1) The Tesla project in the United Kingdom is apparently identical in size to the one that Tesla put into service at the end of 2017 in South Australia to stabilize the network. It is mainly used for primary adjustment and works very well because of the rapid response of the batteries. Beyond reasons of network security, the interest of these batteries can also be economic if they make it possible to substitute for the primary adjustment provided by controllable means which, therefore, can operate in base at 100% of their power (and not 95%) to improve their production and profitability. It’s a question of comparative costs,
2) That being said, it is essentially the power that is interesting in this case, hence the type of design 100 MW but only 129 MWh capacity, which allows only to spend very short spikes, less than 1 hour (because the batteries can not be completely discharged),
3) To pass for example the winter tip of 19 h in France, which has a roughly triangular evolution shape amplitude of about 5 GW (or a little more) and whose base has a duration of about 2h30 it would effectively have a useful storage capacity of 6 to 7 GWh. Available capacity with current PSP, but which would be much more expensive with batteries. That being said, occasionally replacing some peak TACs of 400 MW by a few batteries of the same power capable of storing 1 to 2 hours of energy can probably provide a partial solution which could be interesting to quantify comparative profitability, knowing that also avoids CO2 emissions. But this does not make sense as long as the current PSP and TACs are sufficient.