The program, which Con Ed calls mandatory hourly pricing, has been available to owners of the largest buildings since 2007. The impetus for the program is to motivate users to use less power in periods of peak demand, like hot summer afternoons, by showing how much cheaper it is at other times of day.

It will expand in May 2011 to those who consume up to 500 kilowatts a month — buildings of about eight stories and larger. For a fee of about $100, Con Ed will equip participating landlords with “smart” meters, devices that show exactly how much electricity occupants use at different times of day. They then pay prices that change each hour with changes in overall demand. Landlords can save money by using the meters to shift some activities to when electricity is cheaper.