That's a fair question. I think the answer is going to be a bit slippery, though, depending on your household habits, the wattage of the attached fixture, and how long the device would remain installed. I've seen simple plug-mounted motion sensors for as little as $4.00. The U.S. national average for electrical power is about 11 cents per kilowatt. An unattended 100 watt bulb would consume roughly a kilowatt hour if left burning all night. That's a little over $3.00 a month. So you'll clearly save with things like hallway and outdoor security lighting, if they would otherwise be left in constant duty.

I can see what your problem would be there. Short of installing a second sensor which can sweep the shower stall, I can't immediately think of a cost-effective solution. Of course, motion sensors aren't appropriate for every application. We've got some sharp readers -- maybe someone will offer a workable suggestion.