Solar Panel Efficiency
Of the approximately 1000 watts of light energy the Sun shines on a square meter, most Solar panels provide slightly more than 10% of that energy as electicity. The rest turns into heat or is reflected back into space. There are some exotic solar cell materials that have shown up to 30% efficiency, but they are way too expensive to be practical on home roofs. At the other extreme, thin film silicon solar cells and others can be produced at much lower cost than the typical silicon crystaline cells, but they require much more roof area to produce the same amount of electricity. As the solar cells age, they develop internal defects which decreases their efficiency.

It is a difficult balancing act to trade off efficiency, area, cost and availability of different solar panels. Here are a few articles related to solar panel efficiency.
Efficiency Report
11.1% Thin Films: Past, Present, Future
17% Sunpower Introduces High-Efficiency Photovoltaic Modules
19% HelioVolt: Thin Film Technology Efficiency Comparison
20.5% Samsung's Solar Cell Breakthrough
32.3% Photovoltaics: Energy for the New Millennium
72%?? Full Solar Spectrum Photovoltaic Materials Identified