Yep. There is a tradeoff. When you physically attenuate the panel, it attenuates the voltage and amperage which reduces the wattage. My little pump runs on 20 watts, so it's never starved for wattage from the 100-watt panel that is shaded to about 60 watts. There may be a better way to do this but ANYTHING you use (resistors, diodes, etc..) to attenuate the panel is going to attenuate the wattage, voltage and amperage to some extent. Basically, you just have an equipment mismatch that is going to reduce the efficiency of the whole system. You can trade up on your controller/charger or downsize your panels or leave the pancho in place and call it a day. P.S. you can use aluminum foil or an old dashboard shade or plastic matt. I recommend something reflective as the excess heat can damage your panels.