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The first microwave for home use was introduced by Tappan in 1955.

The heating effect of microwaves was discovered accidentally in 1945. Percy Spencer, an American self-taught engineer from Howland, Maine, was building magnetrons for radar sets with the American company Raytheon. He was working on an active radar set when he noticed that a Mr. Goodbar he had in his pocket started to melt. The radar had melted his chocolate bar with microwaves. The first food to be deliberately cooked with Spencer’s microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.

[1][2] To verify his finding, Spencer created a high density electromagnetic field by feeding microwave power into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly.

On October 8, 1945[3] Raytheon filed a US patent for Spencer’s microwave cooking process and an oven that heated food using microwave energy was placed in a Boston restaurant for testing. In 1947, the company built the Radarange, the first commercial microwave oven in the world.[4] It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5000 each. It consumed 3 kilowatts, about three times as much as today’s microwave ovens, and was water-cooled. The first Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2000 to US$3000. Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952.[5] They tried to market a large, 220 volt, wall unit as a home microwave oven in 1955 for a price of US$1295, but it did not sell well.

http://en.wikipedia.org/wiki/Microwave_oven#History