Abstract: A method for analysing the effect of installed applications (A1-An) on battery usage of mobile communications devices (D1-Dm). Each device has monitoring software which monitors battery usage at frequent intervals throughout the day. The monitoring software calculates the average battery discharge and at less frequent intervals, such as once a day, communicates remotely to a server (105) the details of which applications are installed on the device and the details of the state of the battery. Using data from many devices, the server estimates the effect of each application on battery usage. The estimated effects of the applications are updated each time that a further report is received, by revising the existing estimate. Greater weight is given to new data if the effect on battery usage has been consistently over- or under-estimated. It is not necessary to determine whether a particular application has been used on a device. The effect of all applications installed on a device is taken into account jointly.