Power measurement device for a bicycle and/or bicycle trainer, comprising an acceleration or velocity sensor, a microcontroller and a memory, wherein the device is equipped with or connectable to a power source, preferably a battery, and - wherein the device is equipped with a communication facility to enable the device to wirelessly or through wires communicate with an external application device, - wherein the microcontroller and/or memory are provided with a computer program which is arranged to derive a power measurement from signals received from the acceleration or velocity sensor, and - wherein the power measurement device is arranged to use a frictional force or torque that applies to a wheel of the bicycle and/or bicycle trainer on which the power measurement device is mounted together with the signals received from the acceleration or velocity sensor to calculate the power supplied by a user during exercising with the bicycle or bicycle trainer, characterized in that - the microcontroller and/or memory is provided with a calibration functionality to calibrate the power measurement as derived from the signals received from the acceleration or velocity sensor, - the calibration functionality is arranged to derive a frictional force or torque that applies to the wheel of the bicycle and/or bicycle trainer on which the power measurement device is mounted from a measurement on the wheel of the bicycle and/or bicycle trainer, and - the communication facility is equipped to receive parameters defining inertia and wheel diameter of the bicycle and/or bicycle trainer onto which the power measurement device is mounted, and - the calibration functionality is arranged to apply these parameters regarding inertia and wheel diameter to derive from a run out time of the wheel the said frictional force or torque that applies to the wheel of the bicycle and/or bicycle trainer. - wherein the microcontroller and/or memory are provided with a computer program which is arranged to derive a power measurement from signals received from the acceleration or velocity sensor, and - wherein the power measurement device is arranged to use a frictional force or torque that applies to a wheel of the bicycle and/or bicycle trainer on which the power measurement device is mounted together with the signals received from the acceleration or velocity sensor to calculate the power supplied by a user during exercising with the bicycle or bicycle trainer, characterized in that - the microcontroller and/or memory is provided with a calibration functionality to calibrate the power measurement as derived from the signals received from the acceleration or velocity sensor, - the calibration functionality is arranged to derive a frictional force or torque that applies to the wheel of the bicycle and/or bicycle trainer on which the power measurement device is mounted from a measurement on the wheel of the bicycle and/or bicycle trainer, and - the communication facility is equipped to receive parameters defining inertia and wheel diameter of the bicycle and/or bicycle trainer onto which the power measurement device is mounted, and - the calibration functionality is arranged to apply these parameters regarding inertia and wheel diameter to derive from a run out time of the wheel the said frictional force or torque that applies to the wheel of the bicycle and/or bicycle trainer.