how to calculate solar efficiency?

• Determine the surface area of the panel by multiplying the length by the width. For example a SolarWorld 245W Mono panel is 1675 mm long by 1001 mm wide or 1,676.67 square meters. The surface area is aperture area of the solar panel, therefore, this does include the frame. 
• Pull the name plate rating of the panel from the datasheet. For example if it is a 235 W panel, that is the name plate rating at STC (Standard Test Conditions) 
• At STC the watts per meter squred (W/m²) is 1,000 W/m² This is the standard used to determine how many watts of power are produced in a square meter on earth. (Note: Temperature is always a factor in the output of a panel so STC assumes less than 25 degrees C.)
•  Divide the name plate rating by the square meters at 1000 W/m² to get the solar panel efficiency. In our example for the SW 245 mono (245/1.676.67)=14.61% efficiency 
•  Remember to look at the power tolerance as although the name plate rating might be 245W, the power tolerance will tell you how many more or less Watts a the STC the panel produce. In this example the SW 245 Mono have a power tolerance of -3%/+3%.