Focussing on the UK: the mania has in large part been driven by the structural evolution of the grid. The grid has been landed with an ever-more challenging problem of how to sustain our electricity supplies, sandwiched between two unhelpful trends: ever-increasing intermitency from windfarms, distributed generation etc on the one hand, and relenteless closures of reliable coal-fired plants on the other. Well, the engineers are clever chaps and when money is no object (thank you for the guaranteed rate of return, Minister) there is generally a solution to be found. We may be grateful, I suppose, that there is a bit of a competitive flavour to the solutions they are deploying: an auction for Capacity Mechanism contracts and a tender process for the plethora of services the Grid buys in to sustain the system. And to fairly widespread surprise, battery-farms have been among the winners.
Now auctions and tenders are not true markets: there is always a distinct "free-money-on-offer" aspect, and inevitably the Law of Unexpected Consequences cuts in. Ministers, ever keen to pick winners, had been hoping the winners would be new, ultra-efficient CCGTs (gas-fired turbines), and "demand-side response" (DSR) from large industrial energy users. They were advised before they started (by myself and others) that what they would actually get was a new industry of less-efficient OCGTs instead - a fairly easy prediction to make when you looked at the numbers. That's what has happened, to their dismay.
Even more dismaying - and not widely forseen, though it should have been - was the rash of diesel-farms that also won a lot of the early contracts; a third-world "solution" if ever there was one. For the future, that's largely been fixed by changing the rules (the usual trick); although the other dirty secret is that the dissappointingly small DSR sector often turns out to be factories that, in order to reduce their load on the grid at peak times, will simply fall back on diesels of their own - not what DSR is meant to be about. Still, them's the rules.
The final surprise was batteries, the other big winner for capacity- and grid service contracts. This is less dismaying for Ministers, because (a) everyone knows energy storage will play an ever greater role in any realistic scenario, it's just a question of what and when; and (b) unlike diesel gen-sets pouring out black smoke, batteries aren't a manifest abomination. They can even be made to sound kinda green (by someone who doesn't know what goes on, out of sight, in the manufacturing process).
However, it wasn't as benign as all that for the Grid, because batteries of today's technology are a fairly suspect long-term source of the flexibility the Grid requires. They run down in minutes rather than hours; and if hammered, their lifespan is as pathetic as a Mrs May re-shuffle. We all know this will get better over time (battery performance, that is) but we ain't there quite yet.
So, very sensibly, the Grid has re-specified its requirements more exactly, and the rules have been changed (again) accordingly. When the next Capacity Auctions take place later this month and in February, the first wave of battery mania should prove to be over; and I'd personally be quite surprised if the prediction of 12 GW by 2021 comes to pass ...
But strange things happen when public money is on offer. What do all our C@W engineers out there think?