It basically depends on weather and intensity of radiations, the peak efficiency of a solar panel determine by the surface area of the panel by multiplying the length with width and the surface area is aperture area of the solar panel, therefore the measurement does include the frame. Then find the name plate rating of the panel from the datasheet, the name plate rating at STC (Standard Test Conditions).
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 solar panel so STC assumes less than 25 degrees C).
Then divide the name plate rating by the square meters at 1000 W/m² to get the solar panel efficiency, remember to look at the power tolerance, the power tolerance will tell you how many more or less Watts a the STC the panel produce.
The solar cell peak efficiency might be a few percentage points higher when measured at standard test conditions. Those points of efficiency are lost in the movement of energy from the cell to the output of the module.
Here is one more term which is used in deciding the output losses called PR (Performance ratio) which is used to evaluate the quality of a photovoltaic installation because it gives the performance of the installation independently of the orientation, inclination of the panel. It includes all losses like inverter losses, Temperature losses, DC cables losses, AC cables losses, shadings(specific to each site), Losses weak radiation, Losses due to dust, snow etc.
Before you calculate solar panel efficiency you need to get your units in order. How many sq meters of panels do you have, what type are they, what is the tilt, and where are they located? Solar energy may be "free" but it obeys the laws of physics and optics, so the only time that you receive the peak power is at high noon when the sun rays are perpendicular to the panel, something that occurs only twice a year unless you have some form of tracking, at all other times panels will produce only a fraction of their peak rating.
Solar panels have a sweet spot (the maximum power point or MPP). This point is a function of solar irradiance and voltage and current. Depending upon how well your inverter system maintains that MPP it will drastically affect your output. On larger systems that I have worked on they have reference irradiance meters that they use to track system output based on the theoretical calculated maximum based on irradiance and the VI curve for the panel. If you do not have this reference I suggest you add it. That way you can track your system output and see if your inverter system is the issue or just not enough usable sunlight.
Depend on the climatic area of the installation, the solar plant efficiency is expected to be just about 4-5 hours a day. 4 hours is more common which is called as 'sun hour'. Mean, if our installed capacity is 1.5MW, the rule of thumb on calculating expecting power (kWh) produced is just simply multiply by 4. 1.5x4 = 4.5kWh is acceptable. This is due to the panel efficiency which is just about 70-80 percent of actual capacity. Solar panel efficiency will be decrease about 0.5-0.7percent per year and that is expected to be a decreasing of power produced yearly, depend to the panel brand and climate.