Energy Meter energy calculation from power

I have interfaced a smart energy meter IC (ADE7878) to a microcontroller. The datasheet of the IC can be found here, http://www.analog.com/media/en/technical-documentation/data-sheets/ADE7854_7858_7868_7878.pdf. As per the datasheet, on page 50 total active power of each phase can be read from the registers AWATTHR, BWATTHR, and CWATTHR. My question is what parameters I should consider and formula I should use to calculate the kwh value ?