An optical access network (5) comprises optical network units (10) connected to a node (40). A monitoring unit (35) determines information indicative of energy consumption at the optical network unit (10) over a period of time. An optical network unit (10) can operate in operating states/modes which differ in their energy consumption. Monitoring unit (35) can determine the information by determining a time that an optical network unit spends in the different operating states/modes. Monitoring unit (35) can use a state machine (31) at the node (40) which represents the optical network unit (10). An optical network unit (10) can locally record time spent in states/modes and forward this to the monitoring unit (35). An optical network unit (10) can locally monitor energy consumption and forward this to the monitoring unit (35). An operational parameters of the access network (5) can be modified based on the information determined by the monitoring unit (35).