This is the fifth 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 offers us a means to study our electric use in highly granular detail. By looking closely at the data and considering what is happening in the house, we can determine both the magnitude and duration of electric use for a specific activity. Then we can easily calculate the cost and decide whether that cost is good value.
What follows is a close technical analysis of my energy use. Wattvision may automate these sorts of analyses, but until that time nothing beats looking at the data.
For example, the graph below shows a recent day where we ran the clothes dryer starting at about 12:15 PM.
Let’s look at that time period in detail by selecting a range between 12:00 and 2:00 PM.
Looking at the detail above, or referring to a .csv (comma separated values) file downloaded from wattvision, it’s easy to see that the dryer ran from 12:13 PM until 1:20 PM. Before it began, there was a base-load electric use of 1700 watts in the house. We can imagine the drum motor running continuously and a resistance heating coil turning on and off based on a temperature and humidity signal. The drum motor may only use a hundred watts or so, but the resistance heating coil looks it draws about 5000 Watts when it’s on! Since it cycles on and off, the average household demand during the operating period is about 5500 Watts.
With that information we can determine the energy consumption and cost of drying a load of clothes:
5500 Watts while the dryer operates
-1700 Watts base-load activity
3800 Watts, average clothes dryer use rate
From 12:13 until 1:20 is 67 minutes or 1.117 hours
(3800 watts) x (1.117 hours) / (1000 watts per kilowatt) = 4.2446 kilowatt-hours
If our power costs $0.20/kilowatt-hour, then drying that load of clothes cost:
$.20 x 4.2446 = $0.85
If we dry one load of clothes per day for a year, then our annual clothes drying expense is:
$0.85 x 365 days = $310
…and we’ve used:
4.2446 kilowatt-hours x 365 days = 1549 kilowatt-hours
Now we can consider whether using an electric dryer is worth the convenience, cost, and environmental impact or if we’d be willing to save $310 per year and reduce our carbon footprint by hanging our clothes on a line outdoors. Even if we paid $150 for a fancy “sun and wind powered” outdoor clothes dryer, it would have less than a six month payback! So we can easily justify it even if it’s only used in the summer.
