science

Kill A Watt

Kill A Watt is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kill A Watt rather than just read about it. In short: The Kill A Watt (a pun on kilowatt) is an electricity usage monitor manufactured by Prodigit Electronics and sold by P3 International. It measures the energy used by devices plugged directly into the meter, as opposed to in-home energy use displays, which display the energy used by an entire household.

Key takeaways

Kill A Watt belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kill A Watt to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kill A Watt from memory before moving on to harder problems.

Reference excerpt

The Kill A Watt (a pun on kilowatt) is an electricity usage monitor manufactured by Prodigit Electronics and sold by P3 International. It measures the energy used by devices plugged directly into the meter, as opposed to in-home energy use displays, which display the energy used by an entire household. The LCD shows voltage; current; true, reactive, and apparent power; power factor (for sinusoidal waveform); energy consumed in kWh; and hours connected. Some models display estimated cost. Having a NEMA 5-15 plug and receptacle, and rated for 115 VAC (maximum 125 VAC), the Kill A Watt is sold for the North American market. The unit is manufactured by the Taiwanese company Prodigit, which also makes 230 VAC models of similar appearance and functionality for European Schuko, U.K. BS 1363 and Australian AS 3112 receptacles, and a model compatible with 100 VAC for the Japanese market (2022-04, marketed there as the Watt Checker [ワットチェッカー] Plus by other companies). The basic models support current up to 15 A, power up to 1,875 W (the 230 VAC equivalents also allow up to 15 A, corresponding to 3,750 W). The device can give an indication of the standby power used by appliances.

Models There are several models of Kill A Watt meters:

P4400 This is the original, most basic version, based on the Prodigit 2000M. From the time it is plugged in, it measures:

Voltage (Volts) Current (Amperes) Power (Watts) Energy Used (Kilowatt-hours) Frequency (Hertz) Apparent power (Volt-amperes) Power factor Elapsed time The power setting displays instantaneous power, and the kilowatt-hour setting displays energy consumed since last reset. When electricity is disconnected, the P4400's measurements and meters are reset. To reset Kill A Watt models that store values, press Volt and Amp together, or press the Reset key in some models.

P4460 Kill A Watt EZ This is an enhanced version, based on the Prodigit 2022, which includes a backup battery to store measurements even when disconnected from electricity. It has the same capabilities as the P4400, and can be programmed with electricity cost information, which enables it to display the cost of the electricity consumed since reset. From this, it can calculate cost per hour, day, week, month, or year.

P4320 Kill A Watt PS This model, based on the Prodigit 2024, is integrated into an eight-outlet power strip. Unlike the other models, it does not display frequency or apparent power. It protects against surges and EMI, has a configurable overcurrent shutdown limit, and also measures earth leakage current; one version acts as an earth leakage circuit breaker (ELCB). It switches power on or off at an AC zero crossing, minimizing current surges and interference.

Variations Although identical externally, there have been several different versions of the Kill A Watt and Prodigit equivalents, all essentially of the same design, but with different PCB layout, packaging, and components.

User modifications One shortcoming of the Kill-a-Watt range of devices is that they do not have the ability to store, transmit or transfer the readings, thus limiting their usage for any ongoing monitoring purposes. To counter this shortcoming, a couple of openly available modifications have been published on the Web, to enable these devices send data wirelessly to a receiver. A circuit diagram has been drawn up.

Tweet-a-watt The Tweet-a-watt is a hacked version of the standard Kill-A-Watt Plug in Power Meter. By piggybacking on the device's on-board LM2902N op-amp chip, the creator was able to get readings for voltage and current and transmit to a computer, which then sent this to Twitter via handle @tweetawatt. At the time it gained quite a lot of interest on the Web, but interest waned after some time. The last tweet from this handle was in March 2010.

WattMote Following the usefulness of the Tweet-a-Watt, designs for the WattMote were released on the Web by another hobbyist, Felix Rusu at LowPowerLab. The modifications use a customized clone of the Arduino chip known as the Moteino, making this version much cheaper, and requires much less soldering than the original design. Further optimizations on the design were done by Mike Tranchemontage, his designs featured a more robust power supply unit to the moteino chip, avoid problems capacitors which discharged too slowly with the original design.

Standby power Most plug-in wattmeters are not useful for measuring standby power, also called vampire power if the device in standby is not doing anything useful such as being prepared to wake under timer control. Many meters only have a resolution of 1W when reading power; the Kill-a-Watts read down to 0.1W, but this is still too coarse for measuring low standby power. Modification to read standby power has been described and discussed in detail (with oscilloscope waveforms and measurements). Essentially, the meter's shunt resistor, used to generate a voltage proportional to load current, is replaced by a much larger value, typically 100 times larger, with protective diodes. Readings of the modified meter have to be divided by the resistance factor (e.g. 100), and maximum measurable power is reduced by the same factor.

See also Home energy monitor Nonintrusive load monitoring Wattmeter

References

External links P4400 Kill A Watt - official product website Kill A Watt models Sparkfun: Cracking open a Kill A Watt Tweet-a-watt Wireless communications added to Kill A Watt. Saving energy: is it better to start with utility bills or a Kill A Watt? Kill A Watt EZ electricity usage monitor review Archived 2009-05-11 at the Wayback Machine Conner, Steve (2005-08-30). "Kill-A-Watt vs. analog wattmeter". Tesla Coil (Mailing list). Retrieved 2009-07-16. The low power factor means that the DRSSTC was drawing awful peak currents, but the two meters still agreed well. I conclude that the Prodigit wattmeter sold by Maplin works fine for Tesla coiling. It also seems to be fairly resistant to RFI, although I didn't let the coil arc to ground heavily. The Latest Public Library Loan? Electricity Meters Archived 2015-04-11 at the Wayback Machine

Illustrations

Worked examples

Example 1 — a first encounter with Kill A Watt

Start with the simplest possible case. Write down what Kill A Watt claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kill A Watt before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kill A Watt ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kill A Watt

In research: Kill A Watt appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kill A Watt in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kill A Watt is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical test equipment, Electricity meters, so understanding it makes those chapters shorter.
In everyday life: Look for Kill A Watt outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kill A Watt in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kill A Watt means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kill A Watt out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kill A Watt in simple terms?

Why does Kill A Watt matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kill A Watt?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kill A Watt.