Calculating the power by measure the time elapsed between rounds from an electromechanical induction watt-hour meter (in specific a metal disc / turning wheel which is made to rotate at a speed proportional to the power passing through the meter) and send it over to a Raspberry Pi, which logs the data.
The transmitter (C/C++ on a JeeNode) checks the metal disc / turning wheel (in German: Ferrarisscheibe) from a power meter with an reflective optical sensor with transistor output (CNY70) and transmits data to the receiver.
The receiver (PHP on a Raspberry Pi with RFM12Pi) parses the incoming data from the transmitter and logs it. In particular, it'll submit the power value to an EmonCMS-API.