Cost of adding batteries to existing PV system.

QrizB

There's a huge thread about on-grid battery storage if you're interested in how it works, the technology and the terminology.

If the Growatt inverter in that bundle is the one I think it is, it should be supplied with a current sensor that allows it to measure how much power your house is using. It will then discharge the battery to balance that demand, such that your total power used from the grid is essentially zero.

When your solar PV is generating more than you need and is exporting, the same sensor will let the inverter know how much excess generation you have and will charge the battery using that excess. Again, balancing the total energy flow at or near zero.

The Growatt inverter should have connectors for solar PV, so at a later date you could even add extra panels and connect them there to avoid upsetting your FIT.

And it would also let you move to a time-of-use tariff so that, in the winter when your solar can't keep up with demand, you can buy electricity from the grid at cheaper times and store it in the battery to use later.

(I've got a roughly equivalent setup; my inverter is a Sofar ME3000SP that I bought used five years ago, and my battery is currently a 16kWh Fogstar. It works well for me.)

WIAWSNB

Thanks again.

It looks to me as tho' an AC-coupled system is the way to go, so it's an add-on to my existing setup, no change of inverter, doesn't affect the FIT, and a 'normal' sparky can install it.

They are more costly, tho', and I'm back to not considering it worth the investment, unless I go second-hand.

I'll have a read of that thread. Although I like to understand the principles, I'm not concerned about too much technical detail.

QrizB

I would add, though, that if you're not an enthusiast / tinkerer you might be better served by paying a professional to supply and install your system.

Here's an example that's roughly the same as the eBay bundle, installed and with all the correct paperwork plus someone who can provide tech support, all for about £4k:

https://www.itstechnologies.uk/collections/sunsynk-installed-packages/products/installed-solis-5kw-ac-coupled-ai-optimised-fogstar-energy-eco-16-1kwh?variant=57873708876157

(Just an example, not a recommendation - I know nothing about ITS Technologies beyond what's on their website.)

Reed_Richards

I have a solar system with a hybrid inverter and a 6.5 kW battery. This battery is fine for April to September but with a big enough battery I think I might be able to save up to £500 per year during October to March. So adding battery capacity would be hugely tempting if it really cost £1.5k and worth considering if it cost £4k (a Tesla Powerwall would be too expensive). But my existing system was installed under the FIT scheme and I don't want to mess with that. If I added an AC battery with its own inverter i have never been sure if this would "play nicely" with my existing system or whether the two would fight. @QrizB, do I gather that you think two such systems would get along?

QrizB

do I gather that you think two such systems would get along?

I think they'd get along, yes. You might need to be a little bit careful with where you position your current transformers, to stop one battery trying to discharge into the other one, but it should be possible.

WIAWSNB

https://forums.moneysavingexpert.com/discussion/comment/81973905#Comment_81973905

£4k is clearly better than it used to be, but break even would take too long.

I think I'll keep an eye out for young second-hand batteries, 10kW or just above, and a suitable AC-coupled charger/controller.

Most controllers appear to be flexible on battery voltages, usually ranging from 40-60 volts? Any advantage in preferring batteries of a particular voltage in there?

On controllers, are there makes that stand out as worth considering more? I understand Solis is a good make, and their 3kW RAI 48V can be had for £270 or less (pro refurb).

Growatt SPA3000 for £170 (ditto).

I guess some will have more control options and connectivity to Apps and stuff? I'd be looking to solely charge from excess PV, and release on house demand. They can all be set to do that? (Ie, no charging from cheap overnight rates). And all will have control over min/max charge capacity to preserve battery life? That sort of stuff.

Ta.

(Ploughing through that thread... :-(. )

QrizB

Most controllers appear to be flexible on battery voltages, usually ranging from 40-60 volts.

I think you're being misled there.

Most of the generic "solar" batteries - that Eco Worthy one, or. Fogstar one, or the DatouBoss one in your FB link - are 16-cell lithium iron phosphate batteries. Each cell is a nominal 3.2 volts (51.2 volts for 16 cells) but the voltage varies depending on the state of charge.

If you look at the table here you'll see the voltage will vary between 3.65 volts per cell (battery at 58.4 volts) to charge to 100% down to 2.5 volts per cell (battery at 40 volts) when completely flat.

In practice you'll probably only cycle between ~48 volts and ~54 volts.

So when a battery inverter says it's suitable for 40-60 volt batteries, it means a 48V nominal system. (Note you can often run these on lead-acid batteries, four 12V batteries in series, but this is rarely a good idea since lithium batteries have become so affordable.)

There are exceptions to this. Some systems use high-voltage batteries, often made of several 48V batteries in series. These are often manufacturer-specific and are probably best avoided for a DIY or low-cost system.

Or there are small systems intended for mobile use in vans, campers and boats. These might use 12V or 24V batteries. You could use one of these in a house but they're not generally ideal.

PS what we've not mentioned yet in this thread are the plug-in systems widely used in countries like Germany in conjunction with plug-in PV. The Ecoflow products have been mentioned a few times in the plug-in PV threads but there are many manufacturers of these, some relatively well known like Anker. These are typically smaller than the 10+ kWh batteries that you've been looking at and are somewhat more expensive on a price-per-kWh basis, but are pretty much foolproof and will work (not necessarily legally, at present) if plugged in to a 13A socket.

WIAWSNB

Thanks for the clarification.

WIAWSNB

Oookkkaaayy, I have some what I'm sure are basic Qs, please;

1. When a charge controller is rated at 3kW or 3.6kW, that is the max they will charge and discharge at, yes? Is the discharge rate usually adjustable if needed? (Tho' 3kW is fine.)
2. Battery and charger voltage. Still confused. I do get that the battery voltage will fall as it discharges (presumably this is what the charge level meter registers?), but how does this tie in with the stated DC voltage on the charger? This one, for example, says 70VDC. What the? Is there no industry standard for home systems?

https://www.ebay.co.uk/itm/127825675876 says 70-400VDC, so I presume is not compatible with most domestic battery systems which appear to be 48 or 51.2V?

So, the basic principle is - you get a battery unit, say 5kW, and a charge controller, say the Solis RAI-3K-48ES-5G, and that's pretty much it. This gets fitted after the generation meter by a single(?) cable, and the controller automatically diverts spare generation into the battery, and releases it on demand?

How does it know what's 'spare'?! My PV system and inverter is in the garage, and that's where the battery would go too. I will have a regular high power demand at the garage as I have a Phev. So, how does it know that the supply from the PV is going to the car, or to my WM in the house, or to the grid, so it can say whoa, the grid stuff is mine, all mine?

Ta 😶

WIAWSNB

Why is this insanely expensive for what appears to be just an AC-coupled charger?

https://www.ebay.co.uk/itm/306215856530?