Figuring out ways to slash our over-reliance on air conditioners can go a long way in reducing the energy costs and emissions that usually go hand in hand with using these cooling machines. One way to do this is to tap into the key role that windows play when it comes to saving energy.

Recently, for example, we talked about how a team of scientists has developed a transparent material made of wood that is more cost- and thermally-efficient than glass.

Now, researchers at the National Renewable Energy Laboratory (NREL) have developed windows that can change color in response to different degrees of temperature to keep buildings cool. On top of that, the new smart windows are solar panels too.

As New Atlas reports, color-changing glass has been around for a long time, most commonly as transition lenses for eyeglasses that tint automatically under bright light. More recent developments have made it electronic and switchable on-demand and scaled it up to window size. At the same time, transparent solar cells are getting more efficient, to the point where they can be fitted into windows.

In the new study, researchers have taken these innovative technologies a step further by combining them into one window. The “thermochromic photovoltaic” tech, as they call it, reduces the need for cooling by switching colors when heated up by sunlight. When colored, it also starts harvesting energy from the sun.

The windows are made from a film of perovskite wedged in between two panes of glass, with a solvent vapor injected into the gap. When facing hotter temperatures, the vapor causes the perovskite crystals to rearrange themselves, thus changing the window’s color. Between 95 and 115 °, the windows can switch from transparent to yellow, orange, red, and brown, in about seven seconds.

Ideally, the thermochromic photovoltaic windows could help reduce our reliance on air conditioning, and even chip in a little extra electricity to help run it.