Here is a reference design from Atmel describing how to build a single phase energy meter using a XMEGA A microcontroller. The circuit is attached to a mains line, and moniters the energy consumption of it, as well as being powered by it. Each part of the circuit and the firmware is carefully analyzed.
The heart of the meter is the MCU, which calculates active, reactive and apparent energy based on voltage and current measurement. The ADC samples the analog input voltage, current signals and converts into digital form. The digital samples are given to MCU for calculating energy. The MCU accumulates the products of voltage and current samples
over a time. The active power is calculated by dividing this accumulated value by the number of samples. Multiplying the active power by time gives the active energy consumed.