Since connecting a Tesla Powerwall 2 to their two-year-old solar panels a month ago, Andrew has become the power policeman in his family home.

Checking a phone app, he knows at any time how much power is being used in the house and on which appliance.

That is bad news for his teenage kids, who know they'll get dad ringing to ask: "Why are you using the overhead heat lamps when there is ducted heating in the bathroom? Close the door, heat up the bathroom and turn off the lights."

The point of it all is to save money.