An energy meter is a device that measures electricity (KWH) consumed at our home of office or factory.
Electricity Supply companies (Like BSES, NDPL in Delhi) use electric meters installed at customer premises to measure electricity consumption for billing purposes. This consumption is measured in KWH i.e. one kilowatt load running for an hour. A typical example to understand is if a 1000W Geyser is ON for 1 hour, then it results in 1 unit energy consumption.