Different parts of the world receive significantly different amounts of solar energy each year. Off Grid DIY have a nice article explaining how to use NASA’s research spanning 20 years or more, to estimate how much solar energy you can expect at your location on Earth.
They also explain how to use that data to estimate how much usable energy you should expect from a solar power system, based on the PV panel manufacturer’s specifications.
This approach also allows you to validate and compare the claims of solar power installation contractors who may be quoting to supply and install a system for you. Being able to independently estimate how much energy you can realistically expect per year from a given solar system is very useful. When investing in a solar power system, you are effectively paying up-front for all the energy the system will produce over it’s life. A good solar system should last at least 20 years or more, so that could represent a significant amount of energy and money. Therefore it becomes crucial to able to judge what the realistic upper and lower limits would be for estimates of energy generated by that system. Unfortunately, some estimates provided by dubious contractors may be over optimistic, perhaps because that makes their “cost per kWh” more competitive on the quotation.