On using the P3 Kill A Watt

A Kill a Watt digital power meter

In order to reduce standby demand and calculate actual wattage among my plug-in electronic devices, I recently purchased a Kill A Watt digital power meter. The Kill A Watt is a plug-in device calculates and displays various power consumption statistics on anything plugged into it. It is easy to use, and provides a wealth of valuable information about your electronic devices.

There are several devices in the Kill a Watt family. The 4400 is P3’s basic model, and the model I recommend. The 4460 is an advanced version of the ‘brick’ model that calculates the actual cost of the energy and has some built-in memory. They also make a power strip model, which is superfluous since you can plug any power strip into the basic brick module. The additional models are simply ‘value add’ and will set you back about $20 extra. I found that using the existing features and the calculations below, one can overcome their lack on the 4400.

The biggest use of the Kill a Watt family is that you can measure your energy use over a period of time. This fits into the improvement process by giving you solid measurements on which to base your improvements. Armed with this information, you can decide to turn off, unplug, or replace the device in question with a more energy-efficient model.

What does it do?

The device provides information about power consumption. It shows the following information on its LCD screen, which can be manipulated by the labeled buttons on its face:

• Volts (V)
• Amperes (A)
• Watts (W)
• Volt-amperes (VA)
• Hertz (Hz)
• Power Factor (PF)
• Kilowatt-hours (kWh)

All features are accessed by simply pressing a button. Be aware that there is often a slight delay as the module gathers the information you need. The second row is accessed by pressing the button twice.

Voltage and Hertz

In US households, potential (V) is anywhere between 110 to 120 Volts, and Frequency is usually a solid 59.9 Hz. In most other systems of the world, 220 V/50 Hz is common. Voltage on most power systems remains fixed, but sometimes it varies from room to room in a house. Brownouts and power surges can cause dips and spikes in Voltage. I have never seen my home ever run at anything but 59.9 Hz. I think these are a nice pair of diagnostic values, but they don’t really see much daily use.

Wattage and Kilowatt-hours

Wattage is what residential energy customers pay for, and the rates for residential use are applied in cents per kilowatt-hour.

The wattage listed on a device’s spec sheet isn’t always correct, as you can see if you plug an item into the device and read actual wattage. The reading of the Kill a Watt is often lower than the rated value in the spec sheet, and it is sometimes higher. When a device is trickling a very small amount of power, it is called ’standby’ power. Standby power is generally a very small draw in current, but even very small numbers add up over time.

Watts are only a measure of resistive load. Resistive loads convert electricity into other forms of energy directly, such as a light bulb or electric stove generating both heat and visible light.

To measure the wattage, simply plug the device into the Kill a Watt and press the Watt button. After a few moments, the display will show the current wattage, updated every second or so. Read the wattage and use the following formula to calculate expected kilowatt-hour consumption.

kWh / (Hours * 60 + Minutes) / 60

kWh consumption is also found via the KWH button. Leave the device plugged in for any length of time and use it like you normally would. When you want to find the kWh use over the testing period, simply press KWH, and read that value. Then press the button to get the period of measurement and divide one by the other to get the average consumption per hour.

Volt Amperage and Power Factor

This section mainly applies to industrial applications, so I’ll be brief. Residential users see little benefit from this value unless their power provider measures power factor and adjusts billing for low overall efficiency in the home. Industrial users already pay these efficiency fees, so this is an issue.

Volt-Ampereage, as its name implies, is the product of voltage and amperage. VA is the measure of apparent load, which includes any motor and most electronics.

In DC power, Volt Amperage and wattage Wattage are equivalent. In AC power systems, however, they are not. Electronics aren’t perfectly inductive or capacitive and they alter the AC waveform, changing its phase. This causes them to have a different apparent load, and the ratio of apparent to real load is the power factor.

Power factor is not a real measure of efficiency, but the ratio between apparent and real power draw. Again, for residential users, the difference in power factor doesn’t affect the bottom line.

The long term

The best use of the Kill A Watt is to measure the power draw over time in order to make decisions about an appliance in your home. First, monitor a device over a few weeks with the Kill A Watt to measure kWh under normal use. I like to do this for a daily stretch, but it’s easy to leave the Kill-a-Watt plugged in for a month. Divide kWh by days in order to get an idea of the typical daily consumption of your device under test.

Once you have this value, it’s time to check the instantaneous numbers for the device. This means capturing the following:

• Powered off load, which will either be no draw or a standby trickle.
• Idle load, which is when the device is on but not ‘doing something’.
• Peak load, when the device is on and turned up to 11.

This helps you further refine your decision on what to do with the device:

• Turn it off: for devices with low idle or high peak load, but no draw while off. Example: computers, toaster ovens. Savings comes from minimizing use.
• Unplug it: for devices with acceptable load while on, but a higher than normal load when on standby. The standby often has no benefit in using the device, so Savings comes from avoiding stand-by.
• Replace it: devices you have immediate need for, but have the problem of always being in peak load or they draw a lot of standby power.They cost you a fortune in energy. This often applies to electric lights, and older TV set top boxes. Ditching your incandescent lights for alternative technologies can save a big wad of cash.

A word on specifications and typical load. Some devices, like refrigerators, may outperform or under-perform their energyguide tag. The tests involved are self-reported, and based on a ‘typical’ duty cycle. Your use of a device is probably not typical, either to your benefit or detriment.

Conclusion

The Kill A Watt is a wonderful decision tool that can really get to the bottom of your energy problems quickly. I believe it’s worth the $25 to save hundreds over its usable life. It has made me more  more conscious of my energy diet, and it helps me think in the long term costs. with it you can back your decisions up with facts, and it beats guesswork or reading consumptions specs for getting to the bottom of your energy bill.

I highly recommend it.