Bill Childs switched on the airconditioning inside his Williamstown house on Sunday to ward off the sticky heat, just like hundreds of thousands of other Victorians.

But unlike most of us who pay a price for keeping cool on hot days, Mr Childs made a bit of money out of the electricity retailers, as he has on most of the hottest days this summer.

Bill Childs at home with his solar battery. Credit:Jason South

He did this by feeding energy back into the grid from a small solar battery on the side of his house when wholesale electricity prices surged past a certain mark due to high demand.

New software from Canberra-based company Reposit connected to his solar battery and did this automatically.