Data center energy monitoring

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for monitoring energy consumption. In one aspect, a method includes sending, to a power monitor of a circuit board, requests for power consumption data. The requests are sent periodically based on a reference clock that is located external to the circuit board and that is more accurate than a clock of the power monitor. Each request is for an accumulated amount of power for components of the circuit board. In response to each request, power consumption data is received from the power monitor. The power consumption data specifies the accumulated amount of power for the components of the circuit board since a previous request sent to the circuit board. An amount of energy consumed by the components of the circuit board over a time period is determined based on the received power measurements and the time period.