Yes, solar panels do work on cloudy days — but not as effectively as they would on a sunny day. That's because clouds block some of the sun's energy from reaching the ground (or in this case, your roof). The biggest the rated wattage of a solar panel, the more kWh per day it will produce. How Much Sun Do You Get (Peak Sun Hours). Progress has slowed in recent times, but having reached a top efficiency rating of 25%, domestic panels are effective enough to make the most of any space you have on your roof. Due to the many advances in photovoltaic technology over the last decade, the average panel conversion efficiency has increased from 15% to over 24%. Let's take a closer look at the key influences on solar panel. PV panels work best when perpendicular to the sun.