Kinda dismayed to see Elon Musk talking like this, as in
https://www.theguardian.com/australia-news/2017/jul/07/tesla-to-build-worlds-biggest-lithium-ion-battery-in-south-australia.He says that a 80MWh battery would cost $100Million US. Taking the UK as an example, demand averages at around 35GW, so to provide a day's battery backup would require 24x3500 or 84000MWh. Which would cost $105 Billion US. That in itself is about the cost of eight ITER fusion reactors. (Of which, incidentally, only one has to work and problem solved.)
That's just for a day's brownout protection. In the UK we've seen periods of two months over which windfarm output is very low, and three weeks when it's next to nothing. So, a battery bank to cover the real situation (say 60 days of supply) would cost around six trillion dollars. That's more than twice the annual turnover of the entire UK economy, and makes the Apollo moon program look like mere pocket money at $25.4 Billion. (1973 value)
Though, I guess he doesn't intend to let those guys who can actually
measure that leccy stuff
ruin a good 'green energy' scam.