An Essex hospital is fitting 560 windows at a cost of £695,000 in a drive to be more carbon-efficient.
The 1960s windows in the main ward block at Princess Alexandra Hospital in Harlow are being replaced with double glazing over the next nine months.
A spokesman for the hospital said the energy efficient windows stored heat from the sun.
He added that the nine-month initiative would use "solar power to heat our water supplies".
The NHS Trust's energy manager Bill Dickson said: "The windows are the most obvious example of becoming more efficient and reducing our carbon footprint.
"The windows retain heat during the winter and store the sun's heat during the summer.
"They are more cost efficient as well as energy saving."
In summer 2008 solar panels were installed on the roof of the ward block which now heat the water supplies to large parts of the hospital.