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Cosy Octopus - New ASHP tarrif

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New Member Member
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Long time listener, first time caller... 😀 

I've just made the transition from Ovo to Octopus and ultimately to Cosy, which should be live in the next 14 days .

After crunching the numbers it seems that, at worst (and with no change in behaviour on our part), it'll be the same as we're currently paying. December ASHP stats were a little over 1,000 kWh (due to the ten day -8c / 0c cold snap in the south that saw us use around 600kWh for heating) contributing to a total home usage of 1,787 (we have a Tesla as well, so charging figures are also included in that...more on that later).

November, by way of comparison, was a total home usage of 1,100kWh, and 453kWh used by the ASHP. Mitsubishi Ecodan (FTC5). I use WC, with a curve at 40c @ -2c / 25c @ 14c.

Not on mains gas - electric only.

We've been renovating for the past 3 years (new...everything, including double story 8x7m kitchen diner & master bedroom extension), so it's pretty well insulated, as much as a detached 1960's, home-built Colthouse that had no insulation and no heating, other than 2 open fires, can be. EPC D now - a wind turbine for around £13k would have moved us to C, but not seeing a projected return on that until well after I was 6ft in the ground made the decision to say "no" pretty easy. 

Back to (my numbers for) Octopus Cosy. As I said, we're talking pennies per day difference compared to Ovo if we don't change our behaviour; ASHP on at 6am, off at 9:30pm. WC running the show. And it works well - main rooms are 20, bedrooms are 19. What started to swing it was the Tesla.

I am very lucky that I can charge, for free, at the office. So this takes care of most of the transport needs - kids to school & after school clubs / hobbies / shopping trips etc. Sometimes I need to top up at home, but usually just enough to get me to the office (18 miles) where I plug in again. The top ups seemed like nothing, until I took a closer look.

In December I put in 110kWh from home, out of a total 360kWh put into the car. The cost (kWh only) - £37.40. Using the Cosy cheap tariff to top up when required, that same amount would have cost £22.55 - so a £15 saving. October was a 191kWh month for EV charging. All of a sudden, the small "top ups" were beginning to stack up to something approaching over £200 a year. And that's just the car. 

Shifting the heating schedules to work around the "cosy" periods should yield even greater savings. The house holds its heat for a good while, so 4-7pm (the expensive time in the Cosy universe) with the ASHP off shouldn't be a problem. And we have wood burners and a limitless supply of seasoned wood that I've prepped (living in the South Downs in West Sussex).

But even with the central heating on, my calculations and historical data show that I'll end up at almost exactly the same place as with my current Ovo tariff, but better off if I top up the car (and I now see that I've topped up the Tesla since we got it in April, at an average of 153kWh a month, or 5kWh a day. So that behaviour is not going to change)

Quite how I'll end up configuring the heating - I don't yet know, and will need some time with the new tariff to see how things pan out.

But kudos to Octopus for being the first to bring something "new" to a market that, currently, is penalising the 8% of UK households that are choosing to use "green & renewable" energy solutions to heat their homes.


Let's see where we go from here...





This post was modified 1 year ago by keefsloan

Estimable Member Member
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 for anyone who's interested, first day on cosy octopus tariff with comparisons to Flex and Agile

This post was modified 1 year ago by @sand

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Posts: 383

Looking at the Octopus Cosy tariff and set times per day, one thing comes to mind. Octopus' marketing department do not own ASHPs and fundamentally don't understand how ASHPs are designed to heat houses 'low and slow' as opposed to fired boilers scheduled to be on and off twice a day. Oh dear, an opportunity missed.

Hopefully, once day, we'll see those who have committed to renewable forms of heating rewarded with dedicated tariffs based on renewably generated electricity sold at a lower cost than fossil fuel generated electricity. 

HydroS reacted
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Posts: 1457

@allyfish - It is very important that this feedback is given to Octopus, preferably by email so they can pass it internally to the relevant people.

The Cosy tariff will not be a 'package' dreamed up by their Marketing Dept. It will be based on technical strategies under development within the company, and driven using software enhancements made to their Kraken billing system by in-house designers.

I haven't (yet) looked in detail at what you've all been writing over the past fortnight, due to the devastating effects of a virus. 🤒 
But I will do so. In the meantime here's what I'm seeing:

1: The Cosy Tariff is one which currently favours those of us experimenting with large battery storage, such as @chickenbig @batalto and @korwraith
It would benefit consumers whose HPs are supplied from stored electricity.

2: All of Octopus' ToU tariffs are still based on National Grid data. The costs/time-slots may bear little relationship to what's available in your region.
My DNO Region (SW England) often has excess renewable generation which is discarded rather than offered at a lower price.
The UK urgently needs Nodal (Locational) pricing, which is a matter for National Grid ESO and Ofgem - not Octopus.

3: The time-slots against which users are billed, are those defined within your Smart Meter.
Those time periods do not align with UTC. The Smart Meter firmware uses two random numbers internally to arrive at your half-hour time-slots.
I don't yet understand if Octopus are devising a software compensation for this discrepancy, or whether they believe the inaccuracies are within Ofgem's guidelines.
I am awaiting a reply from Octopus and REC on that matter.

Save energy... recycle electrons!

Derek M reacted
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@transparent Its possible Cosy might work for me. But currently it wouldn't - only due to rates I have from Octopus on GO (7.5p/kWh off peak). Cosy is around 20p in my area. Through December and the cold spell my average cost per kW was around 22p and I used a total of 1.8MW for the month on heating and general house demand. 

So at this point I just cant see any upside. I'd love to play around with ToU tariffs, but the only really interesting on Octopus is Intelligent, and its not open to me as I need the right kind of EV charger.

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE

Trusted Member Member
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@Transparent My thoughts exactly, I wish you a speedy recovery. 

@Allyfish When I read the marketing spiel, it does seem a bit overblown/BS to me so I can see where you are coming from saying that they don't have ASHPs. 

Part of me thinks that this tariff was created without heat pumps in mind at all, it's more of a grid balancing thing as Transparent put it and they slapped on the 'Cosy' label to sell it to the public.

@balalto With my more modest battery storage I think/hope it will be better for me and even if/when I get say another ~10kWh I think it will be better. As @sand showed at the moment Agile is better, however I like the stability that Cosy offers, looking at the historic figures for Agile it is generally better, however during the cold snap when the wind was slack and there was little solar, Agile would have been very expensive for me at least.

Mitsubishi Ecodan 8.5kWh
4.4kW Solar PV
5.2 kWh Battery Storage
1983 build, 300mm loft insulation, cavity wall insulation (beads)


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Posted by: @benseb

@sand I found that if you use the app to set the timer, you can have unlimited(?) temperature changes. You can also change between WC/Auto Adapt/Flow rate which is very useful. For example, when electric is cheap you can switch to a high flow rate, then back to WC when its normal rate.

How do you set the flow temperature on the timer in the app? I can see how to set the room temperature, and I can see how to switch between "room" and "flow", but not how to actually set the flow temperature.


ASHP: Mitsubishi Ecodan 8.5kW
PV: 5.2kWp
Battery: 8.2kWh

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In the app go to Actions, unit setting u can change the flow there.

I'm FTC 5 as far as I am aware I can only change the flow when it's set to fixed flow.

I think maybe FTC 6 is a bit more advanced.

Hope that helps

scrchngwsl reacted
Estimable Member Member
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Posts: 67

I agree I don't like gas boiler mentality for a ashp so the time blocks thing not 100% with, however if I had a battery I could buy from the grid in the cheap times then use that cheap energy to power the house, especially in the more expensive times. I could do this without solar initially then add solar when needed.

Reputable Member Member
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Topic starter  

I’ve got 30kW of battery being installed next week so this could work well for me. 

currnetly kid off peak is 8p do more attractive. However I’m the winter even 30kW of battery will only last until midday so the opportunity to fill those batteries again mid afternoon could work very well. 

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.

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Posted by: @allyfish

fundamentally don't understand how ASHPs are designed to heat houses 'low and slow' as opposed to fired boilers scheduled to be on and off twice a day. Oh dear, an opportunity missed.

 I think you may be in danger falling for the "24x7 on" heat pump myth challenged in another discussion on here. While running low longer may be cheaper if costs are constant WWF, ASHPs are usually well able to put extra heat into the home early and use it as a sort of “ heat battery” to cover a peak time switched off. Both the greater efficiency of  extracting heat from warmer early afternoon and the price discount of cosy suggests this is the intention and it doesn't seem outrageous to me.

Derek M and Benseb reacted
Reputable Member Member
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Topic starter  

Agreed. Most efficient doesn’t necessarily equal cheapest to run. 

250sqm house. 30kWh Sunsynk/Pylontech battery system. 14kWp solar. Ecodan 14kW. BMW iX.


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