To add more batteries to solar or not?

[sjroberts]

Hi.
i have a 10kw solar array split through 2 inverters. Each inverter has a Growatt 6.5kw battery.
We use no oil or gas, all our energy is electric, gshp for heating and hot water, electric hobs and oven.
It’s the beginning of April and we are charging both batteries and using appliances and still have more power that we could be utilising. I think i could probably charge another 2 batteries from this time of year onwards.
Now onto the question. At present the battery storage is lasting till about 02:00 so i would need another 5 hours or so of battery storage to not pull anything from the grid. I know the dark hours are getting less and the days are warming up so the heating will be used less but im struggling to make the decision on if the extra batteries are worth the investment.

Advice and opinions please

 

[bladerunnerpv]

Sounds like a good situation to be in
The questions I would be asking myself in your situation would include:-
i) How many kWh are being used for the 5h from 02.00, how much does the grid supply cost for those hours and, assuming a battery could supply it, what would the payback period for a new battery be?
ii) can the existing inverter actually charge some additional battery capacity? Is there enough inverter charge rate and solar PV energy available? How much spare are you exporting to the grid - assuming you're connected?
iii) Could the inverters be linked by a comms cable in any way such that they can manage supply to a single new battery from both of them?
iv) What is the overall consumption in kWh per annum is which is not supplied by the GSHP?

You could benefit from modelling half of your set-up (5kW PV and one inverter) in PVGIS or easy-pv, using the answer to iv) as the demand value,
first with 6.5kWh and then with 13kWh of battery and see how it behaves and whether there's enough additional self-consumption to justify the payback the cost of more battery.

 

[sjroberts]

Thanks for the thorough reply!
From looking at the smart meter I use around £4 or 11kw of electric from the grid before 7:30 when the house starts waking up. Around then I start generating a very small amount.
I do not export back to the grid from this solar system as I already had a 4kw on the property which exports once the solar iboost has heated the hot water tank, I have lots of room for charging more batteries from what I can see. The smart inverters stop generating when battery is full and nothing is drawing. Yesterday 1 of the systems did 20kw more than the other so the room is there for charging extra batteries.
I think I could charge 2 more batteries which would give me another 12kw of useable power, the question is- is it worth it? I’m roughly guessing that 2 batteries are going to cost £5k if I manage to charge those batteries for 200 days a year (weather dependent and probably being a bit optimistic) then I would save myself £4 a day. That looks to me like it would take around 6 years to break even on the expense.
How does that look and sound?

 

[Tomr182]

I’d be looking at what tariff you’re in. Sounds like 34p kWh during the night. Plenty out there for cheap night time power.
I’m on octopus cosy and it’s 19p kWh during 4-7am and 1-4pm, 34p the others except 4-7pm when it’s 55p ish. I partial charge the battery before 7am to hedge against good sun or bad sun. Then fill it by 4pm. If we don’t use the oven and just hob, our 6kwh battery can last until 4/5am depending on whether heating is on.
Personally, I’ve calculated it’s cheaper to export and buy than get additional battery as I’m selling it for 15p so net is only 4p kWh

 

[sjroberts]

I’ll have a look at tariffs, not wanting to commit and tie myself in to any of those at the moment because the moment I do I know that the price will drop!!
Summer months 1 extra battery might suffice as the days are long and nights short but spring and autumn 2 would be needed. I just don’t know which way to go to get me the best return. Also I’d have to factor in the dreaded vat as well as I’d be buying the batteries as an add on and not part of the complete system. That’s a bad crack that

 

[Tomr182]

I’d also think about the winter and when the sun is minimal. Will you end up with batteries that are empty and no way of filling them?
The tariff I’m on there’s no tie in for it. Can leave or change tariff within octopus easy enough. Other providers and other similar tariffs as well

A better tariff reduces the benefit of batteries somewhat as you can play tunes with the inverter and force charge when cheap etc it increases the pay back a lot as the difference from 19p to buy and 15p to sell is no longer a huge saving. You can make it look better by comparing buying at 19p vs buying at 34p of course as the batteries open that option up more.

I didn’t want to sell to the grid but accepted the balance over the course of the year.

 

[sjroberts]

Just checked and tariff does sound appealing.
There would definitely be lots of winter days and spring/autumn where the sun wouldn’t be shining so I wouldn’t be charging 1 battery let alone 4. I don’t export anything back from these panels so get nothing returned on them.
Now I’m thinking if I had the extra batteries and utilised a tariff like yours then during winter months I would be able to buy twice as much cheap electric to store and use. That way the batteries could pay for themselves a bit quicker by being useful in the winter. (Not sure if that’s what you were getting at or not)
Even more food for thought

 

[Tomr182]

It also depends on the motivation for doing it. Good of the planet, save money etc there’s a point too much battery isn’t as efficient as just buying from the grid if you can get a flex tariff. And if less batteries and still works for the house then less precious metals for the battery etc
You can get very flexible tariffs and buy for 19p and sell back at expensive times for 50p. So if you went big on that and had a load of batteries, you could pay back quick and make profit on it

 

[sjroberts]

I’m not looking to profit from it as I don’t export anything from these systems. I see the point of too much battery and I see the point of the environment, my main motivation was my own expense and trying to be as self sufficient as I could be (summer months achievable) then got thinking about the cheaper flexible tariff option. Decisions decisions

 

[Andy1733]

Imo first get a cheap overnight tariff and top the battery's up for 2 or 3 hours.
See how this works with your current set-up
If you feel an additional battery would benefit you, just get one before winter and see how that goes

 

[bladerunnerpv]

You're welcome sjroberts, it's just my 2d worth.

6yrs payback sounds worthwhile on the surface of it. As other posters have commented it sounds like cheaper tariffs are possible, in which case it may make the payback time excessive. Sounds like you have the data you need to make an assessment for your circumstances though.

Only other thing to bear in mind is whether the same type of batteries are still available and how long that will be the case for. Your opportunity may be time-limited by the market. Guessing they're Growattt-GBLI6532 then Tradesparky have some still.

What I read on the forum says you can't mix different batteries within the same chain. I suggest you have a read of the inverter manual or manufacturers current online info and check what batteries are 'compatible' - you may end up needing to move all your existing batteries to one half of your set-up and buy a different type for the other half if the same sort are not available.

Enjoy your decision!

 

[sjroberts]

Thanks everyone for your suggestions, change of supplier and batteries is the plan

 

[Andy1733]

I've received an email from octopus. The Go tariff.. Overnight has been reduced to 9.5p and daytime. 39p