Scientists said Wednesday they had created a window coating that can be switched electrically to regulate the amount of heat and light that enters a building.

A team of molecular and material scientists from the United States and Spain created a transparent film using nanocrystals -- microscopic clusters of atoms that can change the wavelength of light.

"Smart windows and specifically electrochromic windows (which change colour or transparency with an electric charge) have been developed already, but our solution is the first to offer integrated control over heat and visible light," study co-author Delia Milliron of the Lawrence Berkeley National Laboratory in California told AFP.

In the design, published in the journal Nature, the window is an electrochemical cell with two glass panes separated by an electrically conductive electrolyte liquid.

The film is placed on one pane to create an electrode which passes an electric charge to a counter-electrode on the other pane.

With no electric current, the window remains transparent. A charge causes the nanoparticles in the coating to start blocking heat waves while the window remains transparent -- a further charge causes light also to be blocked.

According to University of Texas engineer Brian Korgel, who commented on the study, residential and office buildings account for about 40 percent of energy use and 30 percent of energy-related carbon emissions in the United States.

To save energy, new materials are needed to better regulate buildings' heating and lighting requirements.

The new study represents "a great advance in the development of such materials," said Korgel.

But several issues must be fixed before the material can be used in windows -- including replacing the highly flammable lithium metal used as a counter-electrode, and finding a solid electrolyte.

"The materials needed to build an electrochromic window will be more expensive than conventional window materials, so the extra expense will need to be balanced by the energy and cost savings that can be achieved through their use," Korgel said.

According to Milliron, the material was designed with buildings in mind but could also be useful for car or airplane windows.