OK. So we are now about 9 months in and I have been keeping a very careful eye on grid usage, solar production, solar and the like.
I remain more than happy with the end result.
Just to recap - we have a 4kw solar array, a Tesla powerwall with approx 13kw capacity, an electric car (BMW i3 taking approx 20kwh full charge). We are high energy users, at approx 7000kwh per year (we are running the farm as well as the house on this!!!
The magic ingredient that makes this all work is Octopus Energy EV tariff combined with the smart meter - which gives us 4 hours of supply (0030-0430am) at 5p per unit, 14p for the remainder of the day.
Up to the installation of the Tesla (25 Feb 2020) we were averaging £3.15 per day import from the grid, which dropped to £2.22 after allowing for the FIT payments from the solar array.
Almost from day 1 we dropped significantly. Import charges over the following 9 months have been £0.89 per day (which actually works out at a nett income of £1.53 per day allowing for the FIT and export payments. My average monthly bill from Octopus is about £35 per month (allowing for daily flat rate charges), down from over £100 previously. As we move onto the dark months of winter I expect things to fall back a little, but on a typical day we take very little power from the grid between 0430 and 2200, the powerwall and solar supply everything we need.
The Powerwall takes a little getting used to, it uses some sort of AI-based algorithm to determine how much power to suck in overnight and sometimes gets it wrong. A period of sunny days followed by an overcast one will leave it undercharged in the morning, but overall it does a good jib. I just leave it to get on with things by itself. Obviously we manage our usage during peak hours where possible. Dishwasher and washing machine are set with their timers to only come on after the 5p rate comes into effect, as does the car. If close to empty that sucks a good 15-18kwh on that 4 hour period (we have a fast charger which will pull close to 30amps at peak) but at 5p per unit that is still less than £1 per charge. We do keep an eye on solar production so will happily turn on the disjwasher in the afternoon rather than export power to the grid (least cost effective option!) but we don't fret about it too much. Our export amount has dropped dramatically but as a domestic user we are assumed to export 50% of our production, so I am winning all the way!
There is a balance here. How much power do you use, how much can you control heavy loads, how much solar do you produce, solar output, battery capacity, but the key to it all (IMHO) is the right tariff which give you a burst of cheap electricity which you can store and use later in the day. My average cost per kwh purchased over the last 9 months has been about 6-7p per unit, compared to the full tariff price.
As for payback period I still haven't done the final calculations - will wait for a full year, but I am confident it will prove a good investment. The powerwall is guaranteed for around 10 years as I remember and even then the battery will continue to work, albeit at reduced capacity. I expect electricity prices to go up so as long as I can keep and EV oriented tariff I think I am in good shape. On top of that I sold the standby 5kw generator we had in case of powercuts since the powerwall automatically provide backup power in the case of grid failure (at least until it runs out!). Fortunately we haven't had a major outage thus far, but it is surprising how many short outages you get.
To give an example from today - the weather has been crap with no sun at all.
Total usage - 58.1kwh (31kwh off peak,27.3 peak)
Solar production - 0.6kwh (yuck!)
Powewall - 12.5kwh drawn in peak period, 13.2kwh charging in off peak
From grid - 59.3kwh total - 15.2kwh in peak, 44.1kwh in off peak (car was charged overnight!)
Not a great day by any means, but a darn sight better than it could have been!
All in all I am happy.