Battery Systems - size is everything

Very interesting advice, @BPLightlog.

Would it be up to the occupier to calculate and decide on the appropriate storage size? In my mind a battery supplier would always advocate as big a battery as they think they can get away with recommending. I supposed the main limitations are size and cost?

Perhaps it’s decisions like this that make independent advice from other people such as yourself on this forum so helpful, and also OVO’s energy expert visits:

Absolutely agree. The supplier didn't really ask the appropriate questions. His calculation suggested 3 x 2.4kWh batteries but there was s supply hold-up (confirmed with other suppliers). When the supply eased, we actually chose to have 4. More than enough for summer. We've just added a fifth!  It allows us in winter to top up overnight (economy 7) and run on batteries until the following night.

They also don't tell you that the system is designed to prevent the batteries from discharge below 10%, so your stated capacity is not usable. 

Our power move challenge targets are always 0.1 kWh - what happens when even a kettle is switched on before the system reacts and allows more to flow from the batteries. The monthly % for peak tea time usage is not a problem. 

I’ve often thought about this. The type of battery involved must have both an output inverter (to convert DC 12V? to AC 50Hz 230V) but also the reverse, like the trickle charger we used to have in the garage to take AC 240V and deliver DC 12V to the car battery. As battery prices fall, using Economy 7 to charge up overnight to power the household during the day might soon become a viable option. It surely beats heating up bricks that cool down slowly.

I wonder what happens if the draw exceeds the batteries’ maximum, e.g. if someone switches the 3kW kettle on while you’re having a 9kW shower? Does a fuse blow and the lights go out? Or does some sort of load limiter kick in to reduce the current to the offending appliances? Or can a system like this draw the excess needed from the grid?

What is the efficiency of the inverters? Are they noisy? Do they (or the batteries themselves) get uncomfortably or even dangerously hot?

100A (I think). The shower’s own fuse is 40A.

What is a typical daily total kWh for the property? I realise that your heat pump and EV will take this well over ‘standard’ but if you start with basics, you can start to calculate. 
Then look at what appliances draw the most (of which you want the battery to cover) and how many will be on at the same time. Add those together - or as mentioned earlier, you will need to be aware of what’s already switched on so that you don’t draw more than the inverter/battery can deliver. 
But as the start of this thread begins to point towards, you can ‘never’ have enough battery storage. As I’ve mentioned before, they will also really help your PV system deliver for you. 
I now charge batteries at the cheapest rates overnight and they cover the whole house usage (apart from our 9kw shower) right through

Very interesting posts indeed.  I’ve adopted a slightly different routine: I recharge my small battery during daylight and run the battery and accompanying small inverter to handle all household consumption overnight. 
 

I have a relatively small PV system, ten years old, which in winter delivers only1 kWh per day on average, if I’m lucky. As an example, so far in December for the first five days we’ve had two days where the total generated by PV has been 0.4kWh each day, and one day of 0.2kWh.  This is a straightforward on-grid system with Fronius inverter, no storage associated with the “main” inverter.  For the first five days of December we’ve imported (bought) 8kWh, exported 0.5 kWh, generated 3.5 kWh.  Our house therefore uses an average of 2.5 kWh every 24 hours, say 100 Wh to include occasional use of heavy stuff:  we don’t use electric showers, and other heavy electric stuff is only used once a week or so…there’s a washload today in the (rare!) sunshine.  
 

The key for us is that our electricity consumption for the entire daily winter 17 “dark”  hours between 1600 hrs and 0900 hrs is practically zero, around 3 W per hour, due to our having purchased about six months ago a relatively small 2.4 kWh battery, an additional small zero-export inverter, and a variable Amp charger which I usually keep set at around 10 Amps (nominally 1 Amp at 230 volts AC) so that the battery happily recharges in the hours between 0900 and 1600, usually by about 15.30.  Without the small battery, we found that much of our meagre PV production over the last ten winters was “wasted” in terms of our not being able to use it accurately during the day.  Now, virtually all of that generation, small as it is, can be employed overnight. 

And of course, whether day or nighttime, the grid supply kicks in if and when household consumption demands it. We just don’t demand it at night (I’ve limited our small inverter to supplying a maximum of 600 Watts, and we rarely exceed 300 Watts for a few minutes at a time, when the chest freezer and the upright fridge/freezer both might both come to life at the same time, for example) and during the day we try to limit heavy usage to “sunnier” days. 

I do not necessarily recommend this approach of retro-fitting small devices, suitably qualified installers and electricians can advise on individual solutions, as can house insurers on any implications, but the figures might be interesting for those people thinking about small solutions. The noise level of the little fan when it’s used at all (a demand over about 400 Watts) is minimal, by the way, inaudible through a closed door for example, and the LiFePo4 battery definitely does not warm up or do anything alarming, certainly at our low levels of charge/discharge. 

There are only two of us now in our small household, and in summer my wife and I can besport ourselves with more abandon, as Fronius can sometimes achieve 15 or more kWh per day, and of course the nights then are so short anyway.  So my experience is that even with a small battery, with careful time shifting and careful consumption, a smallish household can hugely augment the somewhat sparse winter PV production, by recharging during the daytime. 
 

Yes, my gaffer has provisionally approved further expansion of battery storage (I’m really looking forward to getting up towards 10 kWh) but she agrees that this will involve greater overheads in terms of how to charge the things. We are looking at the possibility of a few additional panels fixed vertically, and possibly a wind turbine useful in winter particularly, but that’s going to take us a year or so to plan for. And being honest, our consumption doesn’t really warrant it! But I’m not going to let a small consideration like that stand in our way!!

I’ve moved to Octopus Go as Cosy didn’t work for me with the high cost of the peak period. I get 4 hours at 9p per kWh from 00:30 to 4:30 and 31.2p the rest of the time. If I had a compatible ev charger I could be on Go Intelligent 🤭

Yes indeed @BPLightlog, in order to qualify for FiT payments ten years ago, you’re perfectly correct: we had to get an MCS approved installation, which included a G83 approved inverter for connection to the Grid, apply for approval from our DNO for the system, and obtain a satisfactory EPC for the house. In ten years, our Fronius inverter has generated 23,500 kWh, of which nearly 18,500 kWh have been exported back to the grid! 
 

The more recent mini-system add-on, on the other hand, cannot be allowed to export to the grid, so, without enlarging my existing approved system into an integrated and approved design such as might mirror those systems described in this thread, there are only a limited number of options. These might include an off-grid system which isolates the household from the grid whilst in use, or a system which undertakes not to export to the grid whilst in use. 
 

You’re also absolutely correct when you say one cannot have enough battery storage! Our ten year old system is nominally 3.2 kWh split array, but given our location is probably more like 2.2kWh maximum.  I can hardly wait to obtain more battery storage (the luxury of being able to turn on the oven at a moment’s notice!!) but there will always be the overhead of re-charging the batteries.  I’m very interested in the discussion about using low-cost nighttime tariffs, (9p a kWh for 4 hours a night!).  Amazing, I guess that’s the way to go, and I’m sure we’ll see ever more and more smart tariffs for the responsive householder who has the capacity.  But in the meantime, for small levels of consumption (we’re talking winter, of course:  in the summer there’s absolutely no problem with even a small PV array such as my own effortlessly charging quite extensive battery storage for 7 or possibly even 8 months of the UK year) in a small household I’ve been very impressed with my small add-on supplementary battery system. 
 

Of course, the householder must seek expert advice on matters such as approvals and safety.