Batteries connected to the grid make money by reacting to frequency, pushing power out when it sags and soaking it up when it rises. A few hundred of them moving together, on command from someone who should not have the command, would look the same on a control room screen right up to the moment the network starts disconnecting customers. Rafael Narezzi, chief executive of the London firm Centrii, puts the number needed at 1,500 units in Texas, 5.4 percent of the ERCOT fleet, and 400 units in Great Britain, about 29 percent of the fleet there. Those two percentages describe different problems. ERCOT runs roughly 28 GW of storage across about 28,000 one-megawatt units, so an attacker needs a thin slice of a very large fleet, with 30 million people and $12 billion to $65 billion in damage on the other side of it. Great Britain has about 6.8 GW across roughly 1,400 units, 79 percent of that sitting in England, and the modeled outcome is a national blackout hitting 67 million people at a cost of £2 billion to £10 billion.

Full report : Compromise 400 grid batteries and Britain goes dark. A battery storage cyberattack would look like a badly tuned controller.