There are 2 types of photovoltaic panels generally used in the UK - monocrystalline and polycrystalline. Without going into too much detail, monocrystalline panels tend to be most popular and widely available. The technology is improving within the industry and Solar Fast supply a half-cell monocrystalline panel that is one of the most efficient products on the market.     A good installer will ensure the most cost effective panels are part of your system.