The efficiency of a solar cell refers to how effectively it can convert the light that hits its surface into electricity.
With current technology, the efficiency can range anywhere from 6% to 44%. Common residential solar cells have an efficiency range between 15%-20%.
When manufacturers measure the efficiency of their solar cells, the must do so under standard test conditions (STC) to ensure that everyone is playing by the same rules. The STC’s correspond to a clear sunny day with a temperature of 25 degrees Celsius.
Even though the solar cell efficiency is an important factor, most people value the $/watt criteria more than the efficiency. The cost of solar cells increase exponentially as the efficiency rises so the key is to find a good balance between economy and efficiency.
When does it make sense to have a super high efficiency solar cell? The answer is when space is very limited. For example, most solar cells aboard modern spacecrafts will be more than 30% efficient.