Decarbonising in 2022

chrisgeary

Well-known Member
Just about to move into a new house which we intend to stay in for at least 10 years. It's a 60s cavity wall (uninsulated) detached house in Worcestershire with double glazing and good loft insulation and 0.6 acres. It has an old 90s boiler and undersized cylinder so that is up for renewal anyway. I'm thinking it's the ideal time to think about moving away from gas. The oven and hob will also be fully electric.

My car is a BEV and I'll be charging it at home (ideally from solar).

For heating, I'm considering ASHP or A2A splits. For HW, Mixergy just using electric (overnight and/or solar) or ASHP if I go that way. I know for ASHP there is a grant but I can't help feeling that A2A splits will be better value and cheaper to run especially now RHI is no longer a thing.

My intention would be to use Octopus Go and to shift as much load into the cheap rate as possible and to that end, I'm also thinking about buying enough battery storage to run the house's entire daily load. Without the car or electric CH/HW, that is currently about 6800 kWh/pa. With the car that adds about 1700 kWh and a rough estimate based on the EPC for the house is another 26000 kWh on top of that. Given a COP of, say 3, that's 8800 kWh for CH/HW, plus 1700 for the car plus the other normal house loads of 6800 for a total of 17300 kWh/pa or approx 47 kWh per day. Take off the 4 hours cheap rate period for CH/HW and the car and I think a battery of 39 kWh will hold the entire load. 41 kWh and inverters looks to be around £13,300. Payback time looks to be 4 years which is fine.

I'd probably also install Solar into the mix but at the moment I have no idea how much energy that would bring in, payback time or how to size the batteries including solar, so the above I think is worst case.

I'd appreciate any thoughts, anything I've missed etc.
 

neilball

Well-known Member
For hot water, take a look at Sunamp as this would work well with Octopus Go and/or battery storage.

You issue with battery storage is finding an inverter that can charge/discharge at a high enough rate to allow you to charge fully during the 4 hour cheap rate from Octopus Go. With 40kWh of usable storage then you’d need a 10kW capable inverter, and these are neither cheap nor necessarily very efficient at lower rates.

Otherwise you should look at wall insulation and air tightness to minimise your winter heat losses and therefore minimise your heat out requirements too.
 

chrisgeary

Well-known Member
Thanks! I did look at Sunamp but I see a good deal of mixed reviews about how it works in practice.

It may be that 40kWh is way too much, that even in winter solar will provide some help but I figured I should take a worst case scenario of mid winter cloud day etc. For summer and perhaps the majority of the year, maybe the heating system isn't pulling 3kw constantly and maybe a smaller battery will do. Also, the house has three-phase available so I guess I could split the load across multiple smaller inverters?
 

Chester

Well-known Member
We’re with you on a similar journey Chris. After the Powerwall has been installed (hopefully coming very soon), we’re looking into going for a Mixergy tank for similar reasons, solar diversion and topping up off peak.

It will be great to follow you on your journey and see the decisions you make and the reality after a year or so. There will be another annual report from me in June.

Cheers.
 

chrisgeary

Well-known Member
@Chester What made you choose a Powerwall versus other options?
 

Chester

Well-known Member
I didn’t originally. I preferred a solution that reduced the box count and provided DC to DC charging which is more efficient or has less loss. But we also have a BEV so an AC coupled system is beneficial to prevent battery drain when charging the EV. It also helps prioritise where we want SPV power as I believe most hybrid inverters prefer to charge their batteries first. Finally the clincher was the Tesla Gateway which is really useful as I work from home. In the event of a power cut (infrequent but does happen), I’ve effectively got a UPS for the whole house. The Givenergy solution only has a single circuit and limited coverage which doesn’t help the freezer! I wish a I had it a couple of months back as planned. The whole area was out for about 3 hours.
 

chrisgeary

Well-known Member
Yes, I'd want DC-DC charging and backup as well (I also WFH). I'll give the Powerwall more attention.

Insulation is definitely top of the list though. The double glazing is old and ideally I'll upgrade that to triple. I've discounted CWI as being too risky so its between EWI and IWI. All the rooms are going to be redecorated and some walls changed plus IWI I could potentially do myself. I see lots of plusses to IWI including costs, DIYability, not having to scrap all that anaglypta, ability to do it room by room. I guess after all that is done, I need to get a new EPC to see how the house performs?
 

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