argh tesla - those cost an absolute fortune.
i don't see how they are worth it in the slightest. i'm talking about the cheaper solutions offered.
there is no point spending £10K on a battery to save £20 a month for 10 years then shell out another £10K or face that £20 saving dropping to £10 and then degrade rapidly with age.
I see you didn't do the mathematics behind it or know the actual cost of the product.
Lets be inefficient, and assume a terrible input from the Pv system, and also that the cost of electricity remains constant for the next 10 years at 15pp kWh.
6 hours per day, at a maximum of 3.5kWh, that is 21kWh of electricity being generated, getting it pushed back to the grid using current Feed-in-tarrif price of 3.79pp kWh is about 80p per day, or £290 per year.
Now if you plug your car every night, and you've not stored any of that free energy you'll be paying 8-15p per kWh, so lets say you put in 20 kWh of electricity in to your car, that is £1.60 - £3.00 per day, but 13 kWh of that could have been supplied by the battery storage system, that is about £1.04 - £1.95 per day, or £379 - £711 per year. Obviously there will be times when you don't need to charge, but the spare charge can then be consumed by the house for normal electricity use, or fed back into the grid which you are then getting paid for, all the while the PV is still charging providing more charge to ensure it stays topped up.
The cost of adding a Powerwall 2 is £5400 + £500, and then the installation charge which is £1100 (quoted late last year), so £8000 over all cost, or about £13,500 if you take two of the for a total of 27 kWh storage. Worst case scenario is that you only ever use the electricity at night for charging at off-peak rate and you recoup £3790, more likely is that the value of the saved electricity will be at the high end, which is a £7111 return, still short of your initial investment of £8k, but that doesn't include any extra charge pushed back to the grid which you are paid for at the feed in rate, as I stated earlier. These are all based on low PV generation, electricity costs never inflating (not happening) and if it were to go up just 2% per year for 10 years, that would be 18.3pp kWh, making your 13kWh of stored electricity worth £2.38 per day, or £868 per year, and again that's assuming you never feed back in with excess from the battery or PV when not being fully utilised.
The obvious benefit is that you have been generating clean electricity for yourself for the past decade, and that your Powerwall isn't going to just drop dead on day one of year 11. I've not taken any battery loss into account as when not using the stored power at night, the home can be drawing from it as well as the PV system pushing more energy than it could store in the 6 hours previously mentioned, the average home with a family of 4 in a 3 bed house uses ~9 kWh of electricity per day, so 13+9 = 22kWh, and PV will be generating 21kWh of that.
So yes, I agree with you completely not worth it to save £20 a month, but certainly worth it to have a completely uninterruptible power supply, that generates and stores clean electricity for use in your own home, or to charge your own car and not be reliant on electricity supplied by the grid, or indeed feed back to the grid when not in use. I was brought up to believe that it is better to give than take and if everyone did their bit things could be a lot better for the entire country.
On another note, I bet you didn't know that you can actually send electricity back to the grid from your electric car? So you can happily charge your car on the cheap overnight, and if it's not in use that day you can configure it to feed back in to the grid, thus making a profit on the free or cheap electric you charged with even when you are not driving anywhere.
