According to an initial survey of 2.8 million U.S. households conducted by energy monitoring and management firm Opower, which looked at power consumption in homes of Yahoo Mail and Gmail users, the average Yahoo Mail household used 11% more electricity than a Gmail-using household in 2011.

The difference of 939 kilowatt-hours per household is stark: That’s enough juice to accommodate 150 cycles of clothes drying, five hours of television watching every day of the year, and 200 dishwasher loads combined.

Big Houses, Not Big Data

Berry Fischer, Opower’s head writer and author of the study, said that while he and his colleagues expected to find some difference between the homes using various domains, they were surprised the difference was so large.

“We were excited to explore the reasons why there was such a gap,” Fischer said in an interview this week.

And what might some of those reasons be?

In the report on its Outlier blog, Opower ruled out climate as a differentiator, but did notice that Yahoo Mail users appeared to live in larger homes. But Yahoo homes also consumed more energy per square foot, which added to the energy gap.

Data from other surveys conducted by Hunch.com and Experian offered more clues to what might be happening here. Those surveys found that Gmail users tend to be younger and single.

“Being young and single means going out more; less time at home – and fewer occupants – means less electricity usage,” Fischer wrote. “By contrast, Yahoo users are more likely to be in relationships and have children. Additionally, Hunch found that Gmail users are more likely to be active travelers (having journeyed to 5 or more countries), and so might be away from home more often.”

Gmail users also appear to be more attuned to their energy use overall, which helps reduce their overall footprint, the report concluded.

What About AOL and Hotmail?

The report generated so much interest that Opower posted a follow-up analysis Wednesday that included data results for AOL and Hotmail users.

The second report revealed that AOL users used the most energy of any domain tested: 18.5% more energy annually than a Gmail home. Reasons why appear to be location (AOL users tend to live in the suburbs), size of home (they have even bigger houses than Yahoo Mail users) and – curiously – a greater likelihood of owning multiple DVR devices, which are very energy-intensive.

Opower, which focuses on providing customized energy advice to residential customers, gleaned these results from a subset of the 40 million U.S. households from which it has collected extensive data. The goal was to highlight how many factors can influence home energy consumption.

Of course, there are simple ways to save power no matter what email service you use.

“Whether you’re on a laptop or desktop, the next time you’re going to step away from your computer for more than 15 minutes or so, think about putting it in sleep mode… You’ll use a lot less electricity (70% less) and save up to $75 a year,” Fischer suggested.

Image courtesy of Shutterstock.