rectennas. consumers don't think about it, but microwave ovens are fundamentally similar to microwave antennas. The primary difference is that a microwave oven is designed to contain the "broadcast" so it can be converted to heat energy when it encounters mass i.e. your lunch. This process is called dialectic heating. Don't be surprised this gadget was invented in China, the microwave itself was invented in Japan in 1955. It wasn't introduced to Americans until Amana imported it in 1967.
So here's how this widget comes in. Your microwave isn't 100% efficient. Actually if you've ever looked at it's power consumption you already kow it's a bit of a hog when it's running. That's because much of the microwaves just bounce around and dissapate, they are not all absorbed by their intended target. That means a portion of those aren't converted to heat and are totally wasted. An average consumer microwave oven consumes 1100 watts of electricity to produce 700 watts of power. That's an efficiency of 64%, though it'd obviously vary. To be fair a lot of that is just heat loss.
This rectenna receives the microwaves in your microwave oven and converts that surplus energy to electricity. The reason this regenerative converter works only at a frequency of 2.45 GHz, is that it's also the frequency that all consumer microwaves operate on. Commercial microwave ovens operating 915 MHz. I have read that some countries also use 433.92 MHz. This would be most efficient if it were used to reduce the power draw of the microwave itself, but it's as yet unknown if it will even go into commercial production. More here.