Understanding where energy is being used in buildings is an important CPS component that can help improve energy conservation and efficiency. Current approaches for appliance-level energy metering typically require the installation of plug-through power meters, which is often difficult and costly for devices with inaccessible wires or outlets, or appliances that draw large amounts of current. In this paper, we present an energy measurement system that estimates the energy consumption of individual appliances using a wireless sensor network consisting of contactless electromagnetic field (EMF) sensors deployed near each appliance, and a whole-house power meter. We present the design of a battery-operated EMF sensor, which can detect appliance state transitions within close proximity based on magnetic and electric field fluctuations. Each detector wirelessly transmits state change events to a circuit-panel energy meter, in a time-synchronized fashion, so that the overall power measurements can be used to estimate appliance-level energy usage. The time synchronization and data throughput requirements of this problem motivated the development of a new low-power TDMA sensor networking protocol. Our EMF sensors are able to detect significant power state changes from a few inches away, thus making it possible to externally monitor in-wall wiring to devices. We experimentally evaluate our proposed EMF sensor, three-phase power meter and communication protocol in a residential building collecting data for over a week. The system is able to estimate appliance energy consumption with an average accuracy of 95.8%.