Wattvision: What to do with all this cool data?

This is the third in a series of guest posts by Ted Borer, PE, the energy plant manager for Princeton University. He is actively involved in campus and community energy efficiency and carbon emissions reduction efforts. He has over 25 years of experience in the energy industry, is a registered professional engineer, and holds both undergraduate and graduate degrees in Mechanical Engineering as well as the CEM, CEP, and LEEDAP Certifications. He has published numerous magazine articles, technical papers, and a book chapter on topics relating to energy and cogeneration. 

The Wattvision system offers a highly-detailed look at our energy use. It can measure the magnitude and duration of events that last for less than a minute and demand only a few watts, yet the data can be collected over a period of days, months, or even years. This level of detail offers us insight that we could never get by reviewing a monthly electric bill, or even by seeing an hourly total of power use. By carefully observing the patterns and durations of events in our energy use, and superimposing what we know about what is happening at the time, we can determine which activities use the most and least energy and which we may want to modify.  

Let’s break this down into manageable steps.

First it is helpful to develop a simple inventory of the things that use energy around the house. This may be informative when interpreting the Wattvision graphs and data.

We don’t need to be rigorously thorough or gather complete information about every light bulb. We just need to list the energy consumers that either use a lot of energy, or use energy for a long time. If you need ideas about what to look for around your home, the US Department of Energy offers a list of typical appliances and the energy they consume. This gives us a sense of what are large and small energy consumers, but it is important to check your specific equipment since there are great variations in efficiency and personal use.

Once we know what consumes energy, we’ll make a log of energy events for a while.

Then, we can combine this information with the Wattvision graphs and data to determine which activities used an important amount of energy, and which activities used less. The patterns, magnitude, and durations of events we see in the graphs will give us important clues as to what is happening. 

After making the log entries shown above, this graph was created by downloading that morning’s data from the Wattvision history page, opening it with an Excel spreadsheet, and adding notes.

Now we can begin to make a few observations about this snapshot of our energy use:

  1. The biggest energy user is a steady demand of over 3500 watts. It was 20°F outdoors that morning. No doubt  most of this demand is from our electric heat pump that provides a large portion of our heating energy. If this ran at the same rate all day, it would cost: (3500 watts) x (24 hours) x ($0.19/kilowatt-hour) = $15.96/day!
  2. There are random periodic spikes of over 500 watts. They are too many watts and too brief to be lighting. They may be associated with the well pump coming on to restore water pressure after toilet and sink use.
  3. The coffee maker uses about 1000 watts for about five minutes. What does that cost? ($0.19/kilowatt-hour) x (5 minutes) x (1 hour/60 minutes) =  $0.0158, i.e., less than two cents. So the coffee and cream cost more than the electricity to make a latte’.
  4. Cooking on the stove top or in the toaster oven both appear to use about 1900 watts apiece for several minutes.
  5. There are two similar-looking steep downward spikes in energy use of over 1000 watts for about 5 minutes each. They are about 90 minutes apart. After the drop, energy demand increases again gradually over several minutes, not all at once. No specific energy event was noted in the log, so these may be associated with something that runs automatically. They are large enough that they probably relate to the heat pump. They may be associated with the heat pump defrost cycle. This is worth further investigation. Two sources of information would be the equipment nameplate and the owner’s manual.

We’ve made guesses about some of the events in the graph above. But repeated log entries and graph observations will allow us to confirm or correct these. The important thing is to begin by making observations to sort out what is happening. Then we can see what may be worth adjusting and what may not be worth the effort, inconvenience, or expense. What do we have control over? What are we interested in changing? Appliances that use a lot of power for a long time will have the biggest impact. If we have limited time and resources, the graph above suggests we’re better off looking for ways to reduce energy used by our heat-pump than by getting a more efficient toaster, stove, or coffee maker.