@scrchngwsl great advice! I went for the Growatt 6.5kW battery with max charge power of 3kW, charge time is about 2.5hrs as it reduces the charging current as it gets near full charge. I'll be doubling battery capacity to 13kW next week, so will need up to 5hrs to charge from empty. The Cosy Octopus tariff provides 2 x 3hr periods a day of low rate electricity, so that will work for me. A tariff that only offered 2 or 3 hrs at low rate wouldn't.

@allyfish The Tesla Powerwall 2 battery can charge at a rate of 5 kW from the grid (or the PV on a good day!) so this helps to keep charging time down. Regards, Toodles.

@allyfish Yeah, the Cosy tariff is particularly interesting with its two cheap periods: it means that I can charge the same battery twice a day, effectively doubling the amount of cheap rate electricity I can use on a single battery. Really neat concept - I'm planning on switching to this over winter.

@scrchngwsl that's exactly what I do, and it saved 30% off my electricity bill in winter from day 1. It tipped a moderately performing ASHP retrofit into cheaper than oil heating energy bill territory. Any winter solar PV is a bonus, and we get good days where the battery charge is solar assisted for free rather than from low rate grid tariff.

Posted by: @curlykatie

↑

The garage can get quite cold, assume this is not an issue?!

That might be an issue if the air temperature falls below freezing.

It's most likely that the battery chemistry you are being offered is Lithium Ferrous Phosphate (LiPO4).
Those cells will suffer degradation if they are charged below 0°C, but you can discharge them OK at lower temperatures.

Within the battery pack there is a Battery Management System (BMS), which should include temperature sensors.
Some BMS units won't let you charge if the temperature is too low.
Others have a couple of connectors which can be used to operate a heat-mat beneath the cells.

So you really need to be receiving a bit more information from a potential supplier regarding what's actually in the box.

There's more to buying a battery than just evaluating the cost per kWh of capacity.

Forgive me for stating the 'obvious' @curlykatie but I cannot see anywhere that you've said which inverter is being proposed for your new solar panel system.

That's the main device which will connect to the grid, and hence requires approval from your Distribution Network Operator (DNO).

Some batteries are connected directly to that inverter, using DC cabling.
More commonly, storage batteries have their own 240v AC in/out connection to the grid.

If it is possible for the system to allow export to the grid from both an inverter and a battery simultaneously, then the DNO will need to be satisfied that the combined export is below their threshold.
That's usually set at 16A (per phase) or 3.68kW.

If the battery(ies) are full and the PV panels are producing output, then the system must discard excess generation which you're unable to use within the house.
That's stipulated in an engineering standard called G99, which we can explain further if it matters.

I was on a storage-battery trial a couple of years ago. On sunny days the software first filled the battery and then exported to the grid (at the permitted 3.6kW).
That resulted in no export at all during the morning, and electricity being thrown away in the afternoon. Not very useful!

Also, please note:

If there is a power-cut to your house, then all mains-connected batteries and inverters will shut-down (unless they have a connector designated for 'critical loads').
Grid-tied devices are certified under G98, and may not operate independently of the grid.

All of this should be explained within the quotation.

@transparent thank you for your replies. The system we have been quoted for is coming with a 5kW inverter. I’ve just checked the battery info and it won’t charge below 0 (but will discharge as you say)… I can’t imagine there going to be many occasions when the garage gets below that so thinking it’ll be a better option than the loft!

If I were in your position @curlykatie I would be applying online to the DNO for the required G99 Permission, based on the inverter and battery models identified in the quotation.

You do not need to leave this to an installer, and there are no costs involved.
If you make a mess of it, you can just re-apply. 🙂 

The DNO will respond with confirmation of the maximum-permitted level of export.
That might be higher than the usual 3.68kW because it depends on calculations specific to the sub-station supplying your property.

Equally, the G99 application will reveal if the installer has specified a combination of inverter and batteries which has an Export-Limitation System on the ENA approved list.

If you want help with any of this, please let us know who is your DNO and which of the 14 electricity distribution regions you live in.

Don't worry about making mistakes in what you write here.
You can leave the technicalities and regulations for us to interpret!

@transparent thank you! It’s so confusing!!

some more info- not sure if this helps…

I’ve checked and we are supplied by UK power Network. Now you’ve mentioned this, I remember the sales guy saying that if we went above a capacity of 6kW we would need planning so I’m assuming this is the G99 you mention? I know our neighbour had to apply as they have more panels (7kW system).

the installed capacity of the 13 panels at 455W per panel has been quoted at 5.92kWh  

they have just informed us they are upgrading the battery size for no extra cost to an 8.7kw battery with 5kW inverter (brand is Foxess for both), as they have gone over to a new battery supplier!

do you think in light of the above I need to do/ ask anything?!

My other thought is whether we should be trying to negotiate for a larger battery to maximise the potential in the cosy octopus tariff?

hope above makes sense?!

katie 

Hi Katie, I’ve just gone on to the cosy tariff with a 5kWh battery. Remember you have two off peak periods a day so unless you think you will use more than your battery capacity between the two it isn’t worth buying a bigger battery. 

@bontwoody I should think that there is a distinct chance that more than 5 kW/h might be used between the end of one Cosy dip ending and the next one starting if there is a heat pump in operation though. Regards, Toodles.

@toodles I will let you know after the winter 😁