Two can live more energy-efficiently than one, according to this article from New Scientist: 

“Divorced households are smaller than married households, but consume more land, water, and energy per person than married households,” says Jianguo Liu of Michigan State University in East Lansing, US, who carried out the 12-country analysis with colleague Eunice Yu.

In the US, for example, 627 billion gallons of water, the use of 38 million rooms, and 734 billion kilowatt-hours of electricity would have been saved in 2005 alone if no-one had got divorced.

In the same year, divorced households spent 46% more on electricity and 56% more on water per person than if they had stayed married. And following a split, US households consumed 42 to 61% more resources per person than while married.

Being married isn’t the only way to live together, of course.  This article points to our energy-expensive future as one where people will more readily share living spaces.  Master bedrooms in the 1950’s were about 130 square feet.  In moderately priced new homes, master bedrooms now measure 300 square feet.  Right after WWII, the average new house was 750 square feet.  Now, it’s almost 2,500 square feet.  We have insatiable cravings for more stuff and bigger stuff.  If energy continues to rise dramatically in cost, large suburban houses will become more of a challenge to maintain by small families or single people.

What else uses lots of energy?  According to a second article at New Scientist, the answer is storing computer data on a server, which is about as energy efficient as driving an SUV:

Global Action Plan, a UK-based environmental organisation, publishes a report today drawing attention to the carbon footprint of the IT industry in the UK.

“Computers are seen as quite benign things sitting on your desk,” says Trewin Restorick, director of the group. “But, for instance, in our charity we have one server. That server has same carbon footprint as your average SUV doing 15 miles to the gallon. Yet, whereas the SUV is seen as a villain from the environmental perspective, the server is not.”

This second article fails to note an important mitigating point: providing data with computer server actually saves lots of energy compared to providing that information in the form of catalogues, reports and other hard copy paper forms that would need to be delivered by using fossil-fuel burning vehicles.  For example, the server used by Dangerous Intersection is shared by numerous other web sites.  On a typical day, DI alone is visited by 2,500 people who view about 8,000 pages of information.  This total sometimes rises to more than 10,000 people.  Consider whether there is a more energy-efficient method of distributing information.

On the other hand, the report does call attention to the enormous amount of energy consumed by computers: 

The global IT sector is responsible for about 2% of human carbon dioxide emissions each year – a similar figure to the global airline industry.