Forums

Forums
Renewable Heating
Air Source Heat Pum...
Balancing financial...
 
Notifications
Clear all

Balancing financial efficiency and comfort using the Octopus Cosy tariff

 
Page 2 / 2
Air Source Heat Pumps (ASHPs)
Last Post by Judith 3 months ago
15 Posts
6 Users
4 Reactions
3,472 Views
RSS
Toodles
 Toodles
(@toodles)
Illustrious Member Contributor
Joined: 3 years ago
Posts: 2296
24/08/2025 3:57 pm  

@webcmg That crystal ball I mentioned…, well hindsight is wonderful, but who has foresight? I had solar and battery installed in August 2022 - look what has happened to energy prices since then! Rather than ‘payback’ I prefer to think in terms buying my energy as cheaply as I can ATM but, having the security of supply as of great importance too. @old-scientist is enjoying his fling with IOF just now and that looks like a good prospect to improve ‘payback’ too. Regards, Toodles.


Toodles, heats his home with cold draughts and cooks food with magnets.


   
ReplyQuote
 Old_Scientist
(@old_scientist)
Prominent Member Member
Joined: 1 year ago
Posts: 311
24/08/2025 3:58 pm  

Posted by: @webcmg
↑

I do need to track what the peak usage is outside of Cosy slots in order to determine what the smallest battery I could get away with is... that would reduce the investment cost and minimise the breakeven period. I think I will need to contact OVO to get an annual hourly usage breakdown...

I estimated our usage on the following basis.

The most challenging 6h window for us is 4-10pm as it coincides with cooking/meal time (electric oven, air fryer, microwave, kettle all potentially in use) and no solar generation to help out.

I estimate our heat pump runs at around 1.2-1.5kWh input, so heating for the 6h may require somewhere between 7-9kWh. Background use is another 2-300w (~1.5kWh). Then we may want the electric oven on to cook using another 2kWh?? (I know it draws 2.7kW don't really have a firm number on average usage per hour as it kicks in and out). So our 13.5kWh Tesla Powerwall seems ideally sized for us to meet our peak demand in that 4-10pm window. We could reduce the demand slightly by overheating the house by 1C in the run up to 4pm, and then knocking the flow temp back slightly if we know we are going to be cooking a large roast dinner and battery capacity is going to be tight. The house should not cool noticeably with the heating still on low and the additional heat output from cooking.

Laws of diminishing returns. It may be the case that a battery large enough to cover your usage on 80-90% of days is good enough, and doubling it's size for those last 10% of days makes less economic sense. If batteries were only available in 5kWh modules, we'd need 15kWh, but 10kWh would do most of the time and that's what I would go for. The Tesla PW3 at 13.5 kWh just so happened to hit our sweet spot.

 


This post was modified 3 months ago 7 times by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
👍
1
ReplyQuote
 Judith
(@judith)
Noble Member Member
Joined: 2 years ago
Posts: 437
29/08/2025 7:27 pm  

We’re high energy users other than the ashp so the OVO tariff wouldn’t work for us, so cosy was the next best. We averaged 17p per kWh using a 9.5kWh battery to get our 7kW ashp through peak hours. I did calculate the financial payback for a bigger/additional battery but it was just not viable. Too small a saving.


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC


   
👍
1
ReplyQuote
Page 2 / 2
Forum Jump:
  Previous Topic
Next Topic  



Share:

Join Us!

Podcast Picks

Latest Posts

Click to access the login or register cheese
x Powerful Protection for WordPress, from Shield Security
This Site Is Protected By
ShieldPRO →