Calculating how many kWh/day electricity a heat pump will consume (worst case)

ed110220

Hi all, apologies for asking this here as well as in the Green and Ethical section but it's buried away in a thread mainly about something else.

I'm trying to work out what battery capacity I'll need to run an ASHP on off peak electricity on a cold winter day. At the moment I have gas central heating. I have a 15 kWh battery which I charge up on Octopus Go at 8.55p/kWh overnight so we hardly import any peak rate electricity. I'm thinking of switching to ASHP and adding more battery storage to run it.

I've looked through our daily gas consumption for the past two winters and the highest was 50 kWh. With a COP of 3 that would be equivalent to 16.7 kWh of electricity (I'll ignore the fact that the boiler has a COP of less than 1 to be conservative). Does that sound right or are my assumptions off?

QrizB

I think you're about right.

My highest daily gas consumption last winter was almost 100kWh ...

ed110220

QrizB said:

I think you're about right.

My highest daily gas consumption last winter was almost 100kWh ...

Thanks! Doesn't have to be exact. I was thinking of getting an 8 kW hybrid inverter which would allow up to 40 kWh of charging in a 5h overnight window, so up to 25 kWh for the ASHP and 15 kWh for everything else. I think another 16 kWh Seplos battery would be enough but an 8 kW inverter would allow me to add more if necessary.

Netexporter

I've just done my first winter with a 5kW heat pump. Highest daily consumption was about 30kWh, when temperature was sub-zero and the dunkelflaute meany there was virtually no solar input.

I initially bought a 15kWh Fogstar battery but then they had a Black Friday 20%-off deal, which I couldn't resist, so now I have 30kWh. I will be including islanding in my installation, because I live in a remote area with frequent power cuts.30kWh will cover me for a 24 hour outage, with heating from the ASHP. If the notification indicates it would be longer, then I'd fire up the woodburner, and I'd be OK for a week, even without solar input.

I agree with QrizB that 15kWh would probably be enough if you have a reliable supply, except in the few very coldest weeks.

ed110220

Netexporter said:

I've just done my first winter with a 5kW heat pump. Highest daily consumption was about 30kWh, when temperature was sub-zero and the dunkelflaute meany there was virtually no solar input.

I initially bought a 15kWh Fogstar battery but then they had a Black Friday 20%-off deal, which I couldn't resist, so now I have 30kWh. I will be including islanding in my installation, because I live in a remote area with frequent power cuts.30kWh will cover me for a 24 hour outage, with heating from the ASHP. If the notification indicates it would be longer, then I'd fire up the woodburner, and I'd be OK for a week, even without solar input.

I agree with QrizB that 15kWh would probably be enough if you have a reliable supply, except in the few very coldest weeks.

Thanks, I guess like the inverter limit I have at the moment it probably won't add up to much if the very occasional cold day forces me to import some peak rate electricity. Mainly I'm looking at inverter size I'll need because I can always add another battery but don't really want to have to upgrade my inverter a second time. Power cuts aren't really a thing here because it's a suburban location with underground supply (no risk of tree branches being blown onto overhead supply).

Netexporter

I'm hoping to get DNO permission for an 8kW inverter. That will fully charge the batteries withing a 4 hour cheap window and will be more than enough to run the heat pump and at least one "big ticket" item, like cooking, washing machine, dishwasher simultaneously.

ed110220

Netexporter said:

I'm hoping to get DNO permission for an 8kW inverter. That will fully charge the batteries withing a 4 hour cheap window and will be more than enough to run the heat pump and at least one "big ticket" item, like cooking, washing machine, dishwasher simultaneously.

I think this is the way to go if you can get batteries cheaply enough eg Fogstar/Seplos.

matelodave

On a really cold day our leccy consumption gets up to 55-60 kWh in 24 hours (thats all in lighting, heating TV and possibly washing etc) It peaked at 78kWh on 29th December when the whole family descended , but on average its around 40kWh

We've got a 15year old Daikin heatpump

Reed_Richards

I think you would be crazy to buy batteries big enough to cover your worst case.  The worst case is a very rare event and, because of the way heat pumps work, the worst case is liable to be a lot worse than all but a tiny minority of other cases.  

And a flaw in your reasoning is assuming a COP of 3.  Whilst your heat pump should be able to achieve an SCOP of 3 or more, this is a figure averaged over the entire year.  On the coldest days of the year it may well not be able to achieve a COP as high as 3.

I've had a heat pump since late 2020.  After 4 years, it had used an average of 15.3 kWh per day for heating and hot water (my heat pump has a dedicated electricity meter).  The worst consumption I have ever caught it doing was 65.84 kW from 23:30 on 12/12/2022 to 22:30 the following day.  It's set back at 22:30 so the result for the full 24 hours should be the same. 

I reckon that the battery size that you need should be calculated on the basis of your annual usage, not the worst case.  If I took my average figure of 15.3 kW hours and doubled it to say 30 kWh there would be very few days throughout the year when that wasn't sufficient to cover my heat pump usage, 10 to 20 days I estimate.  Do you really want a battery that you will only use for 10 to 20 days each year? 

My recommendation is that you take your annual gas usage and work out from that what your daily average is.  Divide that by 3 (for SCOP of 3) then double that number and see what battery size that gives you.  And even then you have probably made an overestimate because if you have some hours of cheap electricity with which to charge your battery you'll want to be running your heat pump as much as possible during those hours, heating your hot water at the very least.     

Alnat1

I'd go with "try before you buy" and see how you go for a few months with with the battery you already have. If it doesn't work out you can add more storage fairly quickly but it's a chunk of money to fork out if you find you don't need it very often.

ed110220

Alnat1 said:

I'd go with "try before you buy" and see how you go for a few months with with the battery you already have. If it doesn't work out you can add more storage fairly quickly but it's a chunk of money to fork out if you find you don't need it very often.

I didn't word the question very well. I'm really looking at this from the point of view of what inverter to get rather than the battery because I can always add another battery but don't want to find out down the road my inverter is a limitation again. Basically what inverter would be sufficient to charge as much battery as I might need in reasonable worst case... Which seems to be 8 kW if leaving margin for higher consumption.