Design Article

Power monitor measures cumulative and spot energy consumption

Maurice Wright

12/2/2008 12:31 PM EST

For the past several weeks, my family has been testing the Black & Decker Power Monitor. I briefly described the product installation in a blog post a few weeks back. The $100 product is designed to allow consumers to monitor their energy usage and presumably control costs. The product includes a sensor unit that mounts to the AC power meter and a remote display unit that can be used within about 60 feet of the meter according to the product specification. The monitor includes various modes that allow consumers to measure instantaneous power consumption, cumulative consumption, and the consumption of a particular device in the home. The display can report usage in kW or in dollars.

Home automation and security is one of the five industry segments that we follow on the Digital Home DesignLine. Some day we'll presumably all have Zigbee-enabled smart homes and have instantaneous access to power usage data. In fact we've run several articles describing that scenario (See related links below). Today, however, most homes don't have comprehensive home automation networks and power usage is solely gauged by the local electric company.

When I first saw the Black & Decker Power Monitor, I was immediately interested because our utility bills are excessive. Several factors drive the high power bills. Generally speaking, power rates are relatively high in Southern California where I live. I work at home and therefore consume power for computers and other equipment all day. I also have to run the air conditioner to cool my work space more than typical residents in the relatively moderate San Diego climate.

I also know that my family wastes power. I covered the so called vampire power concept in a blog post over on the Power Management DesignLine. Vampire power is the energy wasted by appliances such as TVs that still use power in standby mode waiting for a remote control input, and by chargers for various battery-powered devices that use small amounts of power even when not charging a device. Guidelines such as Energy Star have certainly reduced standby power. And guidelines around the world have mandated more efficient AC/DC power converters. But the typical home still bleeds power. In my house, we have a lot of power converters and we don't have the best habits in making sure that things are unplugged when not in use.


Figure 1: The sensor unit mounts on a power meter and uses a photo detector to sense the rotation of the mechanical wheel inside the meter.
(Click this image to view a larger, more detailed version)

With no smart meter in my immediate future, I installed the Power Monitor sensor on my meter. Black & Decker literature indicated that the product works with most meters in the US. The web site includes a simple graphic that consumers can view to determine if their meter will work with the monitor. In short, the sensor works with meters that have the traditional mechanical wheel that spins as the home consumer power. It also works with some meters that have a single optical port that can transmit power usage information.





gcat

12/10/2008 1:36 PM EST

Generally good information. However I am apalled that the phrase "power factor" is used to describe the energy (not power) pulse constant, Kh, that has a value watthours per revolution. Remember that power factor in engineering terms relates to the phase shift between the voltage waveform and the actual current waveform (cos of the angle = PF). Your excellent baseline power level of 300W would result in a your disk revolving about once every 1.5 minutes. Any low reading like that will take some time to register if those 30 second updates report "no revolution yet". This will also be true if you perform a tare function with a very low baseline and a small incremental load. I am sure the timing if disk rotation is accurate enough and the resolution limit is mostly a display choice. I'll order a unit today.

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