「電気」 (denki) has two main meanings: "electricity" (power) and "light" (an electric lamp). You use it when talking about electrical power in general, or when you want to turn on or off a room light.

「電(でん)気(き)」は、電(でん)力(りょく)を指(さ)したり、電(でん)灯(とう)などの明(あ)かりを指(さ)したりします。