Contrary to belief, the ideal conditions for maximum efficiency are a strong sun but cold weather, much like in the Arctic. Heat levels above 40 degrees Celcius can degrade the panel and reduce efficiency. Solar panels tend to be most efficient at 25 degrees, but are invariably hotter than the weather. On a 45C day, the panel will likely be around 70C. However, there are many factors involved and many different kinds of solar panels.