A single-phase, 3-wire watthour meter which can measure power consumption, alter a contract capacity, control the stop / start of power supply / distribution, and easily update programs from a higher-level control apparatus by providing a central processing unit, a storing means, a communicating means and various interfaces, can measure, for example, every 0.2 sec, a currents (ia and ib) running through 100V T-phase and R-phase, voltages (v1 and v2) between T-N and between R-N and a current (i3) and a voltage (v3) under a 200V load, determine a 100V power consumption by W100=(ia−i3)·v1+(ib−i3)·v2 and a 200V power consumption by W200=i3·v3, and measure the detailed behavior of a power consumption by totaling a power consumption every 30 min, interlocks with a gas leakage detector and a fire alarm, can control the opening / closing of rain doors and shutters and the operation / stop of Internet home electric appliances, and enables low-cost communication by means of dynamic IP Address-based communication.