During the entire first century it was in common use, the telephone was among the simplest and most energy-efficient of all household technologies. It didn't require any electrical current, and the traditional landline setup still doesn't. Anyone who's ever experienced a blackout can tell you that when the juice stops flowing all over town, the telephone is the one gadget that keeps working-unless, of course, you use a cordless model.

Cordless phones have been on the market for over 25 years now, and they are the epitome of convenience, letting us wander all around the house while we chatter away. But that convenience comes at a cost. Because the cordless handset is essentially a radio communicating with a transmitter in the base station, and because the battery in the handset is constantly recharging itself when it's resting in the dock, your cordless phone must remain plugged in at all times. That little red light glowing 24/7, indicating that power is flowing, means the phone is consuming anywhere from 2 to 7 watts of electricity. If you do unplug it or lose power, prepare to be cut off from the world-which is one reason emergency-response officials suggest keeping an old-fashioned corded phone around in case a power outage hits.

Answering machines, by the way, are no better. Often built into phones, they're no longer the VCR-size dual tape decks of years past. But they still require constant power to be ready to pick up a call at any time, day or night.

Hertz so good: choosing a frequency
When you shop for a phone, you'll find models that are billed as 900 MHz, 2.4 GHz, or 5.8 GHz. These numbers refer to the base station's transmission frequency (in millions or billions of hertz, or cycles per second) and have some bearing on its range-300 to 2,000 feet in the case of a 2.4 GHz or 5.8 GHz phone, with actual performance heavily dependent on such factors as your home's construction materials and layout.

The 2.4 and 5.8 GHz phones are digital (your best choice), but the higher number doesn't necessarily mean more power consumption. A 2.4 GHz phone does use more power than a 900 MHz phone, but a 5.8 GHz phone uses no more power than a 2.4 GHz model. In general, you'll get the best mix of price, performance, and energy savings with a 2.4 GHz model. However, since many wireless computer networks operate in that frequency range, choosing a 5.8 GHz model will minimize interference issues if you use a wireless connection at home.

Energy Star rates cordless phones and gives its approval to models that use about one-third the energy of the average cordless phone. How do these models manage it? Energy Star phones typically have better-than-average power supplies that are far more energy-efficient than the norm. You may pay more up front for that higher-quality power supply, but you'll recoup the cost and more in future energy savings.

So if you have your heart set on a cordless phone, an Energy Star-rated 2.4 GHz model is in most cases the way to go. But if you're willing to trade the convenience for greater savings in money and energy, here are some alternative strategies.

Cut the cord: go all-cellular
Do you spend more time on your cell phone than on your good old home phone? Then think about why you're keep your home phone number. If it's for the desirable area code, consider consolidating your phone use under your favorite number and assigning that to your cell-the single bill is bound to save you quite a bit of cash. Going home-phone-free makes sense if you're comfortable with your phone and your plan, and are willing to go through the process of alerting friends and family of your preferred phone number.

Cell phone manufacturers offer countless types of phone docks that let you connect your cell phone to more comfortable handsets when you're at home. Simply place the base unit at the spot in your home where your cell signal is strongest; attach your cell phone when you get home; and pick up your calls on the nice cordless handset. Of course, the dock is electrically powered, so you lose the green advantage when you opt for this approach (be sure to plug both the dock's power brick and your cell phone charger into a power strip so that you can turn them off when they're not in use). But without that landline, you'll still be saving money and avoiding the hassle of an additional monthly phone bill.

A smarter answer: outsource your voicemail
If the only reason you keep your cordless phone plugged in when you're out of the house is so that its built-in answering machine will pick up your calls, then consider pulling the plug and letting your phone company take your messages digitally. Phone companies typically offer voicemail services for under $5 per month, and you can retrieve your messages easily from anywhere. By moving this operation out of your house, you can unplug your phone when you leave for the weekend or go on vacation.

If you still want to have an answering machine at home, get a combination phone/answering machine. There's no need to be separately powering both devices. Shop for an Energy Star model, of course. You shouldn't need to spend more than $70 unless you want more than one base station or several handsets.

Net result: place your calls online
If you have broadband Internet access and tend to leave your computer on all the time (a potentially energy-guzzling behavior you might want to rethink, by the way), perhaps you should try voice over Internet protocol (VoIP), the technology that turns the Internet into your phone system. VoIP companies like Vonage offer impressive phone savings, but the hardware router that you must add to your system demands power, so it's not a particularly green solution. Software-only VoIP, like the most basic services offered by Skype, require nothing more than the computer you already use, but you'll typically need to be seated at your computer with a headset and microphone to use it in its most basic and inexpensive form.

Whether or not you're ready to take this kind of a technological leap, it's important to get to know a bit about VoIP, because this is where future telephone technology is heading. (Get a five-minute tutorial at How Stuff Works). And by the way, don't confuse VoIP with the digital phone service that your cable company may be asking you to try. Digital phone service, which sends your phone signal through the same cable that carries your cable-TV signal and broadband Internet access, is easy to set up and lets you use the phones and jacks you already own. But that's the bad news, too. You'll still be using your energy-drinking cordless phones.

An off-the-hook idea: go back to your old corded phone
This is an impractical and inconvenient idea, and you probably won't do it. But it's definitely the single most inexpensive-not to mention environmentally friendly-way to talk on the phone.

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