You can use these calculators to calculate solar power efficiency.
Solar Collector -- Fin Efficiency Calculator
PV Cell Basics for Hobbyists
Photovoltaic (PV) cells convert sunlight directly into electricity. They are a convenient source of power for certain hobby projects. Effective use of PV cells requires proper matching of cells to the electrical load. Here is a calculator that will help you estimate the electrical current and voltage that can be supplied by various PV cell configurations.
Photovoltaic System Worksheet
Do you have ambitions of putting a Photovoltaic array on top of your house and disconnecting your electric company? Or, of staying attached to the grid and selling electricity back to them? This worksheet does a simple, "back of the envelope" calculation of PV system economics (Please do a more careful sizing before you buy something!), and can help you understand many of the tradeoffs that are involved in using this increasingly popular renewable energy technology.
Solar Collector Efficiency Calculator
Calculate the efficiency and heat output of your solar colector.
Solar Power Calculator | Find Solar
Ever wonder what size a solar panel system you need or how much one costs? Use our solar calculator to estimate the solar cost and size of a solar energy project in your area.