Leave solar batteries out of the system until we have the house up and running?

[saveasteading]

We are fitting  solar and intended to include a battery system sized to suit estimated use.

But our electrician/plumber suggests we leave the batteries until the house is up and running, then size accordingly.

 

I was surprised, as it will take a year until we have reliable figures, and that's a year of not getting the benefit.

Plus we will not be getting in the habit of optimising.

 

The vat reclaim entered my reasoning until I twigged that it is zero anyway.

 

Is the electrician wary of estimating wrongly? Or wise?

Or perhaps undersizing or oversizing is more than a cash issue.

 

He also agreed we should get a 3 phase heat pump, which the solar people were resisting. 

On the principle that as we have 3 phase we should use it.

 

Thoughts please,

[JohnMo]

13 minutes ago, saveasteading said:

agreed we should get a 3 phase heat pump

If your putting a battery to cover your whole winter loads, it going to be huge and not cheap. I would size it now for an average winter day. But your tariff makes a big difference to the size, Cosy gives you 3 charge periods, a typical electric car tariff only one, so you would need a much bigger battery.

 

Just choose a battery that can be expanded, add more later if you want as slave units.

[SimonD]

It's funny you should raise this difference between heat pump installer and electrician. The electrician I use as part of all my heat pump installations called me a while ago pretty p***ed off. He'd completed a new house wiring installation plus solar & batteries on 3 phase. The heat pump people came along and just installed a single phase heat pump cascade, causing a bit of a headache. At least yours are talking about it. What is the size of your heat pump? And I'm guessing it's been confirmed the model is available on 3 phase? Probably a silly question.

 

I've had a similar experience with solar design for my place where the designer didn't seem to understand modular and phased approach. It's a bit infuriating. With the right system design there should be no problem scaling up if you need to, just as @JohnMo suggests. 

Edited 22 hours ago by SimonD

[Russdl]

The battery would be VAT free during the build and maybe VAT free after. They became VAT free (outside of a new build) a couple of years ago and for sure that gift will be removed at some stage in the future.
 

Definitely get the battery when you’re building, they change the game entirely compared to just solar PV.
 

It’s costing us naff all to run this house (and hybrid car) the battery and off peak tariff is a large part of what makes that possible. 

[torre]

I recently changed my view on this - I used to argue you might as well wait, because the cost to buy the battery was coming down fast enough to balance out the usage savings.

 

Right now though the balance has tipped with battery prices look more uncertain and export tariffs making it more cost effective to use your own stored energy.

 

An ancillary benefit of installing now is that it should improve your as built SAP rating by a couple of points.

[saveasteading]

Though we selected them separately, it turns out the solar  heat pump people and M &E know each other, so there is trust.

 

It was my suggestion to use 3 phase, simply based on the principle that big machines use 3 phase, and kndustry uses 3 phase, and that we have it available.

Sparky has the same instinct. 

 

The heat pump will be big,  around 14 or 15 output.

 

It seems that most domestic heat pump installers don't know about 3 phase. They sell what they know. We've instructed them to try harder on this, but need whatever proof/ backup there might be... or the reverse logic if that is the case.

 

Battery logic.... noted, thanks.

In summary so far.

Get it but don't oversize.

 

[SteamyTea]

A MWh of electrify will cost about £250.

Do some simple calculations and see where it all stands.

 

[SimonD]

10 minutes ago, saveasteading said:

The heat pump will be big,  around 14 or 15 output.

Plenty of those around on 3 phase. In fact some of the larger units are only available in 3 phase (e.g Panasonic M series over 12kW), but it all depends on the system design and whether single big unit or cascade is the better way to go. You need a good designer to look at it and justify the decision.

[saveasteading]

7 hours ago, SimonD said:

whether single big unit or cascade is the better way to go. 

Does cascade mean 2 smaller units.? That was the original plan.

 

Presumably a single at 15 suits 3 phase, whereas a couple of 8s are more likely single phase.

If our well respected local installer hasn't ever used 3 phase, and is tied to a supplier other than the names you suggest, then the expertise may not be forthcoming.

[SimonD]

6 hours ago, saveasteading said:

Does cascade mean 2 smaller units.? That was the original plan.

 

Presumably a single at 15 suits 3 phase, whereas a couple of 8s are more likely single phase.

If our well respected local installer hasn't ever used 3 phase, and is tied to a supplier other than the names you suggest, then the expertise may not be forthcoming.

 

Yes, a cascade is 2 or more units connected together. An advantage of a cascade in a large domestic situation is modulation as a very large single unit may not be able to modulate down low enough and therefore cause short cycling during mean temperatures. But this is entirely decided through design of the system and what your typical outdoor temperature is compared to the coldest design temperature. There are obviously other considerations to cascades v single units, but this is a big one. Some manufacturers like Nibe design their cascade systems so you can add new units onto to older ones etc. over time, or even run ASHP with GSHP and pool heating. There is more complexity in the commissioning and control of a cascade.

3 phase shouldn't really be an issue for the heat pump installer. The 3 phase units I've seen are very easy to wire up and it's really up to the electrician (I get my electrician to do all the mains connections because he's got the necessary crimping tools for the terminal connectors and testing tools etc.) including then balancing the phases.

[Russdl]

1 hour ago, SteamyTea said:

 

A MWh of electrify will cost about £250.

Do some simple calculations and see where it all stands.

 

A MWh cost me £52 and I sell it for £120. 
 

The simple calculations get muddied by the various tariffs/self generation etc. 

 

This house really has cost us nothing in energy in the 4.5 years we’ve been in. 

Edited 11 hours ago by Russdl

[SteamyTea]

1 minute ago, Russdl said:

The simple calculations get muddied by the various tariffs/self generation etc. 

Yes, and this is the major problem.

Most of 'the savings' are really just playing about with retail tariffs.

I can reliably store many kWhs per day in my, nearly 40 year old storage heaters. They have, so far, proved to be 100% reliable.

[Russdl]

10 minutes ago, SteamyTea said:

Yes, and this is the major problem.

Most of 'the savings' are really just playing about with retail tariffs.

Most people shop around for most things, it would be a bit odd to not shop around for the best energy tariffs just because you don’t agree with how they are set. 
 

12 minutes ago, SteamyTea said:

I can reliably store many kWhs per day in my, nearly 40 year old storage heaters. They have, so far, proved to be 100% reliable.

Laudable but you can’t cook on a storage heater, or shower, or drive to the shops etc and you probably don’t need that storage facility for half of the year. Batteries are useful all year round. 

[SteamyTea]

27 minutes ago, Russdl said:

Laudable but you can’t cook on a storage heater, or shower

They tend to be the high power, but low energy usage parts of the daily mix.

Well they are if you do it right.

[saveasteading]

 

56 minutes ago, SimonD said:

advantage of a cascade in a large domestic situation is modulation 

So when demand is low, one of the units can turn off and the other work efficiently?

[SimonD]

1 hour ago, saveasteading said:

 

So when demand is low, one of the units can turn off and the other work efficiently?

 

yes and with some controls they're intelligent enough to balance load between 2 units to keep them both in more efficient operating conditions, or separate dhw from ch. 

[Dillsue]

4 hours ago, saveasteading said:

Does casdade mean 2 smaller units.? Thag was the original plan.

 

Presumably a single at 15 suits 3 phase, whereas a couple of 8s are more likely single phase.

If our well respected local installer hasn't ever used 3 phase, and is tied to a supplier other than the names you suggest, then the expertise may not be forthcoming.

If you go for 2 single phase HPs they can likely be wired on separate phases so you'll use 2/3 of your 3 phase supply;) Your spark will sort that out and hopefully balance the third phase with other house loads, as best he can.

 

On the sizing of your heat pump, if its sized for -2 as ours is, it will rarely be running in those conditions. This winter we've had -2 for 1 night, zero for maybe another 4-6 nights and the rest above zero. With a single big unit, especially if it's oversized, you'll likely hit its modulation limit quite often and see short cycling so youll be paying higher bills for the life of the HP. I'd seriously think about getting an independant heat loss calc done from the likes of Heatgeek just to sense check the size of HP needed.

[JohnMo]

9 hours ago, saveasteading said:

heat pump will be big,  around 14 or 15 output.

It does sound a huge heat pump based on a new build even to min building regs.

[saveasteading]

1 hour ago, JohnMo said:

does sound a huge heat pump based on a new build even to min building regs.

It's a double household and very big. 

Just based on kW/m2 that others have published, it feels right. But the teams are checking it out now.