There are a number of different types of solar PV panels, with varying ranges of efficiency. As solar PV installations have increased with popularity, technologies have improved, giving a much wider choice and more efficient modules.
The most efficient type of module uses mono-crystalline cells, which operate at around 15-17% efficiency. For this reason Williams Renewables always opts to install mono-crystalline modules, unless instructed otherwise.
Poly-crystalline cells are very similar to mono-crystalline, however they are generally not made from such high grade, good quality silicon and so the efficiency drops. They are generally said to operate at roughly 12-15% efficiency.
Thin-film modules are the least efficient, but the most malleable and so a very popular choice for designs working on more abstract and innovative projects. They are currently not sufficiently efficient to install on a mass level, however they are potentially the technology for the future.