Calling all Solar experts - need help speccing battery

[ollie]

Hi all taking the plunge with solar.

I've done some homework but could really do with some advice from somebody knowledgeable.

Quick bit of background

House is a well insulated new build, all electric.
ASHP - for heating and domestic hot water
Total annual electricity consumption circa 8500kw 

height of summer - approx monthly usage    530kw

height of winter - approx monthly usage        850kw

Currently on a unit rate with octopus of 22.30p/kWh (I fixed just before the madness) but we in for a shock when our tariff ends in september hence solar...

I'm looking at going with a local company but Zannsui gave an original quote that estimated an
annual solar output of 5,142.00 kw based on 15 x 400 w 

Spec from local company

6.48Kw (16 x 405w)
8.6KW Fox ess hybrid energy cube battery (2x4.3)
£13,906

vs

same panels but 17.2 kW (4x 4.3w battery)
£16906.4 

The crux of my question is how many batteries should I go for?

Reading up on here the general sensible thing to do seems to be have a split tariff and charge your batteries at night on a cheap tariff - 
is the general consensus just get a much battery as you can afford
or would i be overspeccing with the higher battery capacity

one thing to note is i can set my ashp to prioritise operating in offpeak tariff hours (not sure if that relevant to this or not)

any advice greatly appreciated! 

thanks,
Oliver
 

[S2D2]

It depends entirely on your usage, see here for an outline of the method to optimise the size:

 

 

Installers will almost always push you to get the biggest battery they can sell you, but the diminishing return drop off is very aggressive for the consumer.

 

Flux changes the calcs considerably, but could disappear at any time so best not to plan on it but use it if available.

 

 

[DougMLancs]

FWIW the battery owners I know all say they wished they’d gone bigger but it is down to circumstances. It’s worth checking the specs of the inverter to see how quickly it can charge any batteries too. It’s not as useful having 16kW of batteries if the inverter can only charge them at 2.6kWh and you’ve only got a 3 hour cheap rate window. Also worth double-checking on the MCS website that your installer is certified with them so that you can access tariffs like Octopus Flux. A few people on other forums have been caught out on that recently. Now that Octopus Cosy is out as well then it might make a good winter tariff partner for Flux.

Edited April 29, 2023 by DougMLancs
Extra info on Cosy

[RichardL]

3 minutes ago, DougMLancs said:

can only charge them at 2.6kWh and you’ve only got a 3 hour cheap rate window

+1 this -  key rule of thumb => can you charge the batteries from your off peak window - driven by battery kWh and Inverter size.
No point going larger than that.

The above is basically the winter scenario/worst case/most expensive time of year and IMHO the one to optimise.

Summer will be a bonus with solar topping things up meaning less grid charging up to possible no grid charging.

[Conor]

Wish we put batteries in at the time, even though the economics are marginal...

 

I did the calcs for us a month ago. Very similar to yourselves, all electric, last year we used 8.5MWh. 3 of which was self generated and we imported 5.3MWh. Exported 1.5MWh @ 8.5p unit.

 

We've a 5pkW array, and economy 7. Heatpump only runs overnight. Same for dishwasher. Bill for the year after export payments was £1200. I worked out that batteries would save a max of £300 a year at current rates, by shifting to 100% economy 7 and solar. Meaning payback is at least 15 years. I think it's the similar in most other scenarios, the "payback" is much longer than on panels alone. Also think of what that £8k your not spending on batteries could be used for. Do you have an electric car yet?

 

 

 

 

[PhilT]

Agree with everything said above, especially that the financials deteriorate significantly the larger you go, relative to your consumption. The situation in the market is so fluid that justication will vary from positive to negative with changes in tariff schemes, costs and export rates. If you can justify it, go for a small battery to start with (you can always add later), which will give you the most flexibiltiy to adapt to tariff changes and utilisation; a 3kW battery gives me near enough 200% utilisation i.e. it fully charges/discharges twice daily, programmed to user either low tariff rate or solar or both.

[MrTWales]

I got panels last October and 3 x 3.6kWh batteries.  At the time, I thought - if it was binary as to a decent size or nothing - then it was kind of marginal in terms of cost, but there are a load of things to consider. Personally, I didn't care that much about the extra cost so got a decent sized battery and the price was reasonable (based on the other quotes I obtained). If you are prepared to spend extra as relying on the grid a little less is kind of fun, then go for it!

 

As others, who know way more than me, have said though the marginal benefit reduces as the size increases (as it does for panels once you are past the point of getting enough on the best days). In the winter a decent sized battery never gets full and in the summer you find on most days that there is plenty of battery left in the morning. With my size it does mean that if I have a good day and then a bad one I can still usually get by. Eg if I have have 40% left in the morning so if we use 10kWh in the next 24 hours and there is 6kWh generated then it just about works, but this only really matters for a small part of the year. On the other hand, this sized battery covers a typical day even with hardly any solar so it would work perfectly if I can ever get on a Flux tarrif.

[SteamyTea]

On 30/04/2023 at 07:22, Conor said:

Also think of what that £8k your not spending on batteries could be used for. Do you have an electric car yet?

Yes. You spent some of it on knowing the difference between power and energy.

Shame the rest of the comments have the two mixed up.

 

If the difference was learnt, and understood. There would be no need to ask this question.

It is not being picky about units. It is fundamental to the problem and answers the question.