@johnmo Oh what a shame 😧 Toodles.

I have found the details of Cosy tariff as from July 1st 2024:

I have an oversized Samsung 12kW heat pump, 3.64kW solar, but no batteries, and this new Cosy tariff has caught my interest. For anyone with batteries and a ASHP, it seems highly attractive.

I'm currently on Agile. My heap pump is new, so I am yet to see exactly how oversized it is (12kW Samsung, minimum output is ~4kW, my total heat loss may be as low as 5-6kW), as I'm yet to determine my new heat loss since having had internal wall insulation and pitched roof insulation upgrades at the same time the ASHP was installed, but for the vast majority of the year I think my heat pump will be putting out more heat than I need, so I will be cycling via the room stat and unable to achieve continuous low and slow running.

So for a large part of the year, I may be able to meet the majority of my heating needs (and all of my DHW needs, 2x daily scheduled reheats) at 11p/kWh in the Cosy cheap slots. Looking at Agile, that isn't the case on most days. The energy-stats.uk website shows the average Agile prices from the last year for 4-7am as 14.8p (2-5am is generally cheaper at ~12.5p, but is less useful for heating up the house ready for morning), 1-4pm as 15.5p and 10pm-midnight as 15.9p (for our region), all above the ~11p rates on Cosy (albeit a backward looking analysis in a market where prices have fallen over the last 12 months). The peak prices (4-7pm) are similar on both, and Agile is generally cheaper in the remaining overnight 12-4am slots (background usage only for us) and 7-10pm slot in which we would potentially use for heating in winter.

So how comfortable will the house be running the heating during the cheap slots? Heating from 4-7am allows the house to be heated up at cheap rate from the overnight switch off, ready for when we get up, plus heat the DHW tank ready for morning showers. This is followed by a 6h standard rate window, where we may be able to turn off heating, switching back on at the 1-4pm cheap slot, and again reheating the DHW to provide ample hot water for the evening. We can then turn off for the peak period 4-7pm, and in milder weather just use the 10-12pm slot to warm the house before bed. In colder weather we can provide a little additional heat in the standard rate slots by running the ASHP at minimum output (4kW). We will need to see how effective our insulation upgrades have been, and how quickly (or slowly) the house now cools (I'd be happy cycling on/off between 18-20C or 19-21C)

In the summer we can use the 4-7am cheap slot to run the washing machine at 11p (the wife doesn't want to wait until late morning for solar to put the washing machine on, as we've lost half the day for drying [no dryer]) and can then benefit from exporting any solar at 15p. We also have solar generation in the 7am-1pm standard price slot to offset against the ~22p/kWh cost.

Yes, we would lose those occasional super low days when electricity is largely free (or negative), but in exchange we have 8 hours of usage at half price, and they are slots that seem to benefit owners of oversized ASHPs who are unable to run constantly and need to switch off for periods of time. Cosy also benefits those who want simpler scheduling as one no longer needs to check daily half hour pricing and can instead stick with a rigid schedule based on known cheap time slots.

If I had a battery, I would switch now without hesitation. Giving this tariff some serious consideration.

@old_scientist Sounds like that could work for you.

Whilst staying on Agile, it may be worth shifting your usage pattern to what you would intend to do on the new Cosy tariff for a week or two and use the Octopus Compare app [or similar] to see how the costs compare. Only a short time period and obviously right now not representative of how the ASHP would behave in winter so perhaps hold that thought for a few months.

Are you heating with radiators or UFH? Just thinking about the not heating during the peak period... we did this on Cosy last winter and found that the UFH part of our property stayed more comfortable for longer than the part heated by radiators, but even with the radiators the drop-off in temperature wasn't too much. If you've got some decent insulation and draught-proofing, and give the heating a boost in the afternoon Cosy period then you should be able to avoid running the ASHP over the peak and may not really need it again until the new extra evening Cosy period. Comfort-wise, down to about 6degC outside we found that we were just about able to run the ASHP for DHW & heating in Cosy periods only, but cooler than that the ASHP would kick-in to trickle some heat in at the standard rate.

It's not the most efficient way to run the ASHP but in my [admittedly limited] experience I didn't find the drop in efficiency to be that great, but I did find that even shifting the ASHP usage, battery charging and other load-shifting to the old Cosy periods Agile still would have been cheaper overall. Possible consideration also that running the ASHP hard in stop/start cycles may put more wear & tear on the compressor and other components compared with a constant low-and-slow running... but if it is very oversized and will cycle like mad in any case then I guess Cosy running won't be that much different!

Posted by: @old_scientist

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I have an oversized Samsung 12kW heat pump, 3.64kW solar, but no batteries, and this new Cosy tariff has caught my interest.

I have an oversized 12kW Samsung and similar solar PV. I switched from Cosy to Agile in early October 2023 and have monitored the difference in costs since then. These are the results for ASHP consumption only:

We don't run the ASHP between 10pm and midnight (we go to bed reasonably early!), so the new Cosy tariff would not affect this comparison. However, the comparison is a little biased as although the heat pump was used more between 4am and 7am and between 1pm and 4pm, we didn't stick to these times only. We followed the maxim of never allowing the house to cool excessively. We relied on the thermostat to keep the house above 17C during the day and 16C at night and a little warmer for breakfast time and the evening. As a result, the heat pump ran for 1-3 hour cycles at a time. Flow temperature was kept above 33C to prevent short cycling. The end result was a reasonably stable house temperature, no short cycling and reduced cost albeit at the expense of a lower COP.

Your house sounds better insulated than ours, so you may well be able to keep it warm using the heat pump solely in the cheaper periods, but remember that you cannot do space heating and DHW heating at the same time, so you may find that you have less time for space heating than you thought.

Posted by: @old_scientist

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12kW Samsung, minimum output is ~4kW, my total heat loss may be as low as 5-6kW

My 6kW only turns down to 3kW so I wouldn't worry too much. The important bit is you have enough volume of water engaged to keep cycle time away from short cycling.

@old_scientist Oh Yes! I know the feeling, trying to reconcile those very tasty Agile plunges! Reconciling something so dynamic with a constant lowish rate that can be ‘set and forgot’ with Cosy has amused me for many an hour!

Anyway, I have swapped to Cosy and am still fiddling about with the Tesla app so that my battery (27kWh) can be charged at only the three cheapest periods of the day (along with applying grid power for all other needs during those 8 hours. During the other hours, the battery will supply all needs; then, during the somewhat elevated evening hours, I will attempt to help the grid by ‘Exporting Everything’ as Tesla call it. I think it may be a fine art that takes a little mastering but what I’m attempting to do is export my solar and also use the freshly charged battery in the early evening to export stored energy to the grid. I am using three different values in the schedule and tweaking the rates for import and export with the intention that:

A: I only charge at the ‘Super Off-Peak’ rate.

B: I only export from battery during evening peak time.

C: At all  times, I wish to export (15p. kWh via ‘Outgoing Fixed’) any excess energy.

D: I do not charge the battery at the mid-peak or the peak hours.

E: I do not export from the battery other than at the early evening peak time.

F: I find the right balance of Backup Reserve (currently set at 20%) so that when it comes to heat pump turn on again, I’ll still have sufficient energy in the battery to last until 22:00 when the next boost starts.

In addition, I have set Myenergi Eddi to supply energy to the Sunamp Thermino at any of the three ‘Super Off-Peak’ times if required (in between, should the Sunamp need more, the solar may help too.

Once the above aims are achieved, it will no doubt be time to consider an alternative tariff…😉

Regards, Toodles.

@mike-h Out of curiosity, I just looked at the Agile rates for today and note that during all three cheap times, Agile and Cosy are a pretty good match! (There were a few hours last evening where I think the Agile were cheaper, but not today) The ‘set and forget’ factor does have its’ attractions for me anyway.😉 Regards, Toodles.

@steve81uk We are all radiators, no UFH so do not benefit of the thermal mass associated with the latter. Further, our new internal wall insulation (IWI) on our solid stone external walls, which insulating the property has also effectively reduced it's thermal mass as those solid stone walls are now no longer able to heat up and subsequently release that heat (although I'm assuming the benefits of IWI should massively outweigh that).

Our radiators are capable of emitting the minimum heat output from the ASHP at flow temps of 33-34, and this has been observed in practice as the LWT will not drop below this even when the WC curve is set lower. I control the system with a 3rd party room stat so the ASHP cannot short cycle on LWT. Even at -2C in winter, I think we will be running flow temps of 40C or below, and with temps around 2-7C more common during colder months, we should easily be able to match the heat loss at low flow temps and reduced running periods.

I'm guessing a 2C hysteresis (e.g, 18-20C) would be more than acceptable to us if we can run only in cheaper slots and not lose too much heat whilst shut off.

Posted by: @mike-h

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Posted by: @old_scientist

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I have an oversized Samsung 12kW heat pump, 3.64kW solar, but no batteries, and this new Cosy tariff has caught my interest.

I have an oversized 12kW Samsung and similar solar PV. I switched from Cosy to Agile in early October 2023 and have monitored the difference in costs since then. These are the results for ASHP consumption only:

-- Attachment is not available --

We don't run the ASHP between 10pm and midnight (we go to bed reasonably early!), so the new Cosy tariff would not affect this comparison. However, the comparison is a little biased as although the heat pump was used more between 4am and 7am and between 1pm and 4pm, we didn't stick to these times only. We followed the maxim of never allowing the house to cool excessively. We relied on the thermostat to keep the house above 17C during the day and 16C at night and a little warmer for breakfast time and the evening. As a result, the heat pump ran for 1-3 hour cycles at a time. Flow temperature was kept above 33C to prevent short cycling. The end result was a reasonably stable house temperature, no short cycling and reduced cost albeit at the expense of a lower COP.

Your house sounds better insulated than ours, so you may well be able to keep it warm using the heat pump solely in the cheaper periods, but remember that you cannot do space heating and DHW heating at the same time, so you may find that you have less time for space heating than you thought.

Interesting @mike-h. When I first looked at Cosy, it didn't appeal and we went with Agile. I'm not sure if they have changed the time slots and the discount levels, but it now looks far more usable with an ASHP for us. The 4-7am slot is certainly better than Agile, as Agile prices start to increase after 5am for the morning peak, and running the ASHP in the cheaper Agile overnight periods of 1-5am doesn't really fit well for us.

I take your point about reduced costs at the expense of a lower COP. I think it's too easy to be fixated on achieving a great COP and easy to forget that an ASHP costs the least to run when it is turned off! I could easily run my ASHP during summer when I don't need it and achieve a great COP and the expense of higher running costs.

With a massively oversized heat pump, it is inevitable we will have some cycling, so better to control when the heat pump cycles and run during the cheap periods.

Our DHW tank is relatively small at 170L (158L capacity) and the heat pump can reheat during summer from a cold startup in less than 30mins. In winter this should be a lot quicker, as we have a 50L volumiser that will already be at RWT and will not need heating from cold. Our DHW tank temp does not increase in the first 15mins of the DHW cycle during summer as the heat pump is just trying to get the LWT/RWT above the residual DHW tank temp and ends up cooling the tank at the start of the cycle, effectively transferring heat out of the DHW tank and into the volumiser.

Posted by: @johnmo

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Posted by: @old_scientist

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12kW Samsung, minimum output is ~4kW, my total heat loss may be as low as 5-6kW

My 6kW only turns down to 3kW so I wouldn't worry too much. The important bit is you have enough volume of water engaged to keep cycle time away from short cycling.

Yes, we have a 50L volumiser installed, and our radiators can emit the minimum heat output at flow temps of 33-34C, so this is as low as we can go.

@old_scientist Originally, Octopus Cosy tariff had two cheap slots totalling 6 hours; they were not evenly spaced as there was a larger gap between the afternoon slot and the following morning.

What I have referred to as ‘Cosy MK11’ is easier as the 3 slots have smaller gaps and there is a total of 8 hours compared with the previous version’s 6 hours. In addition, the price for these slots’ rates is lower - if you have OE’s Outgoing Fixed tariff, then you will benefit from an approx. 4 pence advantage (Buy Cheap - Sell High’!) for any exported energy. Regards, Toodles.