Hi @ollie and welcome to the forum. 

I think the prices you have are fixed rates, which are a lot more than the current cap, especially for gas.  The current cap is roughly 20p/4p electricity/gas, so about a 5:1 ratio.  I'm not sure that will be a lot different after April. 

You may be able to achieve a SCOP of 4, but that's definitely at the upper end of what most people get. Living in the SE will help though.

In terms of C02, some people say you should calculate the C02 reduction based on 100% of your ASHP electricity being generated by gas.  This is because gas is used at the margins to flex production up and down and your extra few kWh will, in reality, be generated by gas. I don't agree with this approach but it's quite a common one amongst heat pump and EV 'hesitant' folks.

The ultra quiet Ecodan will be just that. Unless you live in an extremely remote place with no traffic, wind or any other outside noise and you don't have a fridge, freezer or any bathroom fans you will hardly notice the ASHP noise unless you really try. You can also time your HW to come on when you want. And when heating water in the summer, the ASHP isn't working particularly hard.  

I think unless the gas/electricity price differential changes a lot then you will struggle to beat gas for price.  But it's good fun trying and however you work it out, you'll be emitting less C02.  

Good luck and please ask more quesions.

@kev-m thanks for the reply. Regarding the pricing of energy, the rates I looked up were British Gas single fuel. The two tarrifs offered were "The Green One v9" (which seems like green washing on a gas tariff) and "The Fixed One v9", both of which are 8.06p per kWh. I did a search on www.moneysupermarket.com and found on the cheapest tariff that the electricity is about 5.4 times that of gas. This compares with 3.95 ratio for British Gas. I also learnt that British Gas are a rip-off (although I knew that already).

The property it is going into is a 1960s house with retrofitted cavity wall insulation, good loft insulation and double glazing. I am planning to have an extension done and that part will be very high spec insulation, UFH, maybe triple glazed. The older parts of the house will be radiators. I am running an experiment right now with dropping the heating flow temperature to 45C for a few days, and we hit -1C outside and all rooms maintained temperature. There is a couple of rooms that were very slow to reach temperature, so have little margin, but those are a bedroom with a small single panel radiator and a bathroom towel radiator. Both will be easy to upgrade.

Regarding the CO2, I can see the thinking behind considering that extra load will be gas based, although I think this short term view doesn't really reflect ongoing changes to the energy mix. The carbon ratio of the energy keeps dropping and as extra capacity is added to meet the new demand of EVs and heating, much of this will cleaner. Certainly with an extra 30GW of wind coming on by 2030, there could be times in the summer that CCGT would be off entirely, much the same as coal is now. Here is hoping. I found

interesting.

@ollie,

Some really interesting info in that attachment.  

That's good that your heating works at 45C at -1C outside.  A couple of new radiators will be an easy upgrade.  

I think it will be hard to beat gas for price with an ASHP in the foreseeable future, although it may get a little less hard.