We present SPOT, a scalable power observation tool that enables in situ measurement of nodal power and energy over a dynamic range exceeding four decades or a temporal resolution of microseconds. Using SPOT, every node in a sensor network can now be instrumented, providing unparalleled visibility into the dynamic power profile of applications and system software. Power metering at every node enables previously impossible empirical evaluation of low power designs at scale. The SPOT architecture and design meet challenges unique to wireless sensor networks and other low power systems, such as orders of magnitude difference in current draws between sleep and active states, short-duration power spikes during periods of brief activity, and the need for minimum perturbation of the system under observation.