Jeff Siegel’s letter on “Automatic Energy Savings” (“Mailbox” in HP159) raises expectations too high for room-occupancy sensor switches. To do their job, these switches must have heat or motion and light sensors, plus a decision-making circuit. Parasitic energy consumption for various switches runs between 0.9 and 5.2 watts—just for the switch. That’s as much as 3.7 kWh a month wasted on having a sensor acting as your electricity nanny. It’s cheaper just to use LED lights and hope you remember to turn them off more than half the time.
Worse, most light switches are wired to the AC “hot” leg only, with no “neutral” present in the switch-box. That means the switch must be designed to either pass a nonstop small current to the load, or else to allow a “load” current in the equipment ground wire, which used to be a safety violation.
Jeff wrote a letter, not a full article, so there was no space to distinguish between switches that are “manual, on/auto, off” and the “auto on/auto off” type. There was no room to discuss the value of prime-time energy savings versus off-peak consumption. The whole question is way too complicated to allow a simple answer. My preference is to skip the sensor switches: “It’s better manually!”
Joel Chinkes • Photon Harvest Company