Hello. I've had solar panels for 12 years and therefore recieve the higher FIT payments - the install cost paid for itself in 6 years even factoring in compound interest. I'm getting a Powerwall fitted next week and, though expensive, it will pay for itself over a similar period. I'll use Intelligent Octopus to charge in winter months. I know what my daily electricity consumption is for the last two years and it rarely goes over 7kWh per day - you can download the data from Octopus. I can see the Powerwall will easily meet my daily requirements whether charged by solar or off peak throughout the year. In summer my electricity should basically cost very little.

I want to install a heat pump which potentially puts my usage up to 50kWh or more per day in winter. Obviously I can't use the battery to power all the house all day so there will be periods in the day when I'm paying higher prices for electricity. If you are using 100kWh per day somedays are you using a heat pump and how do you factor in the higher pricing to your overall costs?

@davesoa you know how much you use, you know how much your battery can hold from a night charge. Assume basically no solar and there is your cost 

(off peak x storage x 0.8) + (peak rate x (demand - (storage x 0.8)) = total cost

0.8 represents an 80% depth of discharge on the battery - as you cant actually run them to empty. It would damage the battery

Thanks that's helpful. Have you come across a daily profile of actual heat pump use in kWh anywhere? I can find indicative monthly totals only. I've been quoted a 12kw heat pump for my house and a full year consumption figure based on industry averages.

@davesoa all I have is my own profile for heat pump use. However it will vary from location and house. What exactly are you looking for? Demand over the course of a day?

Posted by: @davesoa

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Thanks that's helpful. Have you come across a daily profile of actual heat pump use in kWh anywhere? I can find indicative monthly totals only. I've been quoted a 12kw heat pump for my house and a full year consumption figure based on industry averages.

Have you been provided with any heat loss calculations? The size of heat pump required would normally need to be 1.25 to 1.5 times the calculated heat loss, dependent upon the manufacturer's test data.

@derek-m Yes I have had the calculations. I then had some inspiration - I downloaded the Octopus gas consumption data for last year, converted cubic gas usage to kWh, divided that figure by the COP then overlaid my domestic electricity data in a pivot table. That gives a rough daily projected electricity consumption, by day and month. I can now calculate the potential cost of an ASHP for my home, excluding any FIT payments using my current heating profile.

Posted by: @davesoa

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@derek-m Yes I have had the calculations. I then had some inspiration - I downloaded the Octopus gas consumption data for last year, converted cubic gas usage to kWh, divided that figure by the COP then overlaid my domestic electricity data in a pivot table. That gives a rough daily projected electricity consumption, by day and month. I can now calculate the potential cost of an ASHP for my home, excluding any FIT payments using my current heating profile.

One can carry out calculations until one is blue in the face, but if the system design, installation and commissioning is not done correctly, one could just be wasting one's time. 

There are quite a number of factors which can seriously affect the overall efficiency of an ASHP, much more so than with a gas boiler. I would suggest that whilst you still have the gas boiler, if possible, try turning down the central heating water temperature, to see how your present heat emitters cope with the lower operating temperature of an ASHP. Note any rooms that struggle to get to temperature, since this may mean that the heat emitters are undersized.

Read through the posts on the website detailing the important factors that should be considered when selecting a suitable heat pump and system design. If the installer starts suggesting thermostat controls, buffer tanks or low loss headers, secondary water pumps etc, then question why, since it could be that they don't fully understand how an ASHP actually works. 🙄 

Excellent point and well made. The calculations I did for solar panels and the battery were easy with the buying decision simple. With an ASHP I'm finding it much harder. I'm not looking for a simple payback period or ROCE but, before I spend £10k, I need to be (reasonably) sure that I'm not going to hugely increase my fuel bills.If they stay much the same then that's fine. I could just spend an extra £500 per year on gas for the next 20 years and have the heating on for longer at a higher temp for the same cost. Won't help the environment though. In the end I suspect it will be a leap of faith and, as you say, spend time on the commissioning and finessing of the system post installation.

Posted by: @davesoa

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Excellent point and well made. The calculations I did for solar panels and the battery were easy with the buying decision simple. With an ASHP I'm finding it much harder. I'm not looking for a simple payback period or ROCE but, before I spend £10k, I need to be (reasonably) sure that I'm not going to hugely increase my fuel bills.If they stay much the same then that's fine. I could just spend an extra £500 per year on gas for the next 20 years and have the heating on for longer at a higher temp for the same cost. Won't help the environment though. In the end I suspect it will be a leap of faith and, as you say, spend time on the commissioning and finessing of the system post installation.

The most important factor is to ensure that the design and commissioning are done correctly, which will then mean that the optimisation is much easier.

@davesoa have a look at my previous posts. I did a calculation in the very cold spell we had comparing gas/oil/ASHP costs

TL;DR: ASHP is cheaper in all but the very bitterly cold months when oil is slightly cheaper. So overall ASHP should be way cheaper for the overall year.

But that’s based on current prices. I’m hoping electric will get cheaper as renewables come online and gas/oil more expensive. 

It’s a beast!

Some wiring changes pending and a cabinet for the batteries to sit in.