Solar heat gain (also known as solar gain or passive solar gain) is the increased heat from the sun through your windows. Solar heat heats your home through the radiation produced by the sun. While solar heat gain is desirable in the winter to warm up chilly rooms, it’s an unwelcome side effect in the strong summer heat. Air conditioners work extra hard to overcome solar gain, so keeping the sun out is the best way to keep a house cool and reduce air conditioning energy.