@johnmo will be interesting to see. Not sure if thermostat accuracy in ashp and/or immersion will enable a precise figure? May need to run similar test (say) half a dozen times to get confirmation?

As a matter of interest we heat our hot water to a miserly 43deg only. But find that's fine and never run out - and still have to add cold water for showers/bath/washing up. But I realise in your case you may as well heat to 50deg or more when you have excess/free solar. Depending how much you can be bothered to move it around you might try lower temps when importing from grid?

So far I have not used timed slots for DHW heating but now I'm on Agile I guess it should be considered more

@tim441 Working with Agile slots can be a fun game! The actual times do vary but there tends to be core times when the cheapest rates apply. The core time is rarely long enough that one might ‘set and forget’, there is often a need to go a stage further and find the next best cheapest slots. This is where the fixed slots for Cosy cheapest rates are ‘faff free’ and thus attractive. Also, what is the cheapest Agile slot one night/day may not match the following night/day. (Just to keep you on your toes of course!) Regards, Toodles.

Posted by: @tim441

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Not sure if thermostat accuracy in ashp and/or immersion will enable a precise figure?

My cylinder is remote from the heat pump so have a digital thermostat on the cylinder so will be able to make a direct comparison.

Posted by: @tim441

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a matter of interest we heat our hot water to a miserly 43deg only.

Biggest advantage I am seeing from getting away with a single DHW heat, is during the heating season not running out of battery and taking in full cost electric (30p instead of 14p kWh on E7)

Thanks for useful inputs! It's stimulated me to set some DHW schedules on my ashp. Bearing in mind I'm on Agile

I'm trying:

01.15 to 03.45 50deg

04.00 to 15.45 43 deg

15.45 to 19.15 Off

19.15 to 01.15 43 deg

But also do not want to run out regularly (occasional boost is ok) 

Coming back to the original question, @tim441, I think all the key points have been made in this thread but not necessarily articulated together as one complete answer. Obviously the spin one puts on those individual points can vary but my take is as follows.

• Since you have about 25kWh of battery storage, you can store plenty enough energy to tide you over the bulk of the prime time period (i.e. 4pm - 7pm). That's obviously the expensive period on the Agile tariff and certainly not cheap on Cosy either.
• The cheap periods under the Agile tariff have historically almost always been significantly lower unit costs than even the most competitive standard tariffs and at least comparable with the cheap Cosy unit costs.
• Ensuring your batteries are filled at cheap times and used at expensive times will, in practice, mean you have to
  • get clever with charging schedules,
  • manage the battery charging/discharging settings manually or
  • set up some form of automated system (home automation of some kind) to manage the process for you.

If you get all that covered off, being on a TOU tariff and making it work for you will, I believe, give you about 95% of the benefit. Switching to a different tariff for winter might gain a small extra benefit but only a minimal one and for me it's not worth the hassle. Whether or not different methods of running your heat pump (set back or no, for instance) provide a benefit is a question you can only answer by a bit of trying it out since each installation is different as are your own requirements. However, that trial and error does not affect the points above so won't significantly (IMHO) affect the decision of whether to switch tariff or not.

@majordennisbloodnok yes, I think that captures most points.

Probably the only other point to add is that in our case the inverter max charge/discharge rate is only 2.6kwh. This means I cannot maximise the potential as the ashp might be drawing say 5kwh at times. But hopefully careful use can minimise peak usage.

Other people are likely to have different rates especially if using GivEnergy All-in-one or Tesla Powerwall etc. 

Posted by: @johnmo

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As an experiment I have deleted the timed slot in the morning for ASHP heating and instead replaced with immersion heating for 1hr. So will in theory use 3kWh, have the immersion thermostat set to max temperature so it's not getting in the way. Will have a look first thing tomorrow and check the temperature it has settled too. But also check if it reheats in the afternoon heating slot.

So first day ran immersion for an hour and we used 3kWh. But the heat pump DHW cycle still occured in the afternoon. So this morning increase to 1hr and 20 min immersion timer. Heat pump still runs in the afternoon. We used 4kWh on immersion this morning compared to 1.7kWh via heat pump. This afternoon run for DHW, I was able to record CoP (13 degs outside) and it averaged out at 3.5. So will leave on ASHP heating.

@tim441 since you have givenergy batteries you can use Octopus R&D. It was very quick to set up and allows multiple charge slots per day and charging using the agile tariff. No need for HA or Python!

I’ve spent ages on excel modelling cosy tariff and concluded even with round trip efficiency on our9.5kWh battery and assuming annual heat load the same as gas then we’ll pay the lowest cosy rate plus 25%.

If we feel very confident we might move to agile later. 
The ASHP is still being installed so reality might be different from modelled so we’re staying in the certain range rather than two lots of unknown territory by moving to agile too.

@judith thanks for the reminder to look at that again. I need to see if I can achieve fairly simple charging options... 

I think my ideal is something like:

1. Charge battery whenever rates under say 13.5p. Obviously prefer to tweak and maximise at lowest rates - manually if necessary. Depends a bit on solar prod, weather, season etc

2. As I have ashp I will also charge at higher rates if necessary in winter - say up to 19p

Main thing is to maximise battery with cheapest rates while avoiding peak Agile

As things are I use WonderWatt but tweak manually every day. Would ideally have a setup I can set and leave.

With Gen1 inverter max charge/discharge rate is only 2.6kw which limits options for other tariffs I feel.

Always a bit tricky comparing tariffs as it's likely that usage will change to suit. E.g. using setbacks on thermostats at peak times

@judith On Cosy here and I think that with 27 kWh of battery, I’ll only need the cheapest 8 hours of Cosy tariff per day to keep our ASHP in the style to which it has become accustomed along with all the other house demand added. The battery will provide all that is required over the other 16 hours each day; during the charging 8 hours (3, 3 and 2 hours that are spread out over the 24 hours), the grid will be powering the house cutting out the middle man.

I am still trying to evaluate whether the fixed 11.2* pence on Cosy will be a better arrangement than trying to chase the Agile cheapest HH’s, but, my feeling is that allowing for the ‘Faff Factor’ with Agile daily resets, Cosy might be the way to go. A few more months on Cosy and I’ll reflect upon this and report back. Whereas Agile has had some plunges during July and August, it has also had minimum rates well above Cosy lowest.

* 11.1 something pence per kWh.

Regards, Toodles.

@tim441 You might already know this but in case you don’t the information about how to find your giv energy API is on givenergy community and octopus R&D is from a facebook page. You need to be registered on both groups.

To shortcut that here is a screenshot of both. I hope this works (both Octo and attachment)

@toodles 

the comparison between agile and cosy will be close.

From energy stats this is the hour hour slot prices on agile for the last year which are very similar to agile low cost 11p/kWh. If we added a further battery to our inverter then we’ll need to change to get the hours of charging time needed (3kW max).