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New Solar, Batteries, Air Source and Under Floor Heating advice please

 
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Air Source Heat Pumps (ASHPs)
Last Post by Batpred 4 weeks ago
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JamesPa
 JamesPa
(@jamespa)
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26/01/2026 7:54 am  

Posted by: @batpred
↑

Posted by: @papahuhu
↑

.. You will be shocked at how much power the larger units consume on a cold day. I’ve seen folks using my unit consume approaching 200kwh a day, which gives me indigestion just thinking about it. We are frugal with our heating but at -5C we still chewed through 80kwh. My point being you’d have to have very large capacity batteries to get you through 24 hrs.

200kwh..  Would that be warming an outdoors tent or a castle? 😀  I prefer the image of a castle with cold stone walls..

Now seriously, ASHP in most reasonably well insulated homes with a suitably low flow temperature seem to consume a lot less?

 

Consumption is determined principally by house loss!  200kWh at a COP of 2, which is very low, means the house loss is 16kW.  At a more reasonable COP of 2.5 (Im assuming the 200kWh figure is when its very cold!) its 21 kW. 21kW is either a very, very badly insulated or a very, very large house.  Most houses are in the 5-10kW range.

Remember that most of the season heating systems in the UK run at about half design capacity.  So if your house is 8kW at design temperature the heating system will be running at around 4kW = 1.5kW consumed most of the time.  It might be sensible to consider something in between design loss and this figure, and accept some full price import on the very coldest days!

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Papahuhu
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26/01/2026 8:00 am  

Boiler plate 16kW which has an actual output of about 13kW which is what you would select for a heat loss of about 12kW, in Huddersfield Yorkshire, I picked an early Jan day at random, this had UFH if I remember correctly. On this day was 150kwh, plenty of others similar. I think in this period mine was in the 60-80kWh range, even that seems steep for a tight wad like myself. 

image


   
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 Papahuhu
(@papahuhu)
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26/01/2026 8:02 am  

Boiler plate 16kW which has an actual output of about 13kW which is what you would select for a heat loss of about 12kW, in Huddersfield Yorkshire, I picked an early Jan day at random, this had UFH if I remember correctly. On this day was 150kwh, plenty of others similar. I think in this period mine was in the 60-80kWh range, even that seems steep for a tight wad like myself. 

image


   
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 Papahuhu
(@papahuhu)
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26/01/2026 8:30 am  

@jamespa 

I was comparing and contrasting with this installation because it was the same model of heat pump and we are just the other side of the hills from it and it used the same installer as myself.

But the point I was trying to make is that it is a fallacy to assume you can size house batteries to cover you for 24hrs on the super off peak EV tariff with a larger output unit in cold weather. Unless you have very deep pockets and a 3P supply. We were emptying our batteries in this period even with cycling thrice, having to pay the higher tariff rates, it gets expensive. 


This post was modified 4 weeks ago by Papahuhu

   
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JamesPa
 JamesPa
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26/01/2026 8:33 am  

Posted by: @papahuhu
↑

@jamespa 

I was comparing and contrasting with this installation because it was the same model of heat pump and we are just the other side of the hills from it and it used the same installer as myself.

But the point I was trying to make is that it is a fallacy to assume you can size house batteries to cover you for 24hrs on the super off peak EV tariff with a larger output unit in cold weather. Unless you have very deep pockets and a 3P supply. 

Agreed.  Personally I wouldn't try even for my 7kW house.  Thats roughly 50kWh in the high price period which is a lot of battery.  The sweet spot for ROI is a capacity much lower than this!

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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Batpred
 Batpred
(@batpred)
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26/01/2026 1:46 pm  

Posted by: @jamespa
↑

Remember that most of the season heating systems in the UK run at about half design capacity.  So if your house is 8kW at design temperature the heating system will be running at around 4kW = 1.5kW consumed most of the time.  It might be sensible to consider something in between design loss and this figure, and accept some full price import on the very coldest days!

Absolutely makes sense to me.

And should a good export allowance and suitable tariff justify more energy storage to "lower the electricity bill", it would be another factor determining the middle point.

 


8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC


   
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