A computer implemented method monitors a computer to determine values of a plurality of activity metrics of the monitored computer. A weighted combination of functions of the determined values is calculated as a measure of performance of the monitored computer. The weighted combination may be a weighted combination of net values of the activity metrics. In one method, the net values are calculated as the determined values of the plurality of activity metrics of the monitored computer excluding contributions to the values from one or more predetermined activities. In another method, the net values are calculated as total values of the respective activity metrics from which the total values of the one or more predetermined activities are subtracted. The weighted combination may be used to control power consumption or otherwise take action in relation to the computer.