We all want to be "green" and reduce our personal carbon footprints. But I think whoever passes out dispensations includes people who are trying to sell their homes, especially during the summertime.
First, DC has been hotter than Houston lately, and sometimes the humidity is so high that it feels like you can swim through the air. So having a super functional air conditioning system is really important. Have the system serviced before you put your house on the market, and be sure to change the filters often. I recommend setting the thermometer at somewhere around 75 degrees. And yes, your electric bill might go up a bit, but keeping the house warm is a false economy. You want people to think your house is hot, and the only way to do that is to be cool!
Many people are using draperies that, when pulled closed, help keep the room cool. But when your home is for sale, you want to pull them back and let the sun shine in. Brighter is better.
And speaking of brighter, there is another good habit you it's good to change. We recommend that you use the same type of bulbs in all of the lamps and fixtures in the room- and not the new energy saving products. If you've been hoarding incandescent bulbs, this is the time to pull them out and up the wattage. Even if a room gets a reasonable amount of natural light, helping it out with a few lamps is a good idea. If you know there is going to be a showing, turn on every light in the house. If you won't be home and don't know if any agents will show up with buyers that day, leave at least some lights on so they don't feel like they are walking into a cave.
If you follow this advice, your electric bill may go up a big while the house is for sale, but the idea is to help you increase your sales price to way over your expenditures for energy use.
Then, once your house in under contract, you can go back to conserving energy!