Heat pumps with extra batteries

maf20

Hi all

Been debating for a few years now on whether or not to get an ASHP. We had octopus out but they couldn’t make it work. I then tried other suppliers/installers, but their quotes stood out as significantly higher than octopus. 

Anyway, I’m still keen to make it work, especially as our gas boiler is approaching 17 years old! 

I see many people say that an ASHP doesn’t save them any money, and in some cases costs them more to run. I understand the environmental issues, but is that the main driver to install one? Fine if it is. 

My very long-winded way to getting to my question is this. We currently have solar and a home battery. Obviously we charge the battery up overnight with a super cheap tariff. We also charge both cars at the same time.

Has anyone ever added more battery storage to make an ASHP cheaper to run? i.e. enough battery storage to pretty much power the ASHP all day

TIA 

Reed_Richards

I have a 12 kW heat pump and my calculated heat loss is 9 kW at -4 C.  Last January I used 594 kWh of electricity between the hours of 07:00 and 24:00.  That's 19.2 kWh per day.  I already have a 6.5 kWh battery so I guess adding another 20 kWh of battery storage would keep me covered a lot of the time.  My existing battery can only discharge at about 2.5 kW so a second battery would also need a higher maximum discharge rate.

When I have a bit of free time I will try to work out how variable my day-to-day usage is.    

matt_drummer

I have done this.

We have 38kWh of usable battery storage and 13.6kWp of solar panels.

Export payments more than cover imports.

I export @ 15p per kWh with Octopus

I am on Octopus Cosy

Batteries need to cover the 16 hours that aren't a the cheapest rate.

My inverter will supply around 3.6kW of power although I would like to add another GivEnergy AC inverter when they make it possible.

I tend to leave the heat pump running as I like the house warm at all times. It also minimise the power drawn by the heat pump, the less the heat pump draws the more there is available to do other things without importing from the grid.

DHW is scheduled for what is normally the sunniest and warmest part of the day.

It works as we don't actually pay anything for electricity on an annual basis.

Heat pump runs at a SCOP of 4.50 or maybe a bit better this coming year. Including charging and discharging losses I pay around 15p per kWh for imports which is the same as what I give up in export payments, so at 15p per kWh my heat and DHW costs 3.33p per kWh

Obviously, we had to buy the stuff to make it possible though!

maf20

matt_drummer said:

I have done this.

We have 38kWh of usable battery storage and 13.6kWp of solar panels.

Export payments more than cover imports.

I export @ 15p per kWh with Octopus

I am on Octopus Cosy

Batteries need to cover the 16 hours that aren't a the cheapest rate.

My inverter will supply around 3.6kW of power although I would like to add another GivEnergy AC inverter when they make it possible.

I tend to leave the heat pump running as I like the house warm at all times. It also minimise the power drawn by the heat pump, the less the heat pump draws the more there is available to do other things without importing from the grid.

DHW is scheduled for what is normally the sunniest and warmest part of the day.

It works as we don't actually pay anything for electricity on an annual basis.

Heat pump runs at a SCOP of 4.50 or maybe a bit better this coming year. Including charging and discharging losses I pay around 15p per kWh for imports which is the same as what I give up in export payments, so at 15p per kWh my heat and DHW costs 3.33p per kWh

Obviously, we had to buy the stuff to make it possible though!

Thanks for laying all this out. Good to read. 

Although we’ve a large(ish) ground floor, our first floor is much smaller, hence not a massive space for more panels. We only have 4k panels on one side. I’m more than happy to shell out for more on the other side of the house, although the yield will obvs be lower. 

Another consideration we’d need is our cars. We have two EVs and charging them at the cosy rate would make a dent. We’re currently on Intellgent Go, so get 6 hours at 7p per kWh, meaning per mile costs on cars are really, really low; and when you factor in exports, cars arguably run ‘free’ over the summer. 

Do Octopus stipulate that you HAVE to be on their Cosy rate? If I was to push the button on all this my plan would be to put on extra panels, extra batteries and basically store enough electric overnight at 7p to then power the house and ASHP all day. 

How does this logic sound to people?

In the summer months (May(ish) to Sept(ish)) we currently have no bills - ie import + battery charging + car charging + gas usage + all standing charges = yield + export (give or take a few quid)

However, the winter months (and the gas usage during) is killing me. We can easily spend up to £300 a month on gas alone in the winter months, sometimes even more. 

maf20

When I have a bit of free time I will try to work out how variable my day-to-day usage is.    

Thanks!  Appreciate it 

matt_drummer

I charge my car elsewhere so Cosy suits me best, but your tariff would work too. I couldn't get on your tariff in any case as I have no charger at home.

You can be on any tariff you want, Cosy is not compulsory (but you do need to have a heat pump to use it, any heat pump, not just from Octopus). You don't even have to use Octopus obviously.

Your gas costs are another matter, what heat pump you need and what modifications are required in your house to make it efficient enough to run predominantly on batteries is a bigger discussion.

Maybe your boiler is inefficient, you like your house hot or the insulation is poor, maybe a combination of all of that.

Clearly, a heat pump at 300% efficiency or better is going to be cheaper to run if you can buy electricty at 7p and store enough of it.

As always, the balance is between the capital investment required and the running costs.

maf20

@matt_drummer agree with pretty much all that. 

Would say heating costs are largely down to the boiler now being a bit old, plus we have a huge kitchen with a lot (and I mean a lot) of glass. Although, I did get thermal blinds fitted on all of the glass in late Feb, so maybe this coming winter we might see more of the benefit of that

we certainly don’t have our house hot, that’s for sure. I have all of the rooms individually controlled with Hive TRVs

cheers for the perspective. Appreciate it 

I guess what I need to do is workout what extra capital I need to layout to get to near bill neutral and then workout the payback in years and go from there 

maf20

@matt_drummer just realised I’ve got that totally wrong. Gas usage isn’t £300 in peak winter months. It’s more like £180 plus SC

Reed_Richards

maf20 said:

When I have a bit of free time I will try to work out how variable my day-to-day usage is.    

Thanks!  Appreciate it   

In January 2024 the most electricity imported by my house (heat pump and everything else) between 07:00 and 24:00 was 28.8 kWh.  My heat pump actually used 43.3 kW in that 24 hour period. The least imported was 8.9 kWh.  There were 13 days when I imported more than 20 kWh.

Currently my night/EV rate is 6.571p per kWh and the day rate is 27.793 p per kWh (numbers before VAT).  So every kWh I could shift from day to night would save me over 21p.  Back to the spreadsheet to calculate how much I could save in a year.  

matt_drummer

I can guess at what you might need and you can adjust what I get wrong.

In the worst months you use 3,000kWh of gas.

Assuming the boiler is 90% efficient then that is 2,700 kWh of heat

Over 30 days that is 90kWh of heat on average.

Assume the worst day is a third higher than the best so 120 kWh of heat needed on the coldest day.

At an efficiency of 300% with the heat pump you would need 40kWh of electricity to run it.

6 hours would come from the cheap rate so you would need 30 Kwh to run the heat pump outside the cheap rate

Plus whatever you use in the house minus any solar.

Assuming 3kWh per day generation and 10kWh consumption then you need 37kWh of battery.

Which is exactly how I came up with my 38kWh of battery storage.

That is the worst day of course which there aren't may of.

But that is how I worked it out, not very precise but it gives a good idea.

As I said, adjust the calculation as you see fit based on your knowledge of your home and lifestyle.

maf20

matt_drummer said:

I can guess at what you might need and you can adjust what I get wrong.

In the worst months you use 3,000kWh of gas.

Assuming the boiler is 90% efficient then that is 2,700 kWh of heat

Over 30 days that is 90kWh of heat on average.

Assume the worst day is a third higher than the best so 120 kWh of heat needed on the coldest day.

At an efficiency of 300% with the heat pump you would need 40kWh of electricity to run it.

6 hours would come from the cheap rate so you would need 30 Kwh to run the heat pump outside the cheap rate

Plus whatever you use in the house minus any solar.

Assuming 3kWh per day generation and 10kWh consumption then you need 37kWh of battery.

Which is exactly how I came up with my 38kWh of battery storage.

That is the worst day of course which there aren't may of.

But that is how I worked it out, not very precise but it gives a good idea.

As I said, adjust the calculation as you see fit based on your knowledge of your home and lifestyle.

Great minds. I’ve just done similar analysis in my bills - although not to the level of your detail! 😂 

The highest day of usage I can see over the past twelve months topped out at 107kWh worth of gas. That was in March, weirdly! 

I get your calcs. I guess what I’d want to do is balance that out over a year, rather than pay the capital that would make me bill neutral all year round (resulting in a likely profit) - i.e. I wouldn’t mind paying £50 bills in January if July was £50 income, etc…

If I could workout the above I’m guessing that would lessen the capital expenditure needed and would result in less battery storage needed, overall - FYI I currently already have one 6kw battery

Really appreciate everyone’s time on this 

EDIT: Just to add/reiterate. Current boiler is approaching 17 years old. So, that’ll need replacing in the not too distant and I’m wondering where/how best to make that investment