Battery Systems - size is everything

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?

I believe the battery will supply everything up to the limit (of the inverter ?) then any extra draw is supplied by the grid. Same thing for PV charging the car, our charger draws 7kW but the amount of panels we’d get would only supply 4 ish, so the grid supplies the balance.

The inverter has an audible cooling fan as some heat is created within. This is info from the installers not actual experience unfortunately.

Firedog - with your 9 kW shower, what size main fuse do you have ? 

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

Yes, any draw above the capacity of the battery/Inverter system is met from the grid. Ours is rated 3.6kW.

We've never heard any sound at all from either system, nor noticed any particular level of heat when I've been in the loft and touched them.

Our LuxPower monitor shows that the peak temperature inside the system briefly reached 36 after the draw for cooking dinner. The system's cooling radiator reached 34. It's now down at 17 all round. 

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

Yes, say a typical 3.6kw inverter (which won’t necessarily deliver 3.6kw) limits the output and the rest is supplied by the grid. 
We hear nothing from our inverters although they are in the garage, not in the house. 

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!!

Both Julia and I are using a ‘smart’ tariff @waltyboy so we need to make use of the cheaper times to charge/power up devices.

Your own system sounds interesting - I’m wondering how you’ve arranged your export as that normally needs an MSC or similar certification.

Batteries for me just make the most of the PV array. Ours is a 3.75kw split system but does ok for us. During the sunnier days of course, unless you consume all the generated power from the PV arrays, it simply goes back to the grid. Hence my comment that you can never have enough battery storage.

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 🤭

Sounds like a plan - I’m still on Agile at the minute but using Octopus Watch to compare to other tariffs