpump is more efficient than a single phase

3-phase AC motors are slightly more efficient than single-phase, and heat pumps are powered by electric motors.

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/81923726#Comment_81923726

Always cover your back (in case of edge cases), hence why I said 'typically'.

Whilst 3.68kW per phase (1, 2 or 3phase) is typically only 'notify after', there could be local DNO constraints on 1 or more of the phases, so a G99 application may be required, and of course the DNO may come back with less than 3.68kW on one or more of the phases.

When Sis confirmed the deal to have 3phase installed, the DNO also notified her that she could have 11.04kW total / 3.68kW max per phase of export without needing further prior approval.

If the installer knows of local restrictions then they should ask first. The question of course being, if you don't know, should you just do it, rather than give the DNO a chance to say no (beforehand).

Also, and I don't know if this has gone completely, but many years ago (possibly 10(ish)), the DNO's did try to get a rule (probably not the right term, but hopefully explains) passed, that if an installer had already installed PV within your local postcode, then they needed to ask the DNO before installing another 3.68kW array.

The problem with that, of course, is what stops another installer, who doesn't know better, just doing it. So should local install registeries be required and checked beforehand?

It can get complicated fast.