Projections of running Heat Pump on Cosy Octopus tariff [UK]

I’ve been looking into which electricity tariff to switch to after my fixed one runs out next month, and so I’ve been doing detailed simulations using historical data from emoncms in various scenarios. I thought it might be useful to share my findings (with caveats).

Property: a 1950s bungalow with mediocre insulation, 11 kW heat pump, 5 kWp solar panels on east facing roof and 8.65 kWh of (usable) battery storage. No electric car. (I’ve not shown the effect of PV generation here)

Methodology: request hourly consumption for the past year, and simulate grid import and battery usage over time.

Tariff: Cosy Octopus provides two cheap periods between 4am - 7am, and 2pm - 5pm, and an expensive period 5pm to 8pm. Current rates for Yorkshire are 17.56p / 29.26p / 46.82p for cheap / main / peak periods. (Correct for August 2023, likely to be 2p lower in October)

I have plotted the total cost of 1 year’s worth of electricity, excluding standing charge and export:

Flexible Tariff: This is the cost to run my house, with heat pump, for one year, on the standard rate which is mostly dictated by Ofgem’s price cap - this is the benchmark by which all other scenarios are compared to. Ideally, I’d be looking for something that would be cheaper than this.

No Battery: Cost to run the house on Cosy Octopus, with no PV and no Battery, with consumption charged as whatever the rate is for hours of the day. This works out to be more expensive that the fixed rate would have been.

Demand Shift: This attempts to simulate shifting the consumption out of the peak period to when it’s cheaper. So, moving the heating periods to be earlier in the day, and heating the water in the cheap period. Simulation of demand shift is pretty crude, and I’m not entirely convinced it’s correct, however the impact seems to be small.

With Battery: This is where Cosy really starts to be economical. By filling the battery twice a day, we can avoid buying any electricity during the peak period. In fact, more than 75% of consumed electricity is charged at the cheap rate. Shifting heat pump into the cheaper period makes very little difference, as it mostly runs of the battery anyway.

Now, it could be that my simulations are way off, or my usage is atypical, but it’s hard to see how one could benefit from this tariff without the aid of a battery. Perhaps a newer or smaller property with better insulation would work out better, or where the heating is mostly off during the daytime. I’d be interested to hear from anyone else who’s on this tariff or is considering it.

Other observations:

  • adding in the generated solar plus export brings this down massively, unsurprisingly
  • having a bigger battery provides very little benefit, and isn’t an economical upgrade (for me)
  • I ran the same simulation for Octopus Flux, and found it to be a poor choice for my setup

Obligatory referral link with £50 bonus just in case anyone’s looking to join Octopus.


Did you look at Agile? For me (and probably typical non-HP setup) Average over the last 30 days is just below 20p per unit (from the emoncms Agile App) even with some stupidly high peaks this week.

I have been wondering if a Battery only setup would be beneficial. Difference between normal and peak is as a minimum ~25p it seems. For me, it is all about ROI.

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And your point is?

Hi Tim,

Great post, I’ve been mulling over the different tarrifs. I’m currently on agile (May 2023). Same heat pump as you, 1910 property with mediocre insulation, solar panels (2.4kw system) and 5.8kw battery.

Since May 2023 I’ve averaged a unit rate of 14.19p - see report below. I’ve been charging battery twice a day (picking cheapest 2hr rates both night and day) and then accepting I’ll export on some days on the fixed export rate of 15p.

What’s difficult to predict is the combination of winter rates, heat pump use and unit rates - last year’s look frightening at times given long runs of heat pump. I’m going to stick with agile ATM and try and tweak a bit more with timed hot water runs and see if that helps even further.

Not sure if that helps - I have considered additional battery storage but not sure on cost vs benefit ATM when heat pump could be using 1.2kw/hr without even taking into account background house load.

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I had not, but your reply prompted me to look. My gut feel was the the price is cheapest when I don’t need it (i.e. solar PV), and expensive when I need a lot (due to heat pump), so I’d assumed it wouldn’t be worthwhile. This is perhaps more true of the daily Tracker tariff, which doesn’t allow demand shifting and cheap charging.

Considering the last 12 months of my own consumption vs. historical Agile rates with a battery (charging up in cheapest 3 hours of 12 hour window), I found that average unit rate worked out at about 26p, which is somewhat better than the ~31p from government’s Energy Price Guarantee.

  • Whole year on Agile without battery = £2468 (30p / kWh)
  • Whole year on Agile with battery = £2138 (26p / kWh)

Now, I’m mindful that the prices last winter were ridiculously high, and Agile has been much more reasonable throughout 2023. Considering only usage and prices since then (240 days), the unit price came out about 20p. However, there’s no telling what will happen to Agile prices this winter…

The battery results in an annual saving around £300 in the analyses above, with a ROI over 15 years. Could maybe improve this with more intelligent charge management, like Kevin’s been doing. Compared to my analysis with Cosy, which yields a saving of £700 and ROI of 7 years.
Caveat: numbers likely to be very different if without a heat pump, so apply salt liberally.

That’s very useful, thanks. I think charging twice a day is the magic bit that makes the battery really worthwhile, making it effectively double the size for the same installation cost.

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It takes a few seconds with the Octopus Watch (SmartHound) app to work out for both day and night. I just manually set charge times via my ESS app and base it on 3kw/hr charge rate.

Obviously depends on what prices do this winter. As @Zarch reminded me, you can always switch tariffs if it’s too volatile. Although I don’t think you can switch back for another 6 months.

You can switch between their ‘smart’ tariffs every 30 days.
It’s only the tracker tariff that has some 6 month limitation onit.

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I have long wanted more battery storage, as it often runs out before sunrise*, but whenever I simulate that extra capacity I find the savings gained are way smaller than I’d hoped, making me think it’s just not worthwhile (for my numbers at least).

(*I’m aware that with Cosy or Agile or Go, I’d only need the battery to last until the cheap period).

So, I instead consider the Grid to be a battery with at least 50% efficiency: export at 15p, buy it back at 30p (or less). This “battery” costs nothing to install, has infinite capacity, and no self-consumption that a BCM would have. At the end of June I had 75 kWh “stored” in the grid, which lasted though a very wet July.


That is interesting.

With a battery only system you could charge twice a day. Cosy would make the returns easy to calculate as well.

This might be stating the obvious so I say with caution - but in addition to charging the battery you would also be heating etc and may be able to modify household use to benefit from the dips. Although the 3-7am one is more tricky (aside from heat pump use). One thing I’m not sure on is whether if the battery is set to charge from grid, and then drops to say 50% of charge during the charging window, it keeps attempting to charge to 100% so could get additional marginal benefits there?

I guess cosy you have to make the decision based on a more predictable rate albeit affected by overall wholesale price.