Add a battery to my solar?

[Ollyfp]

Dear Solar Nerds,
Okay, so I'm currently running a simple south-facing 2.5kw solar setup that reduces our bill quite significantly, but I'm planning to add a battery in order to maximise the benefits.
I recall the original installation engineer saying that my inverter wouldn't support a battery, so I'm hoping that one of you lovely people (with large foreheads) might be able to tell me how I can achieve my goal... preferably without selling my organs on the black market.
Thanks in advance!

 

[ericmark]

Solar inverters are progressing in leaps and bounds, and it would depend on the battery size, but the tariffs for an inverter which you can allow the billing agent like Octopus to have control can be a lot lower to those where you can't, so I would start at asking the billing agent what they have on offer.

It has reached a point where independent solar installers are having a problem competing with the billing agents, I am just a user, and I started with a 3.2 kWh battery, my inverter will take up to four batteries, and I found I was running out of battery before the night rate started, nearly every day, I use around 10 -12 kWh per day, I doubled up on the battery so now have 6.4 kWh, in summer this is enough to take me to 00:30 when off-peak starts, at 8.5p/kWh off-peak and 15p/kWh export it is better to import overnight and so then export more in the day. My 6 kW panels and 5 kW inverter means summer they make money for me.

Winter is of course different, and the charging of the battery overnight, will extend how long it lasts in the day, but batteries do have a limited life, so no point going silly for just December.

So if I take Octopus there are two tariffs, "Go" and "Flex" go is really designed for EV charging, but it's off-peak is cheaper to payment for export, so no worries about using too much off-peak, the flex is designed for solar panels, but if you run out of battery before 7 pm it can work out rather expensive, so with a large battery say 12.8 kWh the flux is likely the best, but with my small battery 6.4 kWh go likely works out the best. That asked if I had an EV, I answered I have two e-bikes, and that was accepted, with Octopus, and with British Gas before that, they suggested their EV tariff.

My batteries also give me a back-up supply for my central heating and freezers, so 10% held back for the back-up.

Likely, per £ spent, the batteries save me more money than the solar panels. There is a knock on effect, because I have an off-peak supply, I also in Winter run dishwasher, washing machine, and tumble dryer overnight, the latter is heat pump type, so unlikely to have a fire problem.

 

[Murdochcat]

How old is your inverter? Ours doesn’t support batteries and is about 6 years old so I’m not going to change it while it still works as i don’t think the maths add up to justify it

If you have a hot water tank you should seriously consider a solar divertor so any excess electricity provides free hot water

 

[ericmark]

If you have a hot water tank you should seriously consider a solar divertor so any excess electricity provides free hot water

I had one fitted, an iboost+, it was in hindsight an error, as I get 15p/kWh for export, and pay 8.5p/kWh for off-peak, so a simple time switch would have been better.

At around 15 kWh per week, at 8.5p/kWh that's around £65 per year, so not sure if worth the cost of the unit. They were designed when people got paid to make solar energy, it did not matter if exported or not. The other make I see can be also used to heat electric UFH, but as to if you get enough solar in winter, not so sure.

It is not worth me swapping the iboost+ for a time switch, but if it fails, then will be replaced with a simple time switch, the iboost+ does use off-peak as well as solar, but I would be likely better off removing the batteries from the CT on the DNO tails. Also needs distance between CT coils, so it depends on how cramped the space is where the meters are.

Remember it is not "free hot water" as not being paid for that export, it is costing likely depending on tariff around 15p/kWh.

 

[Murdochcat]

I do not get any FIT

 

[Ollyfp]

How old is your inverter? Ours doesn’t support batteries and is about 6 years old so I’m not going to change it while it still works as i don’t think the maths add up to justify it

If you have a hot water tank you should seriously consider a solar divertor so any excess electricity provides free hot water

We had the ASHP and solar panels installed a year ago (simultaneously). The inverter that was installed doesn't support a battery. Recently, a sales rep for another company (there are millions of them) suggested using an AC charger? Currently, the power generated by the solar setup is only useable when the sun shines.

 

[ericmark]

There are stand-alone batteries like this

and also the likes of iboost+ can divert up to 3 kW into storage, there may be one which can divert it into a battery.

But the main problem is grid tie, and the forms required, G99 or G98 and I have not a clue as to how they are issued, but seems it needs a solar installer who has passed the required exams and is a member of the correct club.

So even as an electrical engineer, I had to pay for some firm to fit the hardware, and configure the software, and submit the correct forms. It is not a normal electricians job, it needs a specialist to oversee what is done, even if they don't physically do the work.

 

[nero44]

@op, hypothetically do you have more roof space to make the array bigger? 2.5kw system is rather small IMO. A battery setup needs to be installed correctly with a good quality lithium phosphate. Your ASHP will require a lot, so it would need to be decent sized system. That does require some investment, which I am not sure the returns are there with such a small system. But of course the devil is in the detail, e.g. do you have some sort of export tariff (which then as eric pointed out requires grid connection via G98 or G99).

If a full revamp is too much, then maybe another way is just to use timers etc to have your appliances run during the day to reap most of the benefits.

 

[ericmark]

I want the solar system to provide over 3 kW, as that is the default size for any high-powered plug in appliance, so my inverter can supply 5 kW, the solar panels at 6 kW, but really that does not matter so much, as rare one will get the design output.

However, the battery and inverter needs to be able to provide over 3 kW to really be much good.

In rule of thumb, a lithium battery can deliver about the same in kW as its size in kWh, and can absorb about 2/3rds. So my 3.2 kWh battery would give out 3 kW and charge at 2 kW. But since there is always background use, really need to deliver around 4 kWh, so I doubled up, to 6.4 kWh, which means it can deliver 5 kW (limited by size of inverter) and charge at 4 kW, so rare will power be exported when there is room in the batteries.

As said my inverter is 5 kW, the minium size is around 4 kW, below that and washing machine, or dishwasher will always draw from the grid.

I use around 12 kWh per day, so with a battery of 12 kWh even with no solar, I would only pay off-peak charge. With 6.4 kWh if I have a day with no solar I will end up using peak power, but what I look at is how often will that happen, and I think only December, so having batteries with limited life just for no month, does not seem viable.

60 amp = 13.8 kW, so for 5 hours, one could have 69 kWh of battery, in real terms more like 40 kWh, as everything does not stop at night, but to power a heat pump, the battery size would be silly. So we move into near enough engineering.

So some off-peak can be used by increasing the temperature 00:30 to 05:30 and reducing the temperature in the day, in the day rooms of course, don't want to be sweating cobs all night, but this flies in the face of modern thinking, of only heating rooms when required to temperature required.

The fly in the ointment is the heat pump, my boiler is 20 kW, and I have a three-story house, so 20 kW would be stretching it to heat the whole house, so simply we don't. Each room has a programmable TRV head, and is only heated when required, and only as much as required, whole house electric supply is only 13.8 kW, at least ½ that is required for the house, so around 7 kW for the heat pump, can be 300% efficient, so around same as 20 kW boiler, but that efficiency depends on the boiler being near the 100% output, so summer the maths is easy, but winter it is very complex.

 

[Ollyfp]

Thanks all for your input re this. Having digested this information, I have decided not to go ahead with the battery at this juncture.
It seems that our energy usage with the current setup is very low anyway. We're using about 4kw per day mid-summer (that figure doubles during the winter).
At least we're getting 'free' juice when the sun shines!
Thanks again!

 

[ericmark]

I looked up my solar inverter

and I have 2 batteries 3.2 kWh each, from memory just over £1000 each, so to have a battery backed system looking at £4k. It does mean I rarely use peak power, since moving to Octopus the first 6 weeks, two with no payment for export the total bill was £2.39, that is with 6 kW of solar panels, and we use around 12 kWh per day.

At your 4–8 kWh, you should also be looking in summer at zero bills in summer, without solar, 4 x 23.86p - 8.5p = 61.44p per day, taking 6 kWh average that's £336 per year approx saving as paying off-peak instead of single rate, so around 12 years pay back time, so yes can see how it would not really pay for you.

For me, I used twice as much, so 6 years pay back time, at 73 not sure, I will get my money back.

 

[Ollyfp]

I looked up my solar inverter View attachment 387793and I have 2 batteries 3.2 kWh each, from memory just over £1000 each, so to have a battery backed system looking at £4k. It does mean I rarely use peak power, since moving to Octopus the first 6 weeks, two with no payment for export the total bill was £2.39, that is with 6 kW of solar panels, and we use around 12 kWh per day.

At your 4–8 kWh, you should also be looking in summer at zero bills in summer, without solar, 4 x 23.86p - 8.5p = 61.44p per day, taking 6 kWh average that's £336 per year approx saving as paying off-peak instead of single rate, so around 12 years pay back time, so yes can see how it would not really pay for you.

For me, I used twice as much, so 6 years pay back time, at 73 not sure, I will get my money back.

Really useful! Thank you!!