An electronic device 50 has at least one harvesting unit 52 for harvesting power from ambient energy. At least one circuit 54, including processing circuitry 56, is supplied with power from the harvesting unit 52. Control circuitry 60 is provided to adjust at least one property of the processing circuitry 56 or the at least one harvesting unit 52 to reduce impedance mismatch between an output impedance of the harvesting unit 52 and an input impedance of the at least one circuit 54.