Effective system-level power management requires cheap, accurate and fine-grained power measurement and accounting. Unfortunately current portable hardware does not provide this capability. We advocate software power measurement : estimation of power consumption by modelling it as a function of device state. The approach requires no additional hardware, and allows fine-grained, per-device and per-application power measurement. We describe a design and implementation of software power measurement, and a feasibility study showing significantly better accuracy than power profiling based on time averaging. We conclude with design recommendations for OS designers and portable hardware vendors to improve the ease and accuracy of power measurement.