Way back when, on my way to improving my home’s energy efficiency, I made a plug adapter for an inexpensive digital clamp-on multimeter to start identifying the energy hogs in my home. The adapter was an AC plug wired to an AC socket with three accessible wires between. A digital AC meter clamped around the hot wire to measure the current to a particular device. I would read the amps, then multiply this value by 120 volts to get watts. Then, I’d multiply the watts by the number of hours the appliance was on to calculate watt-hours, dividing by 1,000 to get kWh.
This device worked well enough, but it was limited in its scope and application, and could not accurately and over time measure intermittent loads like refrigerators that cycle on and off. Thankfully, consumer products are available to do this job, helping consumers find the energy wasters in their homes.
One of the simplest consumer devices is the watt/watt-hour meter used to measure the power and energy consumption of an individual appliance. The Kill A Watt meter ($20–$50) simplifies the measuring process and adds many great features, such as calculating energy cost and averaging energy consumption over time. The meter plugs directly into a wall outlet, which can make it difficult to read since it may be low on the wall or even plugged in behind the appliance it’s measuring. So I typically use it with a short extension cord to make it easier to read. Watts Up? and Brand Electronics meters ($95–$235) are a bit pricier, but are easy to read since they have their own power cord. All of these devices record an appliance’s energy use and cost over time, and record accumulated kWh.
But watt-meters only give a small window into a home’s overall power use, since they are limited to measuring 120 VAC appliances—they can’t accommodate 240 VAC loads—and can only measure one appliance at a time instead of the whole household. (They can measure several appliances if used with a plug strip.) So about four years ago, I bought a HOBO data logger that records information from up to four sensors, such as temperature, voltage, and current. A simple monitoring package runs about $300 (logger, $100; Lite software package, $35; and a couple of current sensors, $90 each). This is a science-nerd tool and requires some patience to set up and get used to. The HOBO is programmed via a computer’s USB port and can run for months on its own internal battery. It can record at intervals of 1 second up to 1 day. I have found that a 1-minute interval is fine for electrical monitoring. (I also used it to record temperature data for my solar heating system; see Access.)
I installed the HOBO in my breaker box with the current sensors clamped around each leg of the main power cables. I would download the day’s data to my laptop, and use the PLOT function to create graphical views of energy use over time.
Trying to correlate our energy use patterns with the graphed data was like reading a mystery novel. The first thing that I noticed was two “heartbeats” that cycled on and off all day—our two refrigerators. (My wife uses a small under-counter fridge to store fabric dyes for her business.) Other wiggles and bumps in the graph plots were lighting and the well pump (big, short spikes). The HOBO software allows labeling the graphs and customizing them in many ways.
The graphs show a detailed timeline, which you can correlate with specific times of electric usage—and figure out what loads were running. A graph plot that shifts up and remains constant for an hour or more typically corresponds to the lighting load. Electric water heaters typically produce high peaks on a graph. I used my HOBO to help my neighbors identify their big energy hog—a water heater which was constantly cycling throughout the day. I helped them install a timer to shut it off during the day, when they were not home.