Balancing financial efficiency and comfort using the Octopus Cosy tariff

webcmg · 2024-09-29T07:20:59Z

I've been fortunate to have a fixed low energy rate until recently, but this has now expired, so I've opted to move onto the Octopus Cosy tariff. I'm interested in getting views, and learning from others experience, on how to best configure my setup to maximise financial efficiency of running the heat pump whilst seeking to maintain comfort on this time of use tariff. Previously, I've been running 100% weather compensation 24/7 (Vaillant Arotherm 12kw @ 0.4 heat curve, deactivated Neostat thermostats in all zones). This resulted in a consistent 21c house and COP of 4.6 (inc DHW) and 4.9 (exc DHW). In 2023 we used 4,218 kwh according to the Vaillant app. The house is ~300m² with UFH downstairs and oversized radiators upstairs. The Octopus Cosy tariff offers: Three cosy periods of super cheap rates between 04:00 - 07:00, 13:00 - 16:00 and 22:00 - 00:00 every day, 51% cheaper than the Day rate in your region. A peak rate between 16:00 - 19:00, 45% above the Day rate in your region. The current Octopus Cosy rates for my region are as follows: Day rate: 21.93p / kWh (13 hours) Cosy rate (04:00 - 07:00, 13:00 - 16:00 & 22:00 - 00:00): 10.74p / kWh (8 hours) Peak rate (16:00 - 19:00): 31.79p / kWh (3 hours) Standing charge: 53.94p / day These are the options as I see them: Option 1: Continue to run as is 24/7 and benefit from the 8 hours of super cheap rate periods, but incur additional cost during the peak rate period. Option 2: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats), with no change to the heat curve. Option 3: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats) and set non-Cosy rate periods to "setback" temp (on Controller), with no change to the heat curve. Option 4: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats) and set non-Cosy rate periods to "setback" temp (on Controller), with a change to the heat curve to offset the impact on comfort. Option 5: Only run the ASHP in the Cosy rate periods (via Neostats), but significantly increase the heat curve. This would reduce efficiency, but may result in lower electricity usage overall. I would be interested in views, whether others have experimented with these options, and what your recommendations might be. I'm hoping to establish the best way to balance use of electricity and comfort, whilst inevitably sacrificing some efficiency. Many thanks

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Toodles

(@toodles)

Illustrious Member Contributor

Joined: 3 years ago

Posts: 2216

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.

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Old_Scientist

(@old_scientist)

Prominent Member Member

Joined: 1 year ago

Posts: 309

24/08/2025 3:58 pm

Posted by: @webcmg

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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 2 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.

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Judith

(@judith)

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Joined: 2 years ago

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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

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