Best battery set-up for large(ish) solar array but with low export limit set by DNO?

nday

Hi, I have really been struggling to get my head around the best battery setup for my situation.

We have had a 9.57kW solar array installed onto south (5.22kW) and south-west (4.35kW) facing roofs but have been limited to a 4kW export by the DNO (we requested 8kW via G99)

We have only been in the property for a few months and only very recently had a smart meter installed so I am not clear on energy usage. It is a 5 bed 1960's property, 2 adults (1 always WFH through the week) and 2 school age kids and a 17kW Grant Aerona ASHP with rads. Would suggest fairly average usage other than the ASHP this winter.

I am now trying to work out the best battery setup to maximise use of the solar and avoid significant clipping in the sunny summer months.

Any advice greatly appreciated.

Newbie_John

I don't really see numbers working for you.
Your ASHP will consume 50%+ of your total electricity and that would be during cloudy and cold winters when you won't have any generated to "consume" later.

Then your solar already cover you for spring/summer/autumn sunny days.

£5k battery with 10kWh makes every kWh 10p more expensive (divide cost of battery by max kWh it can accumlate over lifetime) so you really need to consume a lot during summery nights.

tacpot12

Newbie_John said:

...
£5k battery with 10kWh makes every kWh 10p more expensive (divide cost of battery by max kWh it can accumlate over lifetime) so you really need to consume a lot during summery nights.

Might this be the case if the family use a significant amount of hot water at all times of the year?

Martyn1981

Hi nday and welcome.

That's a big system, do I assume you have something like a 7kW inverter, that's been capped by the installer to 4kW output?

Just wondering about the setup, as you may need a hybrid inverter and DC side batts to prevent clipping. That's what I have on one system, with 6.725kWp running through a 3.68kW inverter.

Edit - Batts* are still expensive, but the lost/clipped units could be worth ~15p/kWh export, plus you mentioned an ASHP, and the batts could be used to charge up on cheap rate leccy, for use during the day.

*Cheaper DIY products are now available, but I assume for this you will need DNO/MCS approved install, but I'm really not sure, and better battery advice than I can give, is needed.

ed110220

OK I realise it's not for everyone because it was a DIY kit, but I have a Seplos 15 kWh battery. My experience (battery storage in general) is pretty positive. Over the winter I have switched to Octopus Go. I don't have an EV but they never asked or checked. My electricity import is averaging just under 10p/kWh as it's almost all on the 8.5p overnight rate, so that's quite a saving over the 24p or so for a flat rate. If you're using a fair amount for ASHP and have a battery that can cover it, that will be a pretty big saving potentially.

One thing to consider is what your inverter can support. Often the battery charge/discharge rate is quite low eg mine is 3 kW even though it's a 5 kW inverter (ie it can output 5 kW from the PV). That's a bit low for using say the oven and hob at the same time, which is why we do still consume some peak rate electricity. If you have ASHP your peak draw will be higher too. Plus it limits how much you can import overnight - say on Octopus Go 3 kW charge rate over 5 hours limits you to 15 kWh of cheap electricity per night which is enough for us but probably wouldn't be for ASHP. So if you do go down the battery route and need a new inverter definitely consider one with a higher charge/discharge limit.Think about your peak power consumption (kW, say when you have the ASHP on and making dinner) and how much off peak electricity you need to store each night.

QrizB

Newbie_John said:

£5k battery with 10kWh makes every kWh 10p more expensive

I don't think batteries have been £500 a kWh any time this decade. Back in 2020, Pylontech 3kW batts were around £1200, if memory serves?

ed110220 said:

OK I realise it's not for everyone because it was a DIY kit, but I have a Seplos 15 kWh battery.

The Fogstar DIY kits are around £100/kWh, but even pre-built they're only £200/kWh. By the same logic, if a £500/kWh battery adds 10p to each kWh of electricity you cycle through it, those Fogstar ones add 2p and 4p respectively.

Reed_Richards

Your use of a battery will depend hugely on whether you have access to cheap-rate electricity.  If so you can charge the battery at the cheap rate and then use it to power your home or even export the electricity for more money than you paid for it.  This could be highly beneficial during the winter months when you will be generating little solar energy but your ASHP will be using a lot of energy.  If you could do this then it would influence the size of battery you might choose.   

ed110220

Reed_Richards said:

Your use of a battery will depend hugely on whether you have access to cheap-rate electricity.  If so you can charge the battery at the cheap rate and then use it to power your home or even export the electricity for more money than you paid for it.  This could be highly beneficial during the winter months when you will be generating little solar energy but your ASHP will be using a lot of energy.  If you could do this then it would influence the size of battery you might choose.   

I would say that if you've got a battery but not on a tariff that offers cheap off peak electricity (either at fixed times or dynamically) at least in the darker half of the year,  you're probably missing one of the main if not the main benefits.

Solarchaser

You have a 4kw export limit, but an array double that size which means you will be quite significantly clipping in the brighter 6 months of the year.

If I were in your shoes id be looking for battery inverters that can run a parallel setup like the lux acs, and using the excess from solar, that would normally be clipped, to fill the batteries, you have a theoretical 4kw of clip, and few battery inverters will take more than 3.6kw of charge, most are 3kw or less, hence why I suggest two in parallel. 
If you have charger all day on clipped solar, which is essentially free since you would otherwise have lost it, and then set the batteries to force export to 4kw in the evening, then you can earn export for that which would normally be clipped.

In the duller months, you can charge the batteries from cheap rate overnight tarrif and use it to power the house, again the parallel will work best to be able to accommodate your ashp as well as washing machine etc.

The fogstar 15kwh battery packs are pretty much the best bang for buck, I have them, and I'd recommend them.
As for how many batteries,  id say that depends on how much you should generate, vs how much you are (due to clipping) and how much electricity you are buying in a year.

If you are buying more than say 12,000kwh a year, id reckon on 2 inverters and 2 batteries on each for a total of 60kwh and maybe £8k.

But really you need to run the numbers for your use and your generation. 

I'm around a 17, 000kwh user including about 4000kwh of car charging and I reckon that with paid export i can reduce my electricity bill to pretty much zero.
Haven't proven that yet, but trying it now

Qyburn

It's not easy to work out how much storage you'd need to handle all the overflow, so I think you'd need to make some rough guesses particularly about your house use. Just for fun I looked at our best day last year when our 8.1kWp system generated 58kWh. Production was over 4kW for 7.5 hours (0930 to 1700). Just skimming over it looks like the total excess, after exporting at 4kW, would be around 17kWh. You could maybe knock 4 or 5 off that for self consumption.

You also need to look at controls to make sure your system prioritises export over battery charge, otherwise you'll find by mid morning you have a full battery, nothing exported yet, and nowhere for the excess to go.

What size and type inverter do you have?
Did your DNO set a total system size limit as well as export limit? Ours did and that ruled out an AC coupled battery.

Newbie_John

QrizB said:

Newbie_John said:

£5k battery with 10kWh makes every kWh 10p more expensive

I don't think batteries have been £500 a kWh any time this decade. Back in 2020, Pylontech 3kW batts were around £1200, if memory serves?

ed110220 said:

OK I realise it's not for everyone because it was a DIY kit, but I have a Seplos 15 kWh battery.

The Fogstar DIY kits are around £100/kWh, but even pre-built they're only £200/kWh. By the same logic, if a £500/kWh battery adds 10p to each kWh of electricity you cycle through it, those Fogstar ones add 2p and 4p respectively.

Sure DIY projects are cheaper but how many of us will do that? Less than 5%. The 95% majority will give a call to multiple installers and get quotes around £5k.
Meaning +10p cost of each charged kWh.
Even DIY for £2500 means that each your solar charged kWh costs 5p more..