Kill A Watt
The Kill A Watt (a pun on kilowatt) is an electricity usage monitor manufactured 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 125Vac), the Kill A Watt is sold for the North American market. The factory is 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 amperes, power up to 1,875 VA (the 230Vac equivalents also allow up to 15A, corresponding to 3,750 VA).
There are several models of Kill A Watt meters:
- 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.
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.
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
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.
The Tweet-a-watt is a hacked version of the standard Kill-A-Watt Plug in Power Meter. By piggybacking on the devices 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.
Following the usefulness of the Tweet-a-Watt, designs for the MoteWatt 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.
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.
- Edwin Kee (2008-05-26). "Cost-saving and handy devices". New Straits Times.
- P4400 specification
- Model 2022-04 Manufacturer's web site.
- "Gadgets that help save energy". Bloomberg News. 2008-05-04. pp. F02.
- "Prodigit 2000M 8-in-1 Plug-in Power Monitor". Retrieved 2009-07-16.
- P4400 Kill A Watt Manufacturer's web site.
- "Prodigit 2022 Energy Cost Monitor". Retrieved 2009-07-16.
- "Prodigit 2024 8 Port Power Safer". Retrieved 2009-07-16.
- P4320 Kill A Watt PS Manufacturer's web site.
- Power Safer — New Safety Revolution For Power Strip Fairly thick chinglish, but informative technical description of the features provided.
- The Kill-a-Watt: Not all versions are created equal
- Kill A Watt P4400/P4460 circuit diagram
- http://www.ladyada.net/make/tweetawatt/. Retrieved 1 November 2013. Missing or empty
- https://twitter.com/tweetawatt. Retrieved 1 November 2013. Missing or empty
- Rusu, Felix. "Meet the WattMote (Moteino based Tweet-A-Watt)". Retrieved 12 December 2013.
- Tranchemontagne, Mike. "Moteino Kill-A-Watt: Hardware". Retrieved 12 December 2013.
- Measuring standby power
- 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
- 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.
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