A system comprises a plurality of network-connected processing devices, for example IoT devices. The respective processing device comprises processor circuitry configured to perform a processing task, a transceiver for communication with other processing devices in the system, and an energy harvesting device configured to provide energy for the processor circuitry and the transceiver. To improve utilization of harvested energy within the system by efficient off-loading of processing tasks between the processing devices, the processor circuitry is further configured to estimate (401) surplus energy in the processing device for an upcoming time period, and the transceiver is configured to transmit (403) forecast data representing the surplus energy to the other processing devices in the system.