IMO theres a world of difference between the two items. Both have to be sealed to prevent electrocution, however, one is going to be less dangerous due to the lower levels of current involved. Also, lights are quite often actually under cover and dont require cleaning to operate or to operate effectively. If I were to clean a light fitting Id like to think it would be turned off, at the mains and not just in an operative sense as they havent been manufactured with that in mind. However, solar panels will have been, simply if only because if they short circuit due to water seepage you've blown a piece of kit worth a sizeable wedge and yourself probably, blow a light and you've popped a bulb worth 60p and probably nothing more.