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

ed110220

Reed_Richards said:

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.     

I didn't word this question very well, what I'm really trying to work out is the inverter power I'll need to charge the amount of battery I might reasonably expect to get. Because I can always assemble another battery kit if I need more storage, but don't want to be in the position again where my inverter is insufficient to charge them in the off peak period. At the moment my inverter has a 3 kW limit, so only enough to get 15 kWh into the battery in a 5 hour window. Which is sufficient at the moment but won't be for ASHP.

You're right about not needing the absolute worst case but was looking at this for simplicity and to be conservative to account for consumption being higher than I expected (over optimistic COP etc) or increasing in the future. I'd estimate that the optimum is probably a cold but not exceptionally cold winter day ie colder than most winter days but not something that happens only a few times a year or more infrequently. The principle seems similar to my current set up where our peak load is above the 3 kW discharge limit of the inverter, but the amount it adds to our average import cost is small (off peak for us is 8.55p and our average import price is < 10p so our peak usage is not non existent, but not worth worrying about).

Heating the hot water off peak is a good point I hadn't thought of, I already shift larger loads like the dishwasher off peak where possible even with the battery.

NedS

ed110220 said:

Heating the hot water off peak is a good point I hadn't thought of, I already shift larger loads like the dishwasher off peak where possible even with the battery.

Although you can shift reheating DHW to off peak, it will be most efficient when the outside temperature is warmest so all else being equal reheating the water in the afternoon is most efficient (uses least power and therefore costs least assuming a constant unit price). If there is sufficient battery capacity to heat in the afternoon, this is when I would do so, otherwise reheat during the off peak time when you can pull from the grid rather than battery albeit at slightly lower efficiency.

Actually, it's not a straight comparison as you must also consider the ~10-15% battery and inverter losses heating from battery in the afternoon vs direct from grid overnight, which may offset the loss in heat pump efficiency due to the colder outside temps.

Martyn1981

ed110220 said:

Heating the hot water off peak is a good point I hadn't thought of, I already shift larger loads like the dishwasher off peak where possible even with the battery.

Hi Ed, sorry for such late comment, but this can also apply to your heating demand too.

I totally agree with you thinking about the inverter size, rather than battery size for now, but you may be able to shift some more of your heating to the cheap rate period too, which may impact inverter and battery size considerations too.

We have a 6hr cheap rate window, and I actually dial up the temps on the 2 A2A units (that heat our house) when I go to bed. I also turn up the speed on a small fan that draws heat from one room, into the kitchen via a small interior window. The fan is noisy at that level, so dialled down during the day, and by removing air, also increases the workload on that A2A unit.

In the morning, the house is the warmest it will be for the day, and I turn the A2A temps down about two degrees. And by the evening the house will be starting to cool down even more, but that's normal (I think) compared to GCH in the later few hours before you go to bed(?)

Then the cheap rate powered A2A units warm things back up.

No idea how to fairly evaluate that, but I think the 6hr cheap rate of 1/4 of the day's heating, is probably more fairly expressed as 1/3 (8-10hrs), so a larger share of the day's total heating demand, than it may at first appear. And not impacting on battery capacity, nor power.

Big caveat though, is that I'm playing/experimenting with two A2A units, that heat downstairs, with heat rising to the upstairs. From all the advice I've seen the good people on here posting about full ASHP wet systems, you may not want to vary the temps much during the day (I'm really not sure on best practice), and you'd need to dial down the rads in the bedrooms, so you don't get too warm.


NedS raises an important point that I pondered over most of last winter - the A2A units will have a lower COP during the night than during the day ..... but in cold temps my battery round losses are probably approaching 20%, so maybe a draw?

silverwhistle

Martyn1981 said:

Big caveat though, is that I'm playing/experimenting with two A2A units, that heat downstairs, with heat rising to the upstairs.

I'm happy enough with the idea of going without upstairs heating, beyond an electric towel rail, as I currently tend to benefit from the rising heat of a woodburner and have no need for a heated bedroom. I'm thinking of a similar approach, with PV but without a battery, at least to begin with. But are your two units separate or a split?

Also, is your kitchen also a diner, that needs the assistance of that internal fan?  In my case I foresee no need for an additional heat source in a small galley kitchen, where I'm active and with various heat sources.  

Martyn1981

silverwhistle said:

Martyn1981 said:

Big caveat though, is that I'm playing/experimenting with two A2A units, that heat downstairs, with heat rising to the upstairs.

I'm happy enough with the idea of going without upstairs heating, beyond an electric towel rail, as I currently tend to benefit from the rising heat of a woodburner and have no need for a heated bedroom. I'm thinking of a similar approach, with PV but without a battery, at least to begin with. But are your two units separate or a split?

Also, is your kitchen also a diner, that needs the assistance of that internal fan?  In my case I foresee no need for an additional heat source in a small galley kitchen, where I'm active and with various heat sources.  

Hi. They are both single units. TBH, planning permission aside, I think that works better as 2 single units are probably cheaper. Also they can plug into a socket, whereas a single larger unit may need to be hardwired. Teeny caveat, I was told that you should really only have one per ring main, but that worked for us, as one is on the kitchen ring, and one on the downstairs ring.

Sorry for any confusion about the kitchen 'fan', it's simply a desktop fan, not heated. As there was no direct airflow of heat from the rooms with A2A units, I assumed that additional heat would be needed in the kitchen. My plan was to add an immersion into the existing large double panel radiator. But step one was to see how hard I needed to run a small oil rad, to get an idea on the power needed.

But it was suggested to me by a forum friend to see if a fan would move enough extra heat through the existing small window between the kitchen and the ickle conservatory where one of the A2A units lives. [That unit heats through the living room, and then hallway and up the stairs.]

Here's a pic of the huge, expensive and complicated heating system for the kitchen:



The window 'hole' is about 700mm by 400mm, so pretty small. The blue door is to the living room, directing airflow, and the internal kitchen window also helps to direct airflow as the A2A unit sits inside that room, above and to the left of the window as we are looking.

I have since bought a better and quieter fan, for about £15.

I was utterly shocked at how well this works. Dialling the A2A unit up to 23/24C when I go to bed, with the fan speed set to high, gives me a room temperature in the kitchen of 21C. And that temp is measured at the other end of the kitchen, on a north facing windowsill. During the day, dialling the A2A down a couple of degrees, and the fan speed down to 1 (of 3) to reduce the noise, gives a temp of about 18C, which seems approriate for a kitchen.

ed110220

Martyn1981 said:

silverwhistle said:

Martyn1981 said:

Big caveat though, is that I'm playing/experimenting with two A2A units, that heat downstairs, with heat rising to the upstairs.

I'm happy enough with the idea of going without upstairs heating, beyond an electric towel rail, as I currently tend to benefit from the rising heat of a woodburner and have no need for a heated bedroom. I'm thinking of a similar approach, with PV but without a battery, at least to begin with. But are your two units separate or a split?

Also, is your kitchen also a diner, that needs the assistance of that internal fan?  In my case I foresee no need for an additional heat source in a small galley kitchen, where I'm active and with various heat sources.  

Hi. They are both single units. TBH, planning permission aside, I think that works better as 2 single units are probably cheaper. Also they can plug into a socket, whereas a single larger unit may need to be hardwired. Teeny caveat, I was told that you should really only have one per ring main, but that worked for us, as one is on the kitchen ring, and one on the downstairs ring.

Sorry for any confusion about the kitchen 'fan', it's simply a desktop fan, not heated. As there was no direct airflow of heat from the rooms with A2A units, I assumed that additional heat would be needed in the kitchen. My plan was to add an immersion into the existing large double panel radiator. But step one was to see how hard I needed to run a small oil rad, to get an idea on the power needed.

But it was suggested to me by a forum friend to see if a fan would move enough extra heat through the existing small window between the kitchen and the ickle conservatory where one of the A2A units lives. [That unit heats through the living room, and then hallway and up the stairs.]

Here's a pic of the huge, expensive and complicated heating system for the kitchen:



The window 'hole' is about 700mm by 400mm, so pretty small. The blue door is to the living room, directing airflow, and the internal kitchen window also helps to direct airflow as the A2A unit sits inside that room, above and to the left of the window as we are looking.

I have since bought a better and quieter fan, for about £15.

I was utterly shocked at how well this works. Dialling the A2A unit up to 23/24C when I go to bed, with the fan speed set to high, gives me a room temperature in the kitchen of 21C. And that temp is measured at the other end of the kitchen, on a north facing windowsill. During the day, dialling the A2A down a couple of degrees, and the fan speed down to 1 (of 3) to reduce the noise, gives a temp of about 18C, which seems approriate for a kitchen.

Interesting! I think one A2A unit would be sufficient for our downstairs as it's relatively small (~40 m^2) and quite open plan. 

silverwhistle

Thanks! And there was I thinking you'd be all hi-tech..;-)

Interesting what you say about two singles being cheaper. In my case I was assuming a low powered split with one in the lounge and the other in the dining room above the open door to the small kitchen, but inheritance and the potential for an extension has rather thrown me. 

What make and power are your two units? You've probably said somewhere but it's too hot for working too hard on a search..

Either way I've got the woodburner as a back up/top up for really cold weather which we don't really get down here anyway. I wonder if anybody has any experience of the impact of the replacement of 40 year old double glazing by triple glazing on my north facing aspect? That I could get on and do without impacting other potential developments, so I might go and ask for advice for that elsewhere.

FreeBear

silverwhistle said: I wonder if anybody has any experience of the impact of the replacement of 40 year old double glazing by triple glazing on my north facing aspect? That I could get on and do without impacting other potential developments, so I might go and ask for advice for that elsewhere.

I replaced all the 1970s vintage wood & aluminium framed double glazing (14mm sealed units) here a couple of years back. It has made a huge difference, although some of that might be down to the quality of fitting & plugging of cold draughts. With the south facing windows, I get a noticeable amount of solar gain during the winter months along with better heat retention. This is with low-e double glazing (28mm sealed units), so I can't compare to triple.

Martyn1981

silverwhistle said:

Thanks! And there was I thinking you'd be all hi-tech..;-)

Interesting what you say about two singles being cheaper. In my case I was assuming a low powered split with one in the lounge and the other in the dining room above the open door to the small kitchen, but inheritance and the potential for an extension has rather thrown me. 

What make and power are your two units? You've probably said somewhere but it's too hot for working too hard on a search..

Either way I've got the woodburner as a back up/top up for really cold weather which we don't really get down here anyway. I wonder if anybody has any experience of the impact of the replacement of 40 year old double glazing by triple glazing on my north facing aspect? That I could get on and do without impacting other potential developments, so I might go and ask for advice for that elsewhere.

Hi SW, both my units are 3.5kW LG units, chosen for being cheap, not necessarily the best. But the second install I went for a 'smart' unit with wifi, and I'd definitely recommend getting one where you can set programs, or even switch on when you are out to start heating/cooling.

I can set on / off times, and change the temp on the app, but I can't set a different temp as part of the programming. Don't know if others can, but worth possibly checking. So, the clever one is set to come on at 11.30pm (with cheap rate) and go off about 8 or 9pm in the winter. That way cheap rate does more of the work. Typically, I dial the temp up a few degrees then switch it off in the evening before the programmed 'off' time, so that when it comes on at 11.30pm it will work harder, then I dial the temp down a bit in the morning.

Reed_Richards

I looked into this as I am planning to install an air-to-air heat pump to heat a workroom made at the back of my garage.  I could not find any model that lets you programme beyond on and off times, you can't change the set temperature other than manually.