I often get the question, “how efficient are solar modules?”. A decade and a half ago, if one was doing back of the envelope calculations, they would make the assumption of 10% efficiency as that was a pretty typical industry average, at least within a rounding error.
The 10% assumption stuck around long after it had been surpassed because it was such an easy number to work with. However progress marched on and today the people typically use 15%. While polycrystaline modules dominate leading to the weighted average of all modules sold to be closer to 15%, the trend towards improvement continues with several companies at or near 20%.
SunPower, the current leader offers a glimpse of what the future holds. Last year they introduced a module with 21 1/2% efficiency.