I have been testing three of these devices, the Power Monitor from Black & Decker Corp. (BDK), the very similar PowerCost Monitor from Blue Line Innovations Inc., and the more-sophisticated The Energy Detective 5000 from Energy Inc. In my tests, the Black & Decker model provided the most effortless electricity-tracking service. At $99.99, it is also the least expensive.

The devices provide real-time data about how much power you’re using across the house in terms that are easy to comprehend: cost per hour and cost per month. Turn on the microwave and watch the cost jump from 10 cents to 25 cents an hour. Turn off some lights and see the cost drop a few cents.

The firms say their customers have, over time, seen drops of as much as 20% in power bills by being more mindful of electricity use and making informed purchases. An independent Oxford University study in 2006 found that people getting direct feedback on their power consumption reduced use 5% to 15%.

After I began monitoring, my most-recent electricity bill dropped $10 from the month before—but that could also be due to my living in a city where air conditioning isn’t a summer necessity. I find myself thinking more about electricity, and even running back into the house to make sure the lights are out.

The monitors sold by Blue Line and Black & Decker are almost identical, because they’re both manufactured by Canada-based Blue Line. The Blue Line model costs $109, is a bit larger, and features a slightly longer range for the wireless signal that transmits power use from your electric meter.

Connecting these two devices to my electric meter was simple. First, loop a metal belt around the glass dome covering the meter. Then align a sensor attached to the belt on top of the glass to read the data collected by your meter. On my old-style meter, the Power Monitor’s sensor keeps track of how fast a dial rotates. The companies say their products work with about 90% of meters in North America.

The sensor you attach to the electric meter wirelessly sends raw data to a digital monitor that is kept inside the house. Before using the monitor, you have to enter data from your electric bill, but finding the right data can be tricky. Black & Decker’s instructions on this are relatively clear, and entering the data into the digital monitor involves a process similar to setting an alarm clock.

The digital monitors, about the size of a large remote control, can sit in one room or travel about the house. A button labeled “tare” on the Black & Decker model helps you calculate how much electricity is being used by any single appliance that you can turn off and on.

The Black & Decker model features a rudimentary display that only reports the aggregate power use for your house at any given time. It can’t go back and show you changes over time.

But the latest model from The Energy Detective, known as TED, connects directly to a house’s power supply for a more-precise read than the Black & Decker. It comes with software that graphs how use patterns change over time. The TED 5000 costs $199.

But installing TED requires turning off your home’s main power line and inserting a sensor into your circuit breaker—a process that the company says should be done “by qualified personnel only.” I sought help from a friend who has a lot of wiring experience, but after several hours, we were unable to make TED work. My issue was likely a decades-old circuit breaker. The company said my configuration is atypical and that problems like this are rare. A colleague has been using TED for several weeks after hiring an electrician to install it.

While TED 5000 offers many more advanced tools for sleuthing your home’s electricity waste than the other models, all of its sophistication won’t necessarily help the average user do much of a better job remembering to turn off the lights. For most of us, the large cost-an-hour sign on the Black & Decker Power Monitor offers the only feedback we really need.

And before buying any of these devices, keep in mind that many utility companies are installing a new generation of so-called “smart” meters, which not only measure real-time power use, but also offer two-way communication with the power company to help cut costs. And Google Inc.’s (GOOG) nonprofit foundation is working with power companies on a free service that connects data about your power use into an online widget.

Walt Mossberg is on vacation. Email Geoffrey A. Fowler at geoffrey.fowler@wsj.com

This alternative Kindle, called the DX, is a super-size version of the popular Kindle 2, which arrived earlier this year. It sports a 9.7-inch screen, some 2.5 times as big as the surface area of the Kindle 2’s 6-inch display. It also sports a higher price tag — $489, versus $359 for its smaller sibling, which remains on the market.

Amazon’s Kindle DX

Like previous models, the Kindle DX allows you to shop for, and wirelessly download, any e-book in Amazon’s (AMZN) growing catalog — now about 275,000 titles. It uses the same easy-on-the-eyes screen technology as the smaller model but at a higher resolution. It still lacks color, and renders images only in gray scale. But the new DX adds a new capability: auto-rotation, which allows you to read in landscape mode.

This new, larger model isn’t primarily aimed at readers of standard books. It is targeted at three markets: textbooks, newspapers and other periodicals, and business documents in either Microsoft’s (MSFT) Word format or Adobe’s (ADBE) PDF format.

Unfortunately, I’ve been testing the Kindle DX and I didn’t like it nearly as much as the Kindle 2, which I own and enjoy using daily. While it performs its promised tasks adequately, I found that its size and weight made it awkward and tiring to hold for long periods of reading. It’s still fairly thin and light, but it’s 85% larger and heavier than the standard Kindle.

In addition, Amazon has degraded the user interface. To prevent the device from being even larger, the company had to remove the left-side page-turning buttons, confining all the controls to a vertical strip on the right. The keyboard at the bottom is also more vertically cramped.

If you’re left-handed, you have to spin the device around and rotate the screen to get buttons on the left, where they appear with their labels upside-down.

Similarly, if you choose to read in landscape mode, all the navigation controls, including the joystick for moving the cursor, will be awkwardly placed at either the top or bottom, far from where your hands are holding the device, and the keyboard essentially will be unusable.

In my view, the Kindle DX would have been a better product with on-screen touch controls that could instantly adapt to its size and orientation.

In reading standard books on the DX, I also encountered instances where the text on a page varied in shade from light gray to black.

I had mixed results with business documents. As with previous Kindles, you can either email personal documents to your device, for a fee, or drag them onto the Kindle via a cable, for free. But, unlike the smaller models, the new DX has PDF display capability built in, so it renders PDF files much more accurately than the older Kindles. That is a big improvement. It also allows you to view Excel and PowerPoint files if you save them in PDF format before sending them to your Kindle DX.

The Kindle DX, right, is a large-screen version of the popular Kindle e-reader, left.

I tried a variety of documents, and in many cases the results were great. The text was crisp, and the tables and graphics looked like they should. But I found that on some of these PDF documents, the text was too small to read. Yet, the Kindle lacks the ability to zoom in on PDF documents. You often can make the type larger by rotating to landscape mode, but this splits the PDFs into multiple pages, sometimes breaking them awkwardly.

Also, Amazon has raised its fees for converting and delivering business documents via email to all Kindles. The charge was formerly 10 cents a document. Now, it’s 15 cents per megabyte, which can add up if you load up your Kindle with lots of large documents. Most of my test documents, which were fairly small, cost over $1 each.

Newspapers looked about the same on the DX as they do on the smaller Kindles. Despite the larger screen, they don’t use traditional print or Web layouts, but a special Kindle layout that some users like a lot, but which I find annoying because it makes it harder to quickly scan multiple headlines.

The Kindle DX does have some nice touches. For the extra money, you not only get a larger screen, but also about twice the storage capacity. Also, because the screen is wider, you can adjust the margins on the DX, to obtain a line length that’s comfortable for your eyes and optimal for reading speed.

Amazon claims the same multiday battery life for the DX as for the Kindle 2. In my tests, I was able to go for several days of moderate reading without recharging, and much more if I turned off the wireless capability.

Although I wasn’t able to test college textbooks, I suspect they may be the killer app for this product. Many already are so expensive and heavy they could make the weight and price of the Kindle DX seem trivial in comparison.

But for standard books, I’d stick with the smaller, more comfortable Kindle 2.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.

While these location-sharing services have some interesting possibilities, they also raise some disturbing implications for privacy — or maybe it just seems that way if, like me, you’re over 35 years old. Lately I’ve been testing a cellphone location-sharing service that I found simple, useful and non-creepy enough that I can imagine people thirtysomething and older using it.

Send a ‘Glympse’ to share your location

The free service is called Glympse, from a company of the same name that has designed it to share your location with friends and colleagues in small increments of time — glimpses, as the name implies, of your whereabouts. Glympse just released a test version of the service as an application for the G1, a phone offered by T-Mobile that runs Google’s (GOOG) Android operating system.

The company will release versions of Glympse for BlackBerry, Windows Mobile, iPhone and Nokia (NOK) phones in the coming weeks. Users can download the Glympse software onto G1 phones through Android Market, the online clearinghouse for applications for Android phones.

I used Glympse on an iPhone and a G1 and, for comparison, tried out a couple of other location-sharing applications, Google Latitude and Loopt. When you start the Glympse application, it identifies where you are on a map using a combination of location technologies in cellphones, including GPS satellites, Wi-Fi hot spots and phone towers.

Your location isn’t shared with others until you “send a Glympse” to someone. The software allowed me to send a Glympse with my location for selected chunks of time lasting anywhere from zero minutes to four hours. Picking zero minutes shared only my location at the moment of sending, while selecting four hours meant the recipients of my Glympse could track me for that period of time, wherever I went.

The sender of a Glympse can address it by entering a recipient’s email address, or a mobile-phone number for a text message. Recipients get a message with a link to a map on a Web page. A nice thing about the service is that it doesn’t require recipients to have the Glympse software, though the experience is richer when they do.

Sending a Glympse can be helpful anytime a friend, family member or colleague is expecting you. You can send a Glympse that lets a friend know you’re stuck in heavy traffic (although it’s wise to do that before you’re on the road to stay safe and, in some states, to avoid breaking the law). Parents can insist that teens send a Glympse when they go out for the evening.

Bryan Trussel, the CEO of Glympse, sent me several Glympses with short messages like “late lunch meeting” and “headed home” to explain where he was going. On the G1, I could see an icon representing Mr. Trussel moving around a map as he drove through the streets of Redmond, Wash., including his speed. I was also able to see traffic conditions, which would have been helpful if he had been running late. Glympse gave me a similar view of his location through a Web browser running on my PC. On the iPhone, the experience was more static, forcing me to reload a Web page to get a fresh view of Mr. Trussel’s whereabouts. Glympse says it will fix this so users of the iPhone can watch someone’s location in real time.

I encountered a bug with the service when I sent a couple Glympses to my own cellphone and that of a colleague — both BlackBerrys and neither of which had Glympse software. Both Glympses linked to high-level maps suggesting I was somewhere in North America, which wasn’t very helpful, even if it was technically accurate. The company couldn’t figure out what the problem was, and it eventually stopped happening for me too.

Once a Glympse expires, the service no longer tracks the sender’s location. It will show the sender’s last known location for 48 hours after the Glympse expires. Glympse may some day use your location information to target advertising to you, but the company isn’t doing that for now.

I’m far more comfortable with this form of finite location-sharing than the approach used by other services. Google Latitude lets you share your location to only a list of friends. It gives you the option of turning location sharing off or, through a “city level” option, it reveals only your general whereabouts. Loopt similarly allows you to set options to show, or hide your location from all or some online friends. A feature called Loopt Mix lists strangers you can send messages to in your general vicinity — many of whom seemed to be looking for romantic partners in my area.

Despite these controls, I easily forgot I was sharing my location with these two services. This might be fine for someone who enjoys being tracked down by friends during a night out on the town or is diligent about changing privacy settings when they want to go stealth.

I’m too lazy to manage my privacy so closely though. I don’t mind giving friends a look at my location, but only if I know the invitation isn’t open-ended.

• Email nick.wingfield@wsj.com.