Apparatuses, methods, and systems are disclosed for power headroom report generation. One method includes aggregating multiple serving cells. The method includes determining that a power headroom report is triggered. The method includes determining that an uplink resource for a new transmission on a serving cell of the multiple serving cells is allocated at a first time after the power headroom report is triggered. The method includes determining a power headroom value for each serving cell of the multiple serving cells being activated based on information received prior to and including a predetermined time before the start of the uplink resource at the first time. The method includes generating a power headroom report medium access control control element including at least the power headroom value for each serving cell of the multiple serving cells being activated.