Kill A Watt P4400: find out what's quietly running up your power bill

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The Kill A Watt P4400 is a plug-in meter. You plug it into the wall, plug a device into it, and it shows watts, volts, amps, and cumulative kilowatt-hours. That last number is the useful one for saving money, because it's what your utility bills.

What it does well

• Shows real draw in watts, so you stop guessing which devices are costly.
• Tracks usage over time in kilowatt-hours, so you can measure a fridge across a full day or week.
• Needs no app, account, or Wi-Fi. You read the screen.

Where it falls short

• It measures one plug at a time, and only devices that plug into a standard wall outlet. Hardwired appliances like some dryers and water heaters are out.
• Not a "set and forget" device. You use it, learn, and put it in a drawer.
• The display can be awkward to read when the outlet is behind furniture.

The money math (use your own numbers)

Cost per year = watts ÷ 1,000 × hours per year × your rate per kWh. For a device left on all year, hours = 8,760.

Illustration, assuming a 17¢ per kWh rate (check your bill): a console or set-top box idling at 10 watts costs about 10 ÷ 1,000 × 8,760 × 0.17 ≈ $15 a year. A desktop PC with monitor and speakers idling at 40 watts costs about $60 a year. Several of those across a home adds up.

So the meter pays for itself only if it finds something you can switch off or replace. In a home with an old second fridge or a pile of always-on electronics, it likely will. In a small apartment with little equipment, probably not.

Related: 9 ways to cut your electric bill.

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