FoxESS Community - Owners & Installers Forum

Hello All,

I am at the stage of selecting batteries for my system... Cube is what I have in mind - I was considering EP5, however those cannot be stacked in Tesla fashion, as per recent webinar with Will and the guy from FoxESS. Good price but I simply do not have wall long enough to accommodate those...

There seem to be few different ways of approaching the sizing of those modules. Some say you need only enough to cover your evening peak usage, same say you need enough to cover your daily use, other say it depend - I believe the latter is probably right answer - not very helpful as it does not indicate numbers

So yeah, I would like to hear from you guys. How did you choose the size of your battery and how's the experience??

Yes there are a lot of variables, one of the key variables is whether you have an EV (or if not a low overnight tariff like Eco 7) as that will drive the commercial thinking - in fact I just read that Eon Next no longer require an EV to be mandated (like the original Octopus Go) and so this low overnight tariff would make a tremendous difference.

The next variable is how much solar generation you will normally get per day - obviously in winter this can be almost zero on the worst days but in summer it's very different.

Then you need to get a good export tariff Octopus have a 15p/kWh I think Eon is ~ 16p/kWh so in theory on a low tariff you can buy at almost half the price you sell at.

If you do have a low overnight tariff (lets say between 00:30 and 05:30) then the best strategy is to make sure you have enough battery to get you through peak period in winter so that you charge your batteries overnight and then use the low tariff to get you through the day.

In summer use the low tariff to charge the batteries as you do in winter but export all of your solar (set feed-in first work mode).

With a low tariff, (just my view) you want as much storage as your worst case daily usage - you're only going to get 90% (depth of discharge) on whichever batteries you buy so your batteries want to be rated at least 10% higher - probably 20% to take into account ageing, but all of this costs money so it's best to build a spreadsheet model and work out the returns on each scenario.

Without a low overnight tariff then the batteries are largely being used to charge from solar and to meet your overnight usage, and so the batteries wouldn't need to be as big.

I have a low tariff (Octopus Intelligent), I can get 6 hours overnight at 7p/kWh and 15p/kWh to sell - i'm quite a high power user, approx 13kWh in summer, 18 in winter and so I have a 20.48 kWh battery pack which gives me approx 18kWh usable. (in all honesty I would have a bigger pack but that was the biggest available when I bought them).

My strategy is to charge overnight every night (summer or winter) and export everything I generate to the grid - in summer I export much more than I use, and so i'm in profit, in winter I flatten the batteries most days but at least every kWh I use is at 7p.

There is a final thing to think about which is often missed, to achieve maximum power from your inverter (5kW) the batteries have to be capable of doing that - if you only have a small stack of batteries the voltage will be low and their max discharge rate will limit the amount of power they can provide at their recommended discharge rate. For example with the ECS4100 you would need a min of 3 batteries (nominal voltage 172.8 * 35A discharge rate = 6048watts ) to achieve your max inverter output.

Also, this all assumes you're using gas for heating. If you have (or might ever get) a heat pump, and you have access to a cheap rate tariff, you just want as much battery as you can afford since it will pay you back every winter...

We use about 11kWh per day on average, so I went for 5 HV25s (12.28kWh). I would have liked 6 but was worried about too much weight in the loft.

I've found by putting things like the dishwasher on timer so it coincides with the cheap period, I'm easily covered in winter.

I'm looking at getting an EV soon so things will get complicated. We have 9.42kW of PV now, so we export a lot in summer. I'm currently on Octopus Flux so force discharge to 50% at 4-7pm which combined with the solar also going out during this period, can be good for £2.00-£2.50 per day. We average around 17-18p per unit exported at the moment and can charge for 12.7p per kWh in the cheap time. With an EV I need to do the sums to see if staying on Flux will pay off, as our exports will always be a lot more than import even with the added EV usage.

The point I'm making is it depends on your use. I went for daily use plus 10-20% as mentioned above and have found it to be about right. I do not currently charge from 2-5am from April - October but with an EV charging I'd have to charge the battery anyway to stop it being drained.

Hello All,

Thank you very much for the comments.

Things that were mentioned in the posts:

1. We are with Octopus on Go Intelligent. We have EV, so charging on the cheap overnight tariff - high consumption devices are already moved over to night cycle (dishwasher, washing machine). Cooking is the one thing that cannot be moved outside of peak for now (kids etc.)
2. We have 4.6kWp solar array - in the sunny day in the summer, we are generating 25-30kWh on a regular basis, winter obviously is lower (looking at the statistics we are getting well below 100kWh in winter months).
3. For the export, I am with Octopus...
4. Heating is gas boiler currently with plans to go Heat Pump within next two years...
5. Our daily energy usage is between 15-20kWh (lots of electronics, computers, TV, game consoler etc..)

We have decided to go for: Fox ECS HV ECM4300, 16.59kWh 1x Master 3x Slave for starters. We might upgrade if necessary...

The plan is to cover peak use mostly for now and monitor the usage via Home Assistant. Home Assistant will also be used to run the automation that will prevent storage battery discharge into the EV battery...

Comments welcome.

FWIW we have an air source heatpump in a family home, in the winter daily usage can be as much as 60kWh for heating alone, and is quite often above 30kWh. We have 13kWh of usable storage which was sized to comfortably handle our electricity usage excluding ASHP use. Anything outside of that is of course charged at peak rate, so you save roughly the difference between peak and offpeak where you can drive that from battery storage. If I was specifying this system again from scratch I'd probably go for 20kWh usable capacity, given your baseline usage is higher I could see the use case for more for you, but of course there's a significant upfront spend associated with that. But buying into a platform where you have room to expand definitely seems prudent.