I too have wrestled with this question with the race against break even time and anno domini, and I just can't make it work with my sensible head on. I can see a good case in usage terms with my geek head on but the harsh financial reality wins out.  I would love to see how that pay back time you quote is worked out. It seems very optimistic. I also suspect that you have a fairly large heat pump so running it from battery storage is going to be for a short period with a 3 or 4 kWh draw.   I have never found if battery storage can supply the HP start up current. I assume that to be okay as many here report having both HP and storage.

Alfapat, Our annual overall consumption is approx. 10,500 kWh with an 8 kW Daikin ASHP. We live in a 1930’s 3.5 bedroom semi with good insulation including underfloor but solid floor in kitchen. Double glazing throughout and cavity wall insulation plus 300 mm+ of loft insulation. We are on Cosy tariff (I recently did a write-up about having battery and how Agile worked compared to Cosy for us).

Now to the point, we have 8.1 kWp. of less-than-ideally orientated panels on roofs and in garden. We export during the more productive months and last year produced 6 Megawatts, most of which was exported to OE Outgoing Fixed at 15p. per kWh. The more productive months of course are during the time we are not using our heating (DHW is separately handled by Sunamp Thermino heat battery). The outcome of this is that as we have 27 kWh’s of Powerwall storage, we can ‘tank up’ during the 8 cheapest hours of Cosy offerings and let the battery take the strain for the other 16 hours each day.

As I see it, with your consumption, Octopus Go with its’ 5 hours of cheap rate will allow you to charge the 13.5 kWh of battery you are considering but with 19 hours left in the day, you will be using a fair percentage of your consumption outside of those 5 hours and I suspect that would entail a fair few kWh’s being purchased daily at the higher rate. In our case, the PV provides no more than a few kWh’s per day from November to the end of January so not a lot of help to the heat pump! I have also written an article on our set-up:

https://renewableheatinghub.co.uk/green-transformation-my-journey-to-a-sustainable-and-energy-efficient-home

I am thinking of changing to Octopus Go once the higher winter consumption has dropped again as we will be able to recharge at the Go rate and still export during the day for approx. twice as much!

Regards, Toodles.

@abernyte I can send the quote , if you can understand the figures on it I would love your opinion , I’m just a bit in the dark .

Posted by: @alfapat

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I have had three quotes for a system installation , a big decision I feel at £14300. A 7.29 kw System size comprising of 18 x Q.peakDuo BLK405w panels . 5 kw Q.Volt invertor and 13.6kw Q. save Battery storage. I use about 14671 kw / year as I have Air source Heating . 

Apparently the pay back time by using Octopus Go would be 6 years 4mths  I am retired but is this a worthwhile saving as presently spending £324 monthly but could very well go down as I am only in with Octopus this month being January.

Hi @alfapat, I'm getting together figures for a self-built battery storage system (will post soon on another thread) but like others, was intrigued by the claimed payback period of 6 years 4 months - particularly since that's what I am expecting with a home-built battery and self install!

A quick back-of-the-envelope calculation yields the following:

• Assume off-peak rates of 7p (7 hours) vs day of 25p are not used currently. 
• Assume night hours consumption annually is 0.36 of total (this is my measured example with an ASHP for heating and hot water, solar water panels provide help in summer)
• Just switching to a night tariff e.g. E.On Next Drive (if you have an EV) will save (25-7)*0.36*14671 = £950 per year, possibly more of you can shift more load into the night hours. This contributes almost half of the system cost without having to spend anything, so it's worth checking how the proposed savings are calculated, and how much Octopus will save. 
• Extending this, if the battery were able to support all of your daytime load (which it is not likely to on coldest days), and there is no solar contribution (so we're just extending what happens on cloudy winter's day over the whole year), then all of your electricity consumption is provided at the cheap rate (the inverter size to charge and discharge needs checking here too). The total savings, ignoring conversion losses for the moment, are then (25-7)*14671 = £2640 per year. (Your current costs, including standing charge you state as 12*324 = £3888. )
• So, without solar panel contributions, 6 years 4 months of savings is 6.33*2640 = £16,711
• Solar PV contribution will surely offset a 20% 2-way conversion loss, so the figures do look in line with the quoted price of 14.3k

The real driver of the speed of the payback is the level of consumption at, assumed, full rate. Your experience with an Octopus tariff may help a lot. 

My own house calculations are based around an annual consumption of 6000kWh, and I'm basing the payback period on savings in addition to running on E.On Next Drive, which automatically brings a 25% reduction in cost, so to get a 6 year payback from that starting point the equipment costs need to be well under half your quotation. 

Hope this helps

Posted by: @abernyte

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 I have never found if battery storage can supply the HP start up current. I assume that to be okay as many here report having both HP and storage.

Yes, we've just been discussing this on this topic  - it really concerns the inverter, not the battery per se. A 12kW ASHP can easily take up to 6kW in cold weather, so an inverter needs to supply that plus other domestic loads. Depending on whether the inverter is set up to let the grid supplement extra power it cannot supply (grid-tied) or if it needs to run when the power is out will lead to very different inverter choices.  

Oh my , , I am completely out of my depth here, if I read you right it’s not a good idea ? 
I you like , I can send you my proposal . 

Posted by: @alfapat

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Oh my , , I am completely out of my depth here, if I read you right it’s not a good idea ? 
I you like , I can send you my proposal . 

Hi, no, quite the reverse, the quotation, based on your historic bills, is probably right in the 6+ year payback, but you should be able to get a decent saving by a new tariff (I don't have the details), and then from that baseline, the further savings will have a longer payback. 

Posted by: @s_gatorator

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Posted by: @abernyte

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 I have never found if battery storage can supply the HP start up current. I assume that to be okay as many here report having both HP and storage.

Yes, we've just been discussing this on this topic  - it really concerns the inverter, not the battery per se. A 12kW ASHP can easily take up to 6kW in cold weather, so an inverter needs to supply that plus other domestic loads. Depending on whether the inverter is set up to let the grid supplement extra power it cannot supply (grid-tied) or if it needs to run when the power is out will lead to very different inverter choices.  

I'm not sure I follow. I have a 12kW Samsung ASHP, and starting from cold, which we do most mornings as we often switch off overnight, the system normally pulls around 2-3kW for around 30mins until the system is up to temperature, then it idles at around 1-1.2kW.

I have seen it pull 4.5kW max during the DHW run, but normally I run this in quiet mode which limits output to ~70% or around 3.3kW, so well within the capabilities of a decent inverter? And again, this is only for a very short period of time, maybe 10-15mins.

According to Samsung's spec sheet, I'd need to be running a LWT of 50C at -7C outside temps to pull 6kW of input.

Edit - there may be a short surge upon start up - my observations above are based on observing my smart meter IHD which only updates every 10 seconds, so my time frame of reference is more like minutes than seconds, and I'm not thinking of being off grid so am happy for the grid to pick up any short (less than a minute) surge.