Posted by: @mikeavison

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Hi

I have been using a heat pump for 18 months and just got some PV panels. Unfortunately I can only fit 8 on a roof. I also had a 9KWh battery installed. Because of the limited PV output I don't think I will be exporting very much. I am currently on the Cosy tariff (for heat pumps) which has 2 cheap periods per day. I find the Octopus web site a bit vague about feed in tariffs. I don't know if I could stay on Cosy and export as well. Any ideas? Or suggestions as to which tariff might suit this system best? 

You may well find that Agile Import is cheaper than Cosy. There are various apps (such as Octopus Compare) which will download your half hourly usage and then tell you what you would have paid on the various tariffs. This will give you an idea of which is cheapest.

@ivanopinion Another consideration being that Cosy offers 2 x 3 hour intervals that are not evenly spread over the 24 hours thus leaving a larger lump during the evening and night before the next 3 hour slot of the cheapest rate. I find that generally there are more (somewhat cheaper) half hours spread around the clock which with a battery storage system available, allow one to charge up at the cheapest times and be better off than with Cosy. Admittedly, I have 27 kWh of Powerwall and am retired with a First Class Honours in Scrooge Economics with Nerdism.😉  Regards, Toodles

Thanks both of you for your helpful tips

@mikeavison Here is a table of Octopus import and export tariffs you can combine

@mikeavison Perhaps I might add a few observations? I should declare they are my opinions only - they may be shared with others but please have a supply of salt to hand nevertheless.

Agile  vs. Fixed Outgoing: With Fixed, the rate is always 15 pence per kWh but Agile outgoing is variable and generally less than 15 pence, in fact I have rarely seen any HH’s where the rate is more than about 13-14 pence - usually less.

Now, Cosy, this is approximately 17p p kWh for just 6 hours per day (unevenly placed at that) the other 18 hours are somewhat more expensive.

Now Agile import, most days, there are some HH’s where the rate is below 15p p kWh and often sub 10 pence. There are also times where the rate falls below perhaps 7p p kWh., there are also times where’plunge pricing’ are applied - when OE pay you to use energy! Having storage in the form of a Tesla 27 kWh powerwall, I import at the lowest rates and completely shun the grid for the rest of the day allowing the battery to supply everything that the PV cannot. I also set the Tesla app so that I try to start the Solar PV generation time with a full battery and allow the panel’s output to be exported (at 15p p kWh). On the rare days that the cheapest Agile HH’s are 16p p kWh or more, I will use the PV energy to power the home instead. Basically, if I can buy for 12 pence or less and have a ‘full tank’, then exporting is very worthwhile.

Without battery storage, much of the foregoing would be tricky and of course, with a heat pump running on a very cold day, every Agile HH has to be carefully scrutinised! (That degree in Scrooge Economics and Nerdism I’ve mentioned comes into play!)

One more thing (Sorry Dr. Michael Moseley) during the winter, Savings Sessions can be very useful and the Surprise Sessions sometimes pay as much as £4 per kWh. In the 23/24 winter period, our bill was credited with over £300 for our efforts. For some of that time, the Tesla app had not been set to allow ‘export everything’ or we might have helped out a little more during the peaker plants running times. Regards, Toodles. 

Posted by: @mikeavison

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just got some PV panels. Unfortunately I can only fit 8 on a roof.

I'm jumping back in to the starter for this topic, because I'd appreciate some clarification...

1: What's the max output of those 8 PV panels?

2: Are they connected to a solar-input on the 9KWh battery (DC connected), or to a separate 'string' inverter (AC connected)?

3: If AC connected, then are the battery and PV inverter 'known to each other' and operating as a matched-pair, or is there no intelligence shared between them via a data-link?

That last question will have affected the application made by your installer to the regional Distribution Network Operator.
An AC-coupled system has two devices with export capability at the same site, and that requires G99 approval from the DNO.

Even if you don't know how the battery and inverter are connected, the installer should've given you the letters from your DNO.

As an aside, it's interesting to me that the discussion above has been dominated by the pricing structure of importing/exporting electricity.

As we move towards Net Zero by 2050, we will need to migrate towards a mechanism based on energy measured in joules rather than £-sterling.
The present tariffs aren't a good match for what's actually being generated from renewable sources.

Making choices based on price is presently increasing our reliance on gas-fired generation plants.
That's why the Dept Energy Security and Net Zero (DESNZ) changed the rules on 10th March to facilitate more gas-fired generation plants without any requirement that they be fitted with carbon-capture technology.

The British Energy Suppliers are currently running circles around us.
Too few consumers understand enough about energy supply to realise what's happening.

Hi @transparent the answers to your questions are 

1 about 3300

2 DC coupled

In answer to your last paragraph. I agree and count myself in, but the system around energy pricing and generation switching is too complicated, while in this state  (no doubt held there by the fossil fuels suppliers) you cannot expect many consumers to understand it! It just won't happen. We need a complete reform of our energy strategy we also need an industrial strategy. Afaik none of the parties offer either, though I confess I haven't read the manifesti yet. I'll be interested to see what GB Energy will be 

Posted by: @mikeavison

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We need a complete reform of our energy strategy we also need an industrial strategy. Afaik none of the parties offer either, though I confess I haven't read the manifesti yet.

I attended a National Grid Community Energy workshop last week.
The organisers (Regen) held a brainstorming session which resulted in a document to be sent to the next Secretary of State for Energy.

If you want to have a say in the energy future of GB, there's still 4 days to respond to the DESNZ consultation Delivering a Smart and Secure Electricity System.

The number of respondents to these is pifflingly small. So those that do, have significant impact.

You don't have to answer the all questions, and the responses are free-form text format.
That means you can write what you want.
I've taken the opportunity to revise the question in a couple of instances where I felt it hadn't quite asked what they needed to know.

@toodles found this thread and was reading it when noticed it’s a year old. 
Did Octopus cosy only have 6h of cheap power last year?

This year 2025, it’s 3+3+2

I use these periods to boost the HP up 0.5C so it saturates the UFH slab with heat which takes me thru to the next period of cheap electricity.

i use these periods battery charge up on cheap periods and discharge during the eye watering 4-7pm period.

So far this year I’ve managed to achieve over 95% usage at the cheapest tariff, only December left to get a picture how useful cosy tariff with fixed export, 5kW PV and 5kW battery is for our well insulated 4 bed semi

@agentgeorge I can’t recall when and the details but Cosy did increase to 8 hours at some point. I cannot recall more than that though. Regards, Toodles.