Welcome to the forums @alphabeta.

The cost-effectiveness is a tricky one to answer due to your battery storage – 19.5kW is a very good amount. As an average, our ASHP does about 50kWh per day in the winter, rising to 75-80kW when it approaches 0C.

Are you on a time of use tariff? There are a lot of users on the site that have batteries running alongside their ASHPs, so they'll provide you with some better insights.

We've got a heat pump and oil boiler running bivalently which worked really well this past winter (after some initial teething issues). Kerosene provides 10.35kW of heat per litre. So if you multiply your average consumption per month, that'll give you a rough idea of your heating requirements.

For our house, given our current tariffs, kerosene will cost us significantly less to heat our house this winter: https://myhomefarm.co.uk/air-source-heat-pump-vs-oil-boiler-running-costs

Hi @alphabeta.

Any reason why you want a high temp ASHP?  Although they can achieve higher temps than conventional ones, the lower the flow temp the more efficient an ASHP will be. Any heat pump will achieve 50C and if you need anything like that or higher to heat your house, any ASHP is going to be very expensive to run; much more than oil.  The only way an ASHP will compete with fossil fuels on price is if it's running at lower temperatures with suitable emitters, normally larger radiators or ufh.  

@editor 

Thanks for your reply - really useful to get some examples.

To your questions, yes I'm on Octopus Go, so I get cheap energy for four hours every night that I use to charge up batteries etc.

It's interesting that you consider kerosene significantly cheaper - is this even at the astronomical rates of oil currently?

KevM - high temp is mainly as I thought I could run this much like a traditional boiler as Mars has covered in his posts - ie quick bursts of heat for a few hours Vs 24/7 running.

We have a log burner and smart thermostats, so we could quite smartly target rooms needing heat at certain times if needed.

Plus it'd be a straight swap for our oil boiler instead of new radiators, plumbing (we have microbore) etc et

It sounds like ASHPs aren't really well rated by your reviews??

@alphabeta 

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I am a fan of ASHP and have one but you can't use them like a boiler very efficiently. They are better than the naysayers say though and you can set back temperatures and use TRVs and other room heaters.

Really useful stats Kev - so I guess the secret might be in focusing on the energy cost. Eg I'm wasting lots of solar energy currently as generating more than I can use or store in 20kw of batteries. So that's effectively free energy that I can't 'release' with oil.

Then with octopus energy I can get electricity at 7.5p per kW - the limit here is inverter and batteries.

So as a curious thought experiment - I could double my inverter and batteries to give myself another 20kw of energy @ 7.5kw and then top up with solar to aim for 5-10 KW extra per day (obviously way more in hot months, but less needed).

So if I got 30kw of free or cheap energy, then that might bring an ASHP to a more reasonable frame. Naturally there's investment costs there (around 6-7k), but it'd pay off in longer term. 

The obvious challenge to this is swapping oil for petrol - ie we have a Leaf and also a Model Y on order, so thisll give us a other 100kwh of batteries to fill up, so if we pointed free energy at heating, it might then require high cost energy to charge cars.

Oh the conundrums! I can't quite tell if ASHP are the future or the 'WAP' of mobile internet - ie we'll intentioned, but destined to be replaced by a better solution....!

@alphabeta

The first thing that I would say is 'don't get carried away with your present solar generation, you will get nothing like that amount in the Winter when you need it'. Having said that there are various ways in which you can better use what solar energy is available.  Do you have a hot water cylinder with immersion heater? If so then have a power diverter installed to provide hot water from Spring through to Autumn.

Your 19.5kWh of batteries will provide approximately 15.6kWh of usable energy at a maximum Depth of Discharge (DoD) of 80%.

Have you considered electric storage radiators to store some of your excess solar energy? It may be possible to use them to provide a portion of your heating, particularly during Spring and Autumn.

Another way would be to install an Air to Air (A2A) ASHP rather than a Air to Water (A2W) one, to be used in a similar way as electric storage heaters to help reduce the use of your oil boiler. I have a small A2A heat pump which provides all our heating and cooling needs from Spring to Autumn, and also assists our gas boiler on sunny days in the Winter. I estimate our gas consumption has been reduced by approximately 40%.

I commend you on installing solar and batteries, we have had a 4kW solar system for over 10 years, which has reduced our electricity consumption by approximately 50%. After further insulation, a battery storage system will be our next priority.

Finally, since you are with Octopus you should ask them about selling your excess solar, they have I think it is an Agile export tariff, about which maybe someone on the forum will be able to provide more information.

Please ask any further questions.

Hi Derek - great points, much appreciated! Few answers:

- solar carried away - yes, totally - I've used forecast APIs to model this - expect 5-9kw in winter. Ie better than nothing, but not transformational - think the batteries and cheap energy will be more useful in those months!

- batteries - actually these are 100% DOD ones, so should get more from that. Think 4% is min boundary. Pondering a third or fourth battery though - even now producing way more than I can store...

- electric storage - interesting ideas, hadn't thought of that, I'll take a look. We've actually got electric underfloor in the hall and kitchen which is slightly related - ie thought in spring/ autumn we could turn this on to take the edge off the oil heating. The hall is under the central column of three stories, so heat could flow upwards.

- air to air - hadn't thought of this either, I'll have a look at this too. Need to understand how this works, but could be a useful add-on also!

- octopus agile export - yes, you can't be on octopus go at the same time unfortunately on this, so have opted for go for now. As octopus let you change after 30 days without penalty, I might look at this in spring as the generation ramps again!

Thanks again for all the tips!

Finally I'm looking at wind generation with some of these vertical turbines from firms like Tesup that might be interesting to test!

@alphabeta

Looking at my data from last December my 4kW system was averaging 1kWh to 1.5kWh, so you could probably expect twice that amount. November and January were just over double the December figure. Our panels are East facing so you may get more if you have better orientation.

I would be interested in the details of your batteries and inverter as I am seriously considering installing a system. Do you know if your system is capable of operating in 'Island Mode' in the event of a power cut?

I think most power diverters can connect to up to two individual loads of up to 3kW. So you could have a power diverter that feeds the immersion heater as its primary load and then feeds your electric UFH as a secondary load, obviously via a thermostat. In that way you can utilise excess solar generation without importing power when the clouds appear. For several years I have been using a 2kW convection heater in the hallway, fed via the power diverter, to help reduce or gas burn.

For A2A think AC Unit, in fact that is what it is, but can run in reverse to heat.

I looked at Tesup wind turbines, but I seem to remember someone on the forum said they had received bad reports. I was only looking yesterday at a unit called a Powerpod, which is being developed by an American company. They are not yet available, but look very promising, and may not require planning permission since they don't have external blades.

Hi Derek,

Yes, we're very fortunate in that we have a big 'swiss style' roof that faces directly south and is at 30 degrees so absolutely ideal for solar panels - so our generation seems to be a decent chunk above any estimates (today was 30kw+ despite it raining for the first half of the day and being cloudy for the second chunk).

We've got a Givenergy 5kw hybrid inverter and 2 X Givenergy 9.5kw batteries - yes our system can cover power cuts also, haven't set that up but is equipped for it. Let me know if you need any other details and happy to share!

Good shout re diverter - I've got an Eddi on order, so this will take solar excess and heat water, just not sure how much it'll need and may need to divert as you describe.

The other option is I have home assistant running all my devices across the house - so could likely do it without a diverter and do it smart (ie take energy generation, subtract battery max and level and then smart select other devices to run to use up excess - eg underfloor if temp under 16C, air con if temp over 23C, car battery if level below X% etc etc)

Definitely keen to get another inverter though as this would unlock our maximum draw to a more comprehensive level.

Will look at A2A though, that seems sensible addition.

Likewise will have a nose at powerpod too! Ta for tips!

Posted by: @kev-m

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I still don't understand why the government doesn't extend a cut in levies and duties on HVO to bring it more in line with kerosene. 88% less emissions than kerosene and a great way to cut CO2 emissions for something like 1.8 million rural homes running their central heating on oil. 

Posted by: @editor

Posted by: @kev-m

With a COP of 3 and a price per kW of 54p,an ASHP will cost 18p/kWh. With a 90% efficient boiler and £1/litre, oil will cost about 10.5p/kWh. Sadly oil is a lot cheaper at the moment.  

I still don't understand why the government doesn't extend a cut in levies and duties on HVO to bring it more in line with kerosene. 88% less emissions than kerosene and a great way to cut CO2 emissions for something like 1.8 million rural homes running their central heating on oil. 

Mars, to save me looking, how much is HVO at the moment?