Thanks for this, an interesting exercise. I had a bash at my own calcs this evening, and I think Cosy ought to work for us, with somewhere between 5-20% savings compared to our current bog standard Flexible rates, depending on how well we manage to shift our usage.
I reckon we can shift practically all of our DHW and a reasonable chunk of our space heating demand into the low rate periods (of course offset somewhat by inevitable cookery in the peak period).
And once our battery gets installed, it’ll most definitely work for us.
Just switched from Flux to Cosy at the weekend as we are no longer exporting enough to offset our standing charge + usage.
With the Powerwall and the double dip most usage is being mitigated to the 16.5p rate and there is still some solar top up in between. SE England times are 04:00 - 07:00 & 13:00 - 16:00 and 16.5p/kWh
Due to complaints from “management” the heat pump is only allowed to start heating at 06:00 but as we have a setback of 19 and a daytime running of 21 that seems to be enough to get a good chunk of usage out of the way and start the reheat. With the 5kW inverter once the Daikin HP is running and has done its “start up and use a chunk of power” thing it runs happily on the Powerwall leaving enough capacity for daily usage and appliances.
The battery is able to fully recharge in the window meaning we have ~ 25kw (after some losses) to get us through the day (both WFH so easy to do all the housework during the second dip / on battery) and then through the evening and night. It seems to be working but we currently have a gas oven and hob so evening use is limited.
Once we have a little more sun again (March / April and we are exporting around 50% of our usage, provided prices stay similar we will probably switch back to Flux as we seemed to build up a nice bit of credit for the winter months.
Unfortunately with no EV we cannot use Go or intelligent as with the 13kW battery that would have been ideal. I was looking at a cheap leaf to both reduce CO2 further and allow us to take advantage of the cheaper rates but the MOT reminded me I do circa 2,500 miles a year and most of that is visiting family using longer motorway runs (not something a cheaper leaf would be good at).
Well, based on an [n = 1 day at these specific temps & usage] sample from our first day of Cosy, we managed to save ~21% of our heating cost, purely from shifting the DHW runs into the cheap periods, and boosting the heating in the afternoon cheap period in order to coast across the following peak period (so using the house as a thermal store. No battery yet; hopefully coming in January).
One thing I noticed was that on our current heat curve settings (I’ve aimed for as low as comfortable for efficiency) we didn’t hit the target indoor temp in the afternoon heating period (it would if I let it run longer). So it seems to me that to make best use of the TOU tariff from a cost perspective, there’s a fairly solid case for bumping up the heat curves a bit, so as to store more heat ahead of the peak period - does that make sense? Anyone else using that strategy?
I have my heat pump set to run in Quiet Mode for 21 hours of the day, which limits the power to ~50%, and keeps the house warm enough. During the 3 cheap afternoon hours the heat pump is allowed to use as much power was it needs, though practically it mostly stays at the same level anyway.
If there’s some way to dynamically adjust the WC curve for certain hours of the day, I guess that might help optimise the utilisation of cheap tariffs.
Thinking about it some more with the aid of the heat curve diagrams, mucking about with the heat curves is probably far less effective than simply bumping up the temp by a degree during our “heat charging” sessions. Plus the scheduling for that approach is already built in!
Next question: this morning it didn’t top up the DHW tank, as the measured tank temp hadn’t dropped far enough. But we really need a full tank for the morning showers. So how can I force it to top up? Don’t want to (wake up at 4am and) manually boost it. Given that I have it scheduled for the cheap periods only, should I just tighten up the hysteresis as much as possible, so it has the highest chance of triggering? Does that seem like a plausible solution?
Yup, that seems to be a common strategy that many people use.
If you want to ensure the tank is fully heated every day, then that’s probably the way to go about it. This will likely result in lower COP, but that’s maybe not a concern during the cheap hours.
Thanks. I’m not sure I’d anticipate much drop in CoP on our case, as it’s being run in fairly solid blocks of operation during those specific periods, and then usually emptied a significant amount in between. We have high DHW usage in our household of seven, so the challenge for us is whether even two tankfuls will get us through the day!
It is a bit of a shame the emoncms Octopus APP to compare prices has not been extended. Just has Agile and Glow, but would be really interesting to see what the effective import costs would be if different tariffs had been chosen and adding in any export earnings as well would give folk a really good idea of what tariff would suit them best.
I used this App to determine if I’d be better on on Agile (which I am).
Just in case it’s of interest, I’ve found that with heat-only load shifting, we’ve managed to hit a consistent 14-15% saving over the standard Flexible tariff:
Only one data point, of course (would be harder to hit that on a colder day), but a promising one nonetheless. Looking forward to further improvements with PV now hooked up, and export tariff and sunnier days to come.
Turns out that prices have been much more reasonable this year. Cosy has been great so far, saving £100 in a month over standard tariff, yet Agile would have saved me an additional £50. So, I’ve switched.
I think this works out because I’ve shifted my heating and charging schedule to fit Cosy, which also works well for Agile. Additionally, Agile is cheaper after midnight when my battery has often run out, and has a lower price overall.
We’ve been Octopus Tracker over winter and it worked out really well, but with the recent Tacker price increase of +3.1p/kWh (for our region) I’m now thinking of switching to Agile as well. As long as I don’t use too much in the evening peak period I think it will be better
You really need to use the hamburger menu (top right) to explore all the things it can do.
It’s helped me work out the penalty I’m paying for not being on Agile. We’ve agreed as a family that we’re willing to pay that penalty for a simpler life. I actually moved off Agile and shut down a bunch of my robots.
I’ve spent way too much time thinking about our heat pump this winter
I’ve been considering battery storage for years. Recently I read some of the posts on the Givenergy forum with people struggling with sudden drops in state of charge, batteries not delivering the stated capacity, firmware updates etc. It did make me think that I don’t need another system that takes time for me to try and optimise.
I did see one post asking: do Tesla Powerwall users have the same problems? And that made me think that perhaps paying a premium for a more stable (?), less user-configurable system might be a good choice.
I am pleased we were on Octopus Tracker this winter as it meant I didn’t have to consider (worry?) what time of day our heat pump was running. Now I’m thinking I know enough that I could generally avoid heating during most of the 3 hour peak period in Agile.
I still don’t know what to do with battery storage and a heat pump; I doubt I’d install enough battery capacity to only use off-peak electricity on Octopus Go during most winter days, so Agile would seem the most likely tariff. But that means the cost savings would probably be fairly small.
Probably a topic for another thread, if it doesn’t already exist!
Just jumping in on this thread, I’ve got an 11.2kw Ecodan heat pump and 5.8kw of battery storage (+ solar). I’ve been on agile since May 23. This is my approach:
1.Use the smarthound agile app to find the two cheapest points for 2hr blocks in a 24hr period. (Usually early morning and often late afternoon, just before the peak).
2. Set the battery to charge for those blocks.
3. I’ll often run the heat pump over night (often from about 1am - 6am and do a hot water charge at 5.45am). I do flex this.
4. Avoid the morning peak and evening peak during the day. Often ill leave the heat pump off between 6am and 9am and then use the battery to bridge the rest of the gap / helped by solar from about 10am. By the time I’m out of charge I’m in to cheaper electric prices from about 11am again.
5. Run heat pump during day and then switch off again from 5pm - 8pm to avoid the peak and use the battery to supply household load trying to avoid the peak costs.
So far it’s worked pretty well with Octoprice suggesting I’m saving 54% against standard variable prices and giving the following stats when I compare tarrifs using my data.
Really interesting post, but I just wanted to see if you had calculated the battery cost option in your methodology as I couldn’t see a reference to it.
When calculating the unit cost for a battery it should take into account the installation cost of the battery, divided by the capacity, divided by the predicted number of lifetime cycles, multiplied by the unit cost to charge it. Without factoring in the costs of the battery too, any cost comparisons are pretty meaningless.
I had not factored in the cost of the battery as it was installed as part of the solar PV system, and factored into the ROI of that. So, any additional savings by using the battery for load shifting is separate from the install costs.
I could recalculate the ROI of the initial install with the additional savings of having cheaper periods, which would knock about 1 year off the payback time.
This comes to 1.56 p/kWh, so an extra 10% to the cost of charging from the grid, if that’s the way cost of the battery is being considered.
