We used the Kill-A-Watt meter, shown above, in cumulative kWh mode to measure the power used by various X-10 modules when idle. To increase accuracy, measurements were made over ~100 hour periods. The accuracy of the Kill-A-Watt meter was then confirmed using 1% resistors as the load and by measuring multiple X-10 modules connected to a powerstrip that was plugged into the Kill-A-Watt. The ProDigit datasheet (they make the Kill-A-Watt) indicates 30ppm accuracy in kWh mode.

 
     X-10 Module        OFF 
     -----------------------
     AM486              0.4W
     LM465              0.5W
     TM751              1.2W
     RR501              1.4W
     CM11A              1.3W
     CM15A              0.93W

I also used the Kill-A-Watt to measure the power wasted (when idle) by an X-10 & Insteon friendly +5V switched-mode power supply from Circuit Specialists. It uses 0.2W when idle.

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