Yes, you know what it means. All those devices you leave on standby, and all those wall-warts you leave plugged in and switched on all the time. We are talking lights in rooms that are not being used, televisions left on standby, mobile phone chargers left plugged in, unused printers, etc, etc.
But there's a twist: one 24-hour period is being campaigned for, starting 6PM, 27 February and nicknamed E-Day. The UK's National Grid will assess what difference this mass switch-off will have on overall electricity consumption.
There will also be a Facebook application which will allow you to record what you have left off for E-Day.