1. A method of producing a measure of performance of a computer, comprising: determining, from an operating system on a computer, a total value of an activity metric of the computer; determining, from the operating system, values of contributions of one or more predetermined activities to said total value; subtracting the values of the contributions from the total value to provide a net value representing the measure of performance of the computer; determining a proportion of a time period P in which the net value of said activity metric of the computer exceeds a predetermined threshold value; determining a power consumption of the computer over the time period P; and determining for the time period P, a proportion of power consumption for which the net value of said activity metric exceeds the predetermined threshold value; wherein the determined power consumption is: 
 PowerAtMin+(((PowerAtMax−PowerAtMin)/100)*CPU value) and wherein: PowerAtMin is the power consumption of the CPU at minimum CPU activity, PowerAtMax is the power consumption of the CPU at maximum CPU activity, and CPU value is the average value of the CPU activity metric over the time period P.