That is the reason why a 50W panel may produce up to 3 times its rated output in a sunny day.
Let me clarify this. Solar panels are rated in watts based on standard testing conditions, a certain amount of light at a certain angle to the sun, and at a certain temperature. An internet search will yield the specific numbers. When a panel is rated at say, 50 watts, it is rated under these optimal conditions. Those conditions are seldom met on a boat. The confusion may come from the difference between a watt and a watt hour.
Over the course of a day, panels on a boat are exposed to varying light conditions. Low angle early morning or late afternoon light produces some current, but no where near the rated capacity. Around midday, with the sun directly overhead it will produce power that is closer to its rated capacity because the sunlight is striking the panel at a more optimal angle, i.e, the sunlight is stronger.
In a solar charging system there are 2 key factors, how much power (current) can be produced at any given moment (the panel's rated output) and how much power can be generated over a period of time (watt-hours or amp-hours). A 50 watt solar panel under optimal conditions producing 50 watts for one hour will produce 50 watt hours of electricity. (roughly 4 amp hours at 12v). If it is exposed for 2 hours to these optimal conditions, it will produce 100 watt hours (roughly 8 amp hours at 12v).
In short, a panel can not exceed its rated capacity by any significant amount, but can produce less than its rated capacity due to a number of factors, heat, shading, cloud cover, angle to the sun etc.
When designing a solar system it is important to match power consumption and generation. If the consumption is 100 watt hours, at least 100 watt hours must be generated to recharge the battery. The power can be generated in any number of panel/power combinations, say 20 5 watt panels exposed for an hour, 5 20 watt panels, 2 50 watt panels or 1 100 watt panel. Because of the known inefficiencies in a charging system, it is helpful to have an estimate of the effectiveness of the panels. In a recent article in Professional Boatbuilding, Nigel Calder suggests that panels can be expected to produced three times their rated power over the course of a day. Thus, a 50 watt panel could be expected to produce 150 watt hours of energy. This is different from saying a 50 watt panel produces 150 watts.
The little solar "trickle chargers" are not very efficient. A 10 watt charger can only be expected to produce 30 watt hours a day, which is less than 3 amp-hours a day. Given the inefficiencies of battery charging, especially at higher state of charge, this is an insignificant about of power. Save your money, but a good charger and plug it in.