Most people are familiar with certain aspects of blackout curtains. Those are curtains that make the room exceptionally dark. For some homes which want to brighten their rooms, there are window treatments that allow plenty of light through and around the curtains. But some people need to block as much light as possible. This may be due to the fact they are light sleepers and do not want the morning sky to awaken them too early. Or they may be shift workers that need to sleep during the day. Another possibility is that they have children who nap, or who should be in bed while the sun is still up. Lastly, people in cities use them to block out street lights and lights from traffic.
While those uses are commonly known, there is another benefit to blackout curtains that many people do not think about.
Because of the way blackout curtains are made, they are denser and heavier than regular curtains. Some even have a rubberized lining on the window-side. These make great light-blockers, but they also block heat.
In the summertime, using blackout curtains can reduce the temperature in a room by as much as ten to fifteen degrees. When used in multiple rooms, this can significantly reduce the load on any air conditioner/cooling unit. That, in turn, reduces the electric bill.
In the winter, you want the light to come in through those windows and heat the house as naturally as possible (reducing your heating bill). However, by using blackout curtains at night in the winter, you can reduce the amount of heat exchange that takes place through the window (which is the biggest source of lost heat in the house). Think of it like putting a blanket over the window, keeping the heat in. Again, using the curtains like this becomes a money-saver by keeping your house warmer in winter, lowering your heat bill.