Batteries only?

NedS

Silly question maybe, but is it possible to do a battery only installation, and charge them overnight on a cheaper Economy7 or similar tariff?

We use between 15-20KWh per day, so 12-18KWh battery capacity should in theory allow us to not have to draw any electricity at peak day time prices. Is this something that is feasible / cost effective?

Solarchaser

It's certainly possible.
It's what lux inverters with squirrel pods were marketed as.
https://homeenergygroup.co.uk/lux-squirrel-pod-battery-storage-uhome/

(Not a company endorsement,  definitely do your own due diligence)
I wouldn't recommend going with uhome batteries,  definitely pylontech seem more.... believeable/reliable/straightforward. 
 Something like this
https://voltaconsolar.com/pylontech-us5000-lithium-battery.html (again not an endorsement)

Whether it's cost effective depends on you really.
I guess first you would want to find a deal for peak/off peak rates for a non ev user, and then look at your annual usage, and break it into monthly if you can, as you will use more in winter than summer, and then work on peak rate vs off peak, but x1.2 (as battery inverters are roughly 80% efficient round trip) and from there work out how much you could save per year, then look at the price of inverters plus batteries to see if it make sense.

For instance if you had a peak rate of 40p/kwh and an off peak rate of 10p/kwh then charging and discharging the batteries would (roughly) cost you 12p/kwh.
If you use 5000kwh a year then the cost from peak would be £2,000, the cost from off peak would be £600 and so *in theory* saving you £1400 / year.

And so if you had a battery system based on for instance buying 5 of the batteries I linked to, giving you a useable 20kwh that would come to £8300, let's say you can get two lux 3600acs inverters for £1700 (not sure how much they are these days, they were £750 each when I bought mine 3 years go.) Then you have spent 10k and have payback in a little over 7 years.

Buuuut it depends on what you pay now for electric or what's the best normal rate you can get vs what's the best off peak.
It also depends on peak and off peak rates having a big distance between them for the next 7 years etc etc.

I've prattled on here and it occurs to me that I think @QrizB already has a thread with this stuff listed??

QrizB

Don't mind me, I think @Solarchaser has covered all the bases

ispookie666

Nicely explained 
You might want to check how quickly the inverters respond to changing house loads. Some of them are quick to ramp up and down, resulting in minimal grid draw. If you have induction hobs, they are a bit of pain! 
The two inverters I have had experience with 
- giveenergy - slow to respond

- Victron - faster, more expensive and difficult to find installers

Solarchaser

Induction hobs will cause any battery inverter to lag, as they respond to the surge, the hobs switch back off etc.

The lux acs has good response, the sofar me3000sp had a little quicker response, but less output.
The growatt sph3600 was very slow, but more recent buyers have noted that's improved

NedS

Wow, thanks folks - that's some fantastic information for me to absorb, really appreciate it. Not cheap, and so much depends on future pricing (or pricing differential) which is a complete unknown.