£16K (or even £25K!) would go a fair way towards employing more solar panels and battery capacity if you have the space and stay off-grid - just a thought! Regards, Toodles.

our aim is to be near Passivhaus standards without the certification.

Passivhaus standards are 15kWh/sq m/year of heating demand.

Let's assume that you get to say 20kWh/sq m/year so +33% worse than Passivhaus standards, then for every 100 sqm of floor area that is just 2000 kWh of heat needed per year.

My house needs about 13,000 kWh per year for heating and hot water and would be fine with a 7kW heat pump. The heat loss (at 24C temperature difference i.e. -3C outside 21C inside) is about 6.4kW.

So, either your new build is well over 650 sq m, for which a 7KW heat pump would be about right, or your heat pump requirement is massively over-specified. You need a  detailed heat loss calculation to determine heat pump sizing.

Please could you give us a bit more information e.g. total floor area and the region of the country that you are in. (Scotland for example has colder winters and hence a lager heating requirement due to a lower minimum outdoor temperature).

On the solar, 16A per phase is the G98 limit i.e. 3.68kW export on single phase, and about 11kW on three phase. More than that requires more paperwork (G99) and approval from the DNO before starting work. G98 allows the work to be done and then the DNO notified. With G99 provided that the grid can handle it in the local area (the DNO checks this), you can theoretically have up to 17kW on a 100A supply on single phase. It is unusual to be limited to just 16A. I am on a 1970s estate over 400m from the nearest substation with 50+ yar old wiring in between and our single phase limit is 5.7kW, which is really quite low.

Who told you that you can only have 16A export on single phase? It may be that your builder is trying to do the minimum possible and notify afterward (G98). However, that only works for three phase for your system size, potentially incurring massive extra costs for you. In your situation, I would recommend applying to the DNO for 11kW on single phase and see what they say about that. Worse case, they will tell you what you can actually have, which is likely to be a lot more than 3.68kW (16A) and may be enough, saving you £9k. 

Note you need a hybrid inverter or some extra equipment so that the total export from Solar PV and the batteries is limited to say 11kW, not the sum of what both are capable of (e.g. with separate inverters).

Caveat what I have said, there may be some subtle differences for new builds that I am unaware of. The above is however the basic set of rules.

By EV fast charger, do you mean 7.4kW, in which case single phase is fine or 22kW in which case you need three phase.

@toodles Thanks Toodles, definitely something to consider.

@rob-of-york Thanks for your detailed reply.

Our build will be 144sq mtrs. The calcs were done at a very early stage, the company giving this info did not have the complete picture at the time. We were still deciding of a method of build/insulation etc.

The build will be in East Anglia, so nothing too extreme there.

The 16amp per phase was the information given to me on site by the DNO doing the site visit.

I was looking at 3 phase to exploit the max export back to the grid and the potential to upgrade to a fast charger in the future. The extra cost involved, however, has forced a rethink. I am not technical in this respect and don't know which way to go with it. I'm concerned that by choosing the wrong route may cost far more in the long run should the demands of electric vehicles etc be needed.

Am I right in thinking that there may be higher costs involved by going with 3 phase regarding the equipment needed, also are all electricians familiar with 3 phase?

I've been told by 2 electricians, 3 phase is the way to go at this early stage and 1 has said it's overkill and unnecessary. 

I'm at a loss as to know what route to take, is it a case that there is no right answer?

@e-a-west I think it unlikely that your solar energy export would recoup the additional costs incurred to have 3p wiring. Export rates are dropping at the moment and (without a crystal ball!) I think it unlikely that they will increase dramatically and unlikely to even match the ‘buy-in’ price. We are on Cosy and the export rate is lower than the cheapest import rate currently.

We have 8.1 kWp. of solar panels, non-ideally orientated and subject to some shading at times. Microinverters on each panel and we produce ~35 - 40 kWh on the average summer day. We are able to self power from late spring to mid-autumn and were it not for the standing charge, would have nothing to pay during this period. Overall, we pay OE ~£450 per annum now.

We have an EPC rated as C but have made improvements since carried out and calculate that notionally, we are a ‘B’ now. 97 sq. m. semi detached 4 bed house with an 8 kW Daikin heat pump and we are all electric. No EV as unable to drive but have induction hob, air fryer, 2 x microwave combination cookers, slow cooker etc. All lighting is LED and we enjoy a 22.5 deg. C home 24/7. Regards, Toodles.

We are on three phase power because it's an enormous listed G2 solid stone pile and we really can pull about 80A plus any vehicle charging.

The kit costs a bit more but more of a problem for many I imagine would be that it's generally larger and you have more of things. So the changeover switches are huge, there are three distribution boards, three consumer units, three fuses etc and it all has to be planned and balanced. With a giant victorian basement it all just looks cool if a little Franekstein but I'd hate to be squashing it into a modern house.

Not had any trouble with electricians. Most of them do 3 phase and for the little jobs it's working on one phase only anyway.

Unless you are going to be drawing over 80A I wouldn't bother. Export payments are on the way down and whilst I am sure there is money to be made I can't see it paying off unless the connection costs are similar even with things like AXLE.

Given it's 4 grand for 64kWh of battery at this point I'd be adding an awful lot of solar and going off grid I think if someone quoted me 25 grand for a connection to something near passivehaus grade. Even if I need a generator for a few days a year it'll probably only scrape the standing charge cost each year let alone the 25 grand plus.

To me everything looks out of kilter. 12kWh of battery is stupidly small (it's 400 quid to go from 10kWh to 16kWh using standard 48v batteries so why go small), and the heatpump seems ludicrously large for the house and certainly the battery is inadequate for the heatpump in winter if its sized right (summer when cooling if air/air or underfloor you run off the solar so it's no big deal).

Main issue with 3 phase at low power (16A per phase) is that... you get 16A per phase.

So if you plug two 10A single phase appliances on the same phase (for example, a toaster and a microwave oven) then boom it will trip.

It is a major headache. 16A per phase is okay if the only stuff you got are balanced three phase appliances and lights. But for a home, nope. You will end up with post-its like "do not use washer and oven at the same time". Then subsequently strangled by wife. LOL. And then there's the EV charger. 

If you go with three phase I strongly recommend 32A per phase. It costs more, obviously, but it is usable. Or just go with single phase.

Thanks to modern inverter tech, heat pumps work the same on single vs three phase. Only differences are AC to DC conversion circuit and motor drive voltage. But in the end it's a three phase motor driven by a DC powered three phase inverter.

A nice perk with inverter heat pumps is they do not have the enormous current inrush when the motor starts, so it will most likely be possible to run one on a PV inverter in backup mode. Which is generally useless, since blackouts don't occur on sunny winter days, they occur either on terrible weather days in winter when PV is useless anyway or extra sunny days in summer when the heat pump is useless anyway.

The only justification for three phase is to charge a car at more than 7kW.

The issue with going offgrid is the winter week of terrible weather where 12kWp PV produces like 2 kWh/day. In that case forget modern comforts and you better have a heater that burns fuel not electrons. Same for hot water, and the sun chooses when you do the laundry. Offgrid requires massive oversized PV array and batteries just for that week twice a year, which means the investment is not amortized and return on investment is poor since most of the time the vast majority of the potential production is not used.

IMO grid export is a losing bet. Since everyone wants to do it at the same time there is overproduction, then either kWh spot price will be negative or they will curtail your production. Do not count on theft from the taxpayer grid export to finance your system. I know a few people who were counting on theft from the taxpayer grid export to finance their systems and when the kWh price dropped to "middle finger" level then bitched and moaned about no longer being able to steal from the taxpayer. Well, Karma. Counting on it is trusting the government to honor their promises, and honestly who in their right mind would do that? It has to finance itself on its own, which is not difficult to achieve considering how cheap PV has become. Then you get to enjoy driving for free, air conditioning for free, etc. I love it!

As the other said your battery is ridiculously small. The PV system is supposed to make your life easier and work for you smoothly in the background, not force you to consider charge levels all the time to launch your laundry.

@e-a-west Given that your new build is 144 sqm and that you might end up at about +33% worse than Passivhaus standards for heat loss, then that is just 3000 kWh of heating requirement per year or put another way, a maximum heating demand of about 2kW continuous in worst-case conditions (Passivhaus mandates 10W per sqm, and so I've used 13.3W there). This means you will only need a very small heat pump for space heating, perhaps a Vaillant Aerotherm plus 3.5kW; some extra is needed for your hot water heating, but not much. It's really important not to grossly oversize as heat pumps are not good at modulating down to very low outputs and instead short cycle, switching on and off, which is inefficient and causes premature wear. Also over-sizing costs far more, both for the heat pump itself and everything else sized around a gross heat loss overestimate. With a new build, a detailed heat loss calculation will be done once the design is finished and this will inform your choice of heat pump. Avoid committing to any particular heat pump until you see those figures.

I would be very cautious about going off-grid, it is liable to impact both the resale value of your house and how easy it would be to sell.

As others have said, the only viable reason to have 3 phase is if a 22kW EV charger was a necessity. The usual domestic "fast" chargers are 7.4kW. They are still capable of putting 56kWh into an EV in an 8 hour overnight charge, which is more than the size of most small EV batteries anyway. The compromise would be if you have multiple EVs that are all doing quite high daily mileage. You might end up doing some charging on full price electricity, however, you would have to do a massive amount of that to get anywhere near the £9k+ extra for 3 phase.

Regarding the solar, I am somewhat skeptical about what the DNO has said on the site visit. Without doing the detailed grid calculations, all they would be able to say is max 16A per phase, which is what they have said.

If I was in your position, then I would get a single phase grid connection, make sure the heat pump was correctly sized. And here's the contentious bit, I would then wait before doing anything else. I would enjoy the beautiful super-insulated new build house for a year and see exactly what my electric consumption was. During that time, I'd get an EV and a 7.4kW charge point. Depending on mileage, I'd either go for an EV tariff or a heat pump one. Once I had some real data on electricity consumption, then I would consider a home battery and solar PV. I'd look to size the home battery to cover the daily electricity consumption (not including EV charging) for 90% of the days of the year. With the battery, I would charge it overnight on cheap electricity and use it to power the house the rest of the time. The remaining 10% of days that have higher electricity consumption, I would simply accept the possibility of using some extra full price electricity from the grid - no point spending thousands to save pennies. Regarding the solar PV, given a battery and an inverter, the cost of adding solar panels is low. I would put in a fast-track G99 application for 7.2kW and see what the DNO will allow on single phase. Assuming that 7.2kW is permitted, then 16x 450W panels would do the job.

Regarding the economics and environmental arguments, with a Passivhaus-lite and a heat pump, you have already de-carbonised your heating and hot water. Using Octopus tariffs, you would be zero CO2 emissions on those things. No actual need for solar or a battery to get to that, hence you should really only consider those in terms of the economic arguments for them. It is likely that the price paid for electricity export will continue to fall, while the price for off-peak / overnight electricity remains low. Right now, with an appropriate sized battery, it is actually better to never charge your batteries off solar, but rather only use solar for either direct consumption or export.

One issue may be the changes to building regulations coming in 24th March 2027. These mandate that new builds have solar panels covering 40% of the ground floor area of the property. Assuming that your new build has two floors that are the same size, then the ground floor area will be 72 sqm and the solar panels would need to cover 28.8 sqm. That will equate to roughly 7.2kW of panels. It would be a crazy situation where you are mandated to install at least 7.2kW of solar PV, but the grid will only let you export 3.6kW. Not all Govt regulations involve joined up thinking...

@toodles Again, thank you for your advice/experiences.

Given the build costs that are slowly filtering in and the general advice from the forum, it seems single phase is the way to go. 3 phase is too much of a luxury.

@etchedpixels To be fair to the company who had originally given these figures, we were at a very early stage of design. I am certainly learning that the information they have  given us was pretty lax, they didn't ask any searching questions.

@e-a-west Presumably you won’t be facing a bill for a gas supply installation?! 😉 Regards, Toodles.