As I now have well over 12 months actual energy consumption data for my heatpump and many prior years data for the old gas boiler I thought it might be useful to share this and invite others to do the same.
I find it very annoying to hear media reports that heat pumps are more expensive to run when the reverse has been demonstrably true for me. I estimate the saving as between 14% and 25% as you will read below
Clearly, as energy prices vary, it makes most sense to look at kWh purchased for both energy sources and then the ratio of prices over time to get to a savings estimate.
Some background - it was a 220m2 detached house in the South of England which was extended by 15% when the heat pump was installed so the heat pump usage is for a larger house. My 12month COP has averaged 3.6 - middle of the pack for this forum. I’m using 4 fan assisted rads and the rest are the originals from the gas system - all in a single zone and the flow temp varies in a narrow band from 35 to 38c - to suit the fan assists. My house is warmer on average than it was on gas mainly because it’s more efficient to keep the heat coming in slowly and near constantly.
The data below is all accurate for the periods stated - some of the years have been grouped as year-end readings were not taken for some years in the middle of the groupings.
Amount Purchased
Gas kWh
Elect kWh
2011 to 2014 Average/year over 4yrs
20,337
2015 to 2018 Average/year over 4yrs
18,245
2019 to 2022 Repeated meter failure - so incomplete data
2023 Full year Heat Pump
4,963
House extended 15% larger
Ratio Gas Usage 2011-18 vs 2023 Electricity
3.89
Adjusted for larger house area
4.47
Over this period the Electricity price/Gas Price Ratio averaged 3.34 (min 2.76 max 4.21). So for me the Heat pump has been 14% cheaper than gas overall and if adjusted for the extension it is 25% cheaper. Of course I only have 1 year of heatpump data and if 2023 was milder than the average then that should also be taken into account. Nevertheless the heatpump is cheaper than gas and my house is warmer.
Note that the gas boiler installed in 2010 was a Vaillant condensing boiler and I have not measured or included the pump running costs for the gas boiler - so the heat pump saving should be greater than shown.
Really helpful, thanks Colin
Can you fill us in a little on your home insulation levels, HP survey and HP sizing?
I ask since my brother in law is considering a HP but the house is a solid granite walled cottage on a windy Cornish hillside . Insulation will be a challenge… Also I am considering but my house is a large single story with a loft conversion. Also a PITA to insulate well!
The house was built in 2000 with 25 mm insulation in the cavities, double glazing (u 2.8) 50mm floor slab insulation, 200mm plus loft insulation.
HP is supposed to be 10.6kw Daikin Altherma 3 and this would be a perfect fit if it delivered that at minus 2. It is very good at 3 degrees and above but falls over badly when defrosting and only give 8kW max at -2c. Nothing like what it says in the book. Its adequate though with minor supplemental heat at sub zero temps. Other makes seem to be better at the lower temps.
I know the recieved wisdom is to spend money to insulate fully rather than putting in an oversized heat pump but if that is not possible its just a question of getting the same number of kW into your house as you do with gas with a SCOP of better than say 3.5.
That means investing in a very good radiator/ emitter system in order to run it all at well below 40c flow temp. …ideally 30c. Do not get cajoled into a high temp HP with minimal rad changes …false economy. @matt-drummer has a good case study on this…starting with a very inefficient high temp system he has ended up with a very efficient one which will now be saving lots vs gas.
From following this forum id say there are some golden rules for a good heat pump installation.
Do a proper survey and cross check it every way you can. Paying someone else doesnt mean you will get an accurate number. They all make errors some of which can be massive, they usually over estimate ventilation and do things in a hurry. Best to cross check vs actually energy use. Ideally use the daily gas reading from a supplier like Octopus. Take the kwh used on the coldest day, reduce that number by 10% to allow for boiler inefficiency then divide by 24 to get kw and then add 25% for intermittent operation. That number should be close to your survey number. If not then look for the error before you size your heat pump.
Put in the biggest radiators or emitters possible. A 2m long sofa against a wall could have a 2m by 600mm double or triple rad hiding behind it without being noticed. Use fan assists or close spaced underfloor if you want to minimise visual impact. Focus on downstairs to start with and maybe see how existing rads cope upstairs before changing them.
Make sure your rads can deliver the house heat loss at 40c flow temp.
Size your heat pump to match the ACCURATE heat loss plus say 20%. Some makes do not need so much, do not oversize much more than this or be tempted to go up a range as the minimum power consumption matters lot in getting the best COP. Eg Daikins 9 to 16kw range uses 900w minimum continuous and its 5 to 8kw range uses 400w approx. So if you need 7kw choose the 8 but not the 9…
Hope this helps…happy for the above to be corrected or improved on by others.
Thanks @ColinS that’s a great result overall in terms of the boiler comparison and a good summary of the golden rules for a good heat pump installation!
. Insulation will be a challenge… Also I am considering but my house is a large single story with a loft conversion. Also a PITA to insulate well!