Businesses and residents in Derbyshire are being asked to switch off their lights for an hour to conserve energy.
The Derby De-lights event, organised by the city council and South Derbyshire District Council, will run from 2100 and 2200 GMT on Monday.
Non-essential lights and electrical equipment will be switched off in a bid to reduce carbon emissions.
An event in November saved enough energy to power around 200,000 light bulbs for one hour, organisers said.
It is estimated that Derby's energy consumption was reduced by 12,768 kwh last November. The council said street lights would stay on to ensure public safety.
University of Derby students will compete to see which hall of residence can save the most energy.
Residents are advised not to use candles instead of electric light due to the risk of fire.