So some observations 

A setback or off period your heat pump will have to play catchup at the end of that period to replace the energy lost. But there way to many variables to give a black and white answer. Just looking at our garden room (well insulated) and house. The garden room would become uncomfortably cold after 3hrs on a cold day, you wouldn't notice it in the house.

Experiment and see what works for you.

A 12kW heat pump would say big house or leaky (heat) house or an oversized heat pump. Can you supply the required heat from the heat pump in 8 hrs instead of 24 - would have to be massively oversized for that, so would doubt it.

@johnmo Yes, I feel the same @johnmo, the 3 hours of peak might be tolerated without heat but the pump will have to play ‘catchup’ afterwards. The problem with trying to just use the 8 cheapest hours is that you are likely to lose out on comfort due to temperature modulation during the other 16 hours. Again using any setback will need compensation and this will make the heat pump have to work harder and be less efficient.

Mind you, I am an old fusspot and along with my wife, likes 22.5 degrees C. 24/7 with a smart (old Hive fitting from the gas boiler days) TRV reducing our bedroom at night. We ‘cheat’ the system by only using the 8 cosy hours and Powerwall storage and I appreciate not everyone can do this. None of us knows how long such TOU schemes will last or whether investment in storage would be worthwhile long term or not. Sorry I can’t really offer a solution webcmg. Regards, Toodles.

&webcmg Hi. Did you come to a conclusion on whether to use Octopius Cosy or not?

I am currently on an Octopus fixed tariff and looking as to whether it would suit us to change to Cosy.

My situation is that we are at home all day, no PV or battery and like &toodles, we like it warm in the house.

What's making me hesitate moving to Cosy is the 3 hour expensive period from 4pm to 7pm just when we are likely to need the heat in winter and cooking the evening meal.

We ran on Cosy last winter and we switched off during the peak (4-7pm) and set back over night as the house gets too hot if running constantly, even on minimum due to oversized heat pump. For reference, we averaged an import price of 19-20p over the Dec-Feb winter period. Our house did get pretty cold by 7pm - on average we dropped around 1C/hour (others may be better insulated than our 1700's stone cottage). We tried to bump the heat up a little in the preceding cheap 1-4pm period to compensate. We only ever heated hot water during the cheap slots,

As others have said, there are just way too many variables to determine if it was more cost effective, and how much we saved, and was that saving worth it for the discomfort we endured (subjective). So do what works best for your home and it's occupants as there is no clear cut answer here.

What you can do is to try to model what you think your effective import rate will be, and then compare that with other tariffs to try to determine the cheapest option for you.

@old_scientist Although Cosy is (presumably) aimed at heat pump owners, I think the tariff really offers the most efficient utilisation to those who have the means (battery) to suck up their day long energy needs during those 8 cheapest hours whilst leaving the more expensive hours alone but just available in emergency situations should they arise. I feel that Cosy is the best solution for us as we are unable to avail ourselves of an EV tariff with cheaper overnight rates. Come next late spring / early summer I might give IOF another try if it is still available - or any other better deal that becomes available, but my crystal ball seems to be on holiday right now. Regards, Toodles.

@richard24738 as we do not have batteries or solar, I'm sticking with OVO extended fixed 13 month plan to Nove 2026 with their free Heat Pump Plus add-on (type of use tariff). The fixed tariff is 23.87p/kWh and standing charge 54.16p per day. However all ASHP use is currently charged at 15.00p regardless of the time of day.

This allows us to run the ASHP 24/7. Whilst Cosy offers 8 hours at 12.29p, the 3 hours at 37.59p and standard day rate of 25.06p are not favourable given we would typically want to use some electricity at peak times. I like the idea of Cosy, but the reality for us is only heating our house in the dips would not provide the same level of comfort as the OVO addon tariff allows.

I note that the OVO tariff is only available for certain manufactures and models of ASHP.

@toodles I'm not 100% sure about that, I was considering a Fogstar 32kwh battery + inverter. I think this coupled with Cosy would likely reduce all our electricity use to 12.29p, but the ASHP saving of 2.71p and 11.58p p/kwh saving for other usage would mean the payback period would be >10 years. Given I can't guarantee we will live in this property in that timeframe, the battery investment is difficult to justify.

@old_scientist Thanks for the reply.

Both myself and my wife are retired and do not like to be cold so having a set back temperature during the expensive 4pm to 7pm period is not attractive, although like you, we have an overnight set back.

I will trial changing the Ecodan Auto Adapt Room Temperature for the whole period 04:00 > 19:00 to 20c.

I have also moved my 2 x DHW sessions to the notional cheap slots.

The heat pump may well consume some expensive electricity between 16:00 > 19:00 but the house fabric should be pretty warm by 16:00 so maybe minimal.

Whilst a proper comparison will need to wait until the heating season, I will monitor my current usage via the app "Octopus Compare" to compare the current usage cost at my fixed rate versus Cosy rates.

Really, Cosy needs a battery to cover the 3 hour expensive slot. Maybe next year!

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...

@webcmg Yes, the Ovo tariff certainly looks attractive.

Unfortunately, Ovo are not accepting Mitsubishi Ecodans at this time. I am on their waiting list. 

As mentioned, the cosy tariff really needs a battery but the cost at the moment gives a long pay back period.

Technology is moving at quite a pace so I'll wait.

Agreed that Cosy works best for battery owners, and the savings do not always stack up well versus the battery costs over the 10 year warranty period (although hopefully the battery will have a longer usable life, even at reduced capacity).

If your usage is dominated by the heat pump, and you are eligible for a heap pump add-on (type of use tariff) at 15p for the heat pump, then purchasing a battery is never going to be economical. The numbers didn't stack up well for us when we bought our Tesla, going from an average pre-battery import price of ~20p/kWh to ~13p/kWh on Cosy all at cheap rate, it would take a long time to recoup the cost at 7p/kWh saving. But it's not just about cost, comfort is important too, so I also compare against the SVR (ultimate comfort as no time constraints) where Cosy is essentially half price.