Firstly you need to recognise that December through February are very cold and so you will inevitably have MUCH higher bills in these months, probably about 3* a typical bill.  With oil, and to an extent gas, you dont look at daily/monthly figures so you don't notice how expensive these are during the cold months.  With an ASHP you are more likely to look at daily/monthly figures AND the effect is greater - its cold so your house needs more energy (roughly twice as much as the typical amount for the season) and its cold so the heat pump COP is lower.  Together these mean that you can expect roughly three times the consumption on very cold days as that you will see on a typical day/the average for the season.

The key with heat pumps is to get the flow temperature as low as possible, by operating them 24x7 without thermostats switching them on and off (which can be achieved easily by turning the thermostat and any TRVs up to maximum), adjusting the weather compensation curve so the house is just warm enough, and if necessary balancing the radiators.  This is generally both the cheapest and the most comfortable but takes a bit of setting up and a bit of a leap of faith as its the polar opposite of what we have been told (in part incorrectly) to do with boilers.  If you want some 'instructions' for doing that then tell us a bit more about your house and system.

TOU tarifs can be beneficial with heat pumps if either

(a) you can arrange a 'set forward' (ie boost) during the cheap period.  Only some heat pumps support this without resorting to thermostatic control, which is likely to

(b) your heat pump is well oversized and your house has a large thermal mass, particularly if it has UFH in a concrete slab.  In this case you can treat the house like a storage heater, warming the slab when electricity is cheap.

In the absence of these its a balancing act and depends on the price ratio and # hours for the cheap rate- basically does the average work out less than a non ToU tarrif.  Often it does, but not with all tarrifs.

If you would like to try optimising your set up (but bearing in mind that you may not want to conflict with Aira) please provide a bit more info about the heat pump (model/capacity) and house (estimated loss, construction and area) and any controls you have other than the heat pump control itself.  Also what was your annual oil consumption/cost (which gives a sense check on the loss).

Assuming your house was heated previously your oil consumption indicates a loss of more like 4-7kW, so an 8kW unit would probably have been perfect assuming that an '8kW' unit actually does 8kW at your design temp.  Given your house construction I would say 4-7kW is more likely anyway.  I guess that the surveyed figure of 8.9kW (two dp is nonsense, even one is doubtful) probably was just over the capacity of the 8kW unit once the DHW load was taken into account.  Unfortunately there are design cusps like this where the uncertainty in loss can affect the choice.

What are the weather compensation settings?  Key to lowest cost operation is to get these as low as possible consistent with heating your house.  Do you have TRVs, are they fully open  or set well above the target temperature, or set to the target temperature.  If they are set to your target temperature then they are probably fighting your heat pump which can result (paradoxically) in cold rooms.  Unfortunately there is a tendency in the UK to design heating systems with too many bosses (as in control elements in charge).  The correct number of bosses is exactly 1.  Fortunately tweaks to the settings can deal with this. 

If you can answer the questions in the paragraph above I can make some suggestions which you can line up against what Aira say.

Posted by: @wally

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We are on a Good Energy Time Of Use tariff. I am becoming to realise that such a tariff is of little benefit to us unless we have a battery. The only benefit I see from such a tariff is for our own domestic devices, washing machine and dishwasher. It contributes very little to the running of our heat pump.

Just on the tariff point, we find that Agile works for us.  We have 19kWh batteries but they only run the house + Ecodan 14kW for around 5 hours on cold winter days so a cheap overnight tariff is not helpful.  Agile is in the range 10 - 15 overnight, 15 - 20p during the day and  40 - 60p in the peak from 1600 - 1900.

We stayed on Agile throughout 2025, despite some very high rates in January /February when the rate hit 100p/kWh in the peak on a few days.  However, looking at the year overall we have this monthly usage and costs:

and you will see that the blue line (Agile) was always cheaper than the red line (Flexible tariff) - read these off the right axis.

This table summarises the average cost and savings:

We have an EV so the usage includes about 5000 miles @ 3 miles/kWh =  1650kWh approx.  (We do more mileage than that but also have solar so some car charging is effectively zero cost.

Agile works for us but without a battery you would need to be ready to turn off during the peak hours if the rates get too high.  But this is usually only for 3 hours so if your property has a reasonable thermal mass or good insulation it may work for you.

Lots of historical Agile tariff data on this excellent web site:

https://energy-stats.uk/octopus-agile-tariff-pricing/

@jamespa

@downfield Thank you for your response and suggestions. The more I read on TOU tariffs I see the real benefits for those with batteries and/or EV’s. The Good Energy tariff is: 05:00 to 09:00 and 13:00 to 16:00 - 13.00 pence. The rest of the day it is 28.98 pence.

Posted by: @wally

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All the TRV’s are set are their highest settings with the exception of our bedroom which is set mid-way at 3.

Thats sensible then

Posted by: @wally

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As far as I am aware I don’t think Aira has explicit weather compensation settings. Certainly no settings that are user accessible. Instead Aira heat pump ‘learns’ from usage, comfort and efficiency and weather patterns. In saying that I am aware the Aira engineer has the ability to amend the heat curve.

Most heat pumps seem to limit adjustment of the WC settings to installer mode, although some do expose something to ordinary users.  Until all heat pumps can self-optimise I personally think thats a mistake because, like it or not, few installers are going to spend the 2 weeks (elapsed) or so it takes properly to balance radiators and properly to set up the WC curve.

My house is about 7kW, possibly similar to yours.  My December consumption was 650kWh and Jan to date 247kWh.  Jan 2025 was 1022kWh.  My total 2025 consumption was 4700kWh, so Jan 2025 represented over one fifth of the total!  Im in the South so its warmer.  You gave cost but not consumption and of course I dont know what tarrif you are on, you might want to compare the consumption figures .  

@jamespa My December 2025 consumption was 1241kWh. So far for January, 337kWh.

Posted by: @wally

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@jamespa My December 2025 consumption was 1241kWh. So far for January, 337kWh.

That seems high.  Do you know what your WC settings are (even if you cant change them and even roughly?)

@jamespa Regrettably I have no idea. However I am due a call from Aira and will pose that question. As a matter of interest, what would you expect the figure to be?

Posted by: @wally

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As far as I am aware I don’t think Aira has explicit weather compensation settings.

I was supposed to write/comment about this last year, because I was in touch with all major manufacturers towards the end of last year regarding weather compensation controls and how to tweak settings. Aira got back to me and said, "Aira heat pumps don’t have a controller, it’s all app controlled and optimised by ML and AI." I followed up, but never heard back from them, on whether weather compensation tweaks existed at all because it didn't sound like it did, which is a bit crazy to me, leaving everything to AI and ML. 

Any other Aira users here that can confirm this?