@jamespa E7 energy usage seems to vary a lot from year to year probably based on how cold it is and how often we are there.
1638kWh 2024, 5399kWh 2025 & 3343kWh 2026. All taken from energy bill on roughly same date each year and covers storage heaters and immersion heater.

That's tiny.  If it's correct figure for annual use and if you actually heat the house with the storage heaters and assuming you are in the UK it suggests a loss at design outdoor temperature of 2kW max. Can this be true?  If not which assumptions are incorrect?  Is it that you are hardly ever there so don't heat the house?What's the floor area btw?  If it is rarely occupied and only for short periods then an a2w ashp may not be a great choice, you may be better with a2a particularly if you have no plumbing for distribution.

@cathoderay I too remember the Airfix models of my youth but don't think I ever had the patience to do a Lancaster bomber.
I also remember the complex physical plastic scaled models of chemical plants that we used to use before 3D CAD pipe modelling became a thing. We used them to check whether our guesstimated pipe routes would fit prior to producing fabrication Iso's for the new plant.
I've come across many a 'professional' in my working career that thought things were correct because computer model said yes and often they were wrong because of incorrect input.

@jamespa You have to bear in mind that this is a holiday home so is unoccupied for most of the year. We do try and go every month  but the number of days vary from 3 to 7days at a time. I find the 1638kWh hard to believe. I can understand the 3343kWh as due to broken bits I've not been as much this year.

The estimate of heat loss from software for the whole house was 6464w but I believe that was with std air changes so could very well be an over estimate.

Posted by: @paul1098

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You have to bear in mind that this is a holiday home so is unoccupied for most of the year.

I think @jamespa may be onto something here when he says an a2w heat pump may not be the best choice. Because they run 'low and slow' they take ages to heat up a house from cold, especially buildings with a high thermal mass. We're talking days not hours. There are two possible workarounds, firstly start the heat pump remotely say 24 hours before you are due to arrive, or secondly install a high output heat pump, which can be run more like a fossil fuel boiler ie fast and furious on your arrival and then turned down once the building has warmed up.

I agree the historical annual use figures are variable enough to need some explanation. I also think that we need to remember that heat pump capabilities are conventionally quoted in kW (power) whereas use (and bills) are measured in kWh (energy). All other things being equal, if you take two 6 kW heat loss buildings heated by 6kW heat pumps, and run one all year round, and the other for 6 months, then the annual energy use in kWh will be very different. Thus going from annual energy use (kWh) to heat loss (kW) in a partly unoccupied building is likely to be very misleading, unless some sensible adjustments are made.  

Posted by: @paul1098

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@jamespa You have to bear in mind that this is a holiday home so is unoccupied for most of the year. We do try and go every month  but the number of days vary from 3 to 7days at a time. I find the 1638kWh hard to believe. I can understand the 3343kWh as due to broken bits I've not been as much this year.

The estimate of heat loss from software for the whole house was 6464w but I believe that was with std air changes so could very well be an over estimate.

How many rooms does it have and does it have pipework for radiators.  Is the use going to continue in the current pattern?

As I understand it you currently have storage heaters.  Is there a reason to change?  The carbon benefits with the current pattern of usage may not be that great, and the operating cost saving ditto.  However if it's going to be used 'full time' then it's a different matter all together.

@jamespa There are 5 rooms and no pipework for radiators.

We will not be changing how we use the house but we won't own it forever and I'm guessing that a heat pump and radiators will sell better than storage heaters.

Two of the storage units are from the 90's and we probably need to change them for Quantum or similar that can control temperature and energy release better. To install newer storage heaters requires additional wiring so not straight forward and Quantum storage heaters are not cheap either.
The 2 bedrooms only have electric heaters and are not part of the E7 consumer unit.

@cathoderay That is the same problem that we have now with storage heaters. I remotely turn them on 1 or more days before we are turning up to get the temperature up. The only problem being is that I don't have total control, all I can do is turn on the E7 heating/ hot water circuits remotely.
We have to guesstimate when we will be coming again so I can set the number of days the Quantum is in holiday mode before going to normal temperature.
That gets restrictive and we often end up turning up to a house at 15degC rather than 21degC

@paul1098 — rather than using an on site timer, which I think is what you now do, and needs foreknowledge of when you will return, I was thinking of true remote control ie an app on a smartphone anywhere on the planet sort of thing. I can't do it because I don't have a smartphone, but I am sure that in this world of IoT, such things are possible. If not, then they should be!

The RRP for the 2 Quantums is £2056 which I assume the local approved installers will charge. Additional to that is the install cost which will include extra wiring as new storage heaters need to have 2 electrical supplies rather than just E7. As the consumer unit is full I'm not sure whether that also means a new consumer unit.
I'm guessing £2500 to £3000 and still no controlable heating in the bedrooms.

If we can get the BUS then I'm hoping we can get an A2W fully installed for the same outlay from ourselves.

@cathoderay I use honeywell app on my phone to control the E7 circuits. The honeywell app is similar to hive and I can set different times for on / off etc but it has no access to the individual storage heaters, we had contactors installed to control the E7 wiring.
All the storage heaters are dumb and can't connect to anything. A newer version of our quantum can be controlled over wi-fi to a hub which could be controlled via an app but this can't be retro fitted.
Our existing quantum can be set locally to 3 different modes in all day, out all day or holiday mode which is what I fiddle with before leaving, the other 2 90's storage heaters don't even have that facility, they're either on or off.

Posted by: @paul1098

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The RRP for the 2 Quantums is £2056 which I assume the local approved installers will charge. Additional to that is the install cost which will include extra wiring as new storage heaters need to have 2 electrical supplies rather than just E7. As the consumer unit is full I'm not sure whether that also means a new consumer unit.
I'm guessing £2500 to £3000 and still no controlable heating in the bedrooms.

If we can get the BUS then I'm hoping we can get an A2W fully installed for the same outlay from ourselves.

OK, makes sense.  A decent heat pump will have full remote control and can therefore be turned on in anticipation say a couple of days before hand.  Note that there is no point in over-sizing the heat pump for faster warm up unless you also (over) size the emitters appropriately  A heat pump capable of generating 12kW will still only output 6kW if the radiators can only output 6kW.  Of course early in the warm up phase the emitters will be able to emit more because DT room-emitter is initially greater.  However as room temperature approached the target they will only emit what they are designed to emit.

If standard software with standard air changes gets to 6.5kW, then the actual loss is, as you say, probably a bit less.  However the minimum size heat pump is 5-6kW anyway so basically thats more or less what you are fitting unless you plan to oversize a lot.  

Based on what you have said I think I would probably right-size (or just very slightly oversize) everything, use the remote control features, and buy a couple of fan heaters if you ever intend to go at short notice.   You might need to spend some time there in mid winter to get the WC curve and rad balancing right.