Sydney Powers Down For Melbourne Cup

Electricity use across Sydney dropped dramatically as the city powered down to watch the Melbourne Cup.

Ausgrid said electricity use across its Sydney network dropped by 100 megawatts, the equivalent of one million desktop computers going into sleep mode.

The energy supplier said its team measured Sydney's electricity use at 2,688 megawatts about 2.40pm (AEDT) on Tuesday.

Demand dropped over the next 30 minutes to a low of 2,584 megawatts about 3.15pm by the time the horses were out and running - almost a four per cent drop, or 100 megawatts.

It was back up by 60 megawatts by 4pm (AEDT).

"The Melbourne Cup is really the only event where we consistently see this sort of sudden drop in electricity usage," Ausgrid energy efficiency expert Paul Myors said.

Ausgrid supplies power to 1.6 million homes and businesses across Sydney, the Central Coast and the Hunter.