In this context, efficiency is usually a PV panel spec that actions simply how much in the daylight that hits a solar panel is transformed into electrical power. A first caveat is that Carnot efficiency isn't going to consider that some blackbody radiation is inevitably emitted from the photovoltaic gadget; https://cristiano9vw1.loginblogin.com/43323144/how-much-you-need-to-expect-you-ll-pay-for-a-good-cell-solar-charger