Last month, I wrote about the EcoDog home power monitoring system, which lets you see how much electricity your house is pulling, circuit by circuit. Apart from being fun for energy geeks like me who have an insatiable appetite for data, the device lets you discover patterns in your power consumption you might never have known about and that are burning up your money. Soon afterwards, I got a call from EcoDog's competitor Powerhouse Dynamics. I had mentioned its eMonitor system in passing, but hadn't fully appreciated its differences from EcoDog's system.

Both devices came out about a year ago and provide more detail about where you're using electricity than a whole-house monitor such as The Energy Detective (TED) does. You or your electrician install it by opening your electrical service panel, clamping a little magnetic coil around each circuit wire, and attaching the coils to an interface box. eMonitor has two crucial differences from both EcoDog and TED. First, the interface box plugs directly into the home computer network—it doesn't have an intermediate step of sending data over your electrical wiring, which, in my experience, can be flaky.

Second, the eMonitor interface sends the data to an outside server rather than to your own PC. That server does all the calculations and record-keeping, and you visit a web page to view the data. Thus the system works on Windows, Macs, Linux boxes, iPhones, whatever. The interface buffers 24 hours of data (soon to be 14 days, in a coming update) in case your Internet connection or the company's web server goes down. The company has a demo page you can visit to see a sample data display. The server does entail a monthly fee, but the system itself is a bit cheaper than EcoDog's, so the total price works out to be comparable.

The same question that I had for the EcoDog comes up: Is the system really worth it for most people? Tim Durant, the company's vice-president of business development, says they did a customer survey last November and half the respondents reported saving up to 20 percent on their electric bill. An eighth reported saving more than 20 percent. For those people, the system might indeed pay for itself. Obviously, it also depends on how high your electric bill is, although I suspect that people who have high bills are paying mostly for air-conditioning or heating, and an energy monitor isn't going to help much with that.

Some savings come simply from being conscious of how much power you're using and resolving to cut back. A little blinking box can be a useful ally in the eternal battle to get the family to turn off unused lights. Some savings come from detecting suspicious deviations from standard operating procedure. Both EcoDog and eMonitor can alert you if they sense something awry. For instance, Durant recalls how he recently got a warning that his freezer was using an abnormal amount of juice. He went to check on it and found that someone had left the door ajar. Last year, Durant says, one of the company's founders got a message that the kitchen stove had been left on by accident.

If an electric hot water heater begins to use more power, it might be a sign of sediment build-up. If a freezer seems peculiarly thirsty, something might be wrong with the defrost cycle. The system can also watch for safety hazards (and code violations) such as a circuit that routinely draws more than 80 percent of its rated current.

The potential of monitoring devices, as yet untapped, is to let you compare your power usage to other people's, so you can keep up with the eco-Jonses. "There's plenty of opportunity to tie this into social networking," says one of eMonitor's first users, Arnold de Leon. He lives with his wife and daughter in Cupertino and has provided feedback to the company to debug the device and software.

At the same time, energy monitors will probably save countless marriages by revealing that something you thought was a power hog isn't. De Leon says his oven draws a lot of power when it's on, but doesn't even rank in the top-10 loads over the course of a day. He has also found that you might as well leave LED holiday lights on all the time; a timer would use more power than it saved.

Ironically, De Leon says he'd already done so much to conserve energy that the new monitor hasn't really helped him cut his usage. This is the paradox faced by early adopters: We're the ones willing to buy these systems, but get the least benefit from them. For my part, I doubt EcoDog or eMonitor would justify its cost. Gas is by far the bigger fraction of my energy bill. So I got excited about when Durant told me the company plans to release a gas-monitoring system later this year. He says it'll be able to correlate gas and electricity usage to look for patterns. Now that might pay for itself.

Screen shot courtesy of Powerhouse Dynamics