can be disconnected from the batteries without any damage even when in full sun. This is how the solar
regulator/controller works. It turns the solar off and on (very rapidly) to maintain the correct voltage. For example 50% off and 50% on means the panels
are. Delivering 1/2 their potential power to the batteries (ignoring switching losses etc). Therefore the regulator
can adjust the solar panel output from almost 0 to 100% depending on the needs of the battery
So the "lost" power does not really go anywhere. It is never produced. The panel only produces power when connected to battery allowing electrons to flow.
If the regulator
is set to the right voltages it will do its job and keep the batteries healthy.
The "lightbulb" terminals are not much use and are usually not used on a boat. These terminals can do a couple of things depending on how the regulator is programmed or designed.
The most common function If you connect a load to the "lightbulb" terminals is to supply power only when it is dark. The power comes from the battery, the regulator is just acting as smart switch. This is used to automatically turn on a light like an anchor
light. The second most common function is to supply power only when the battery voltage is OK. This can be used to say turn off the fridge if the voltage drops down because the battery is getting flat. You need to be careful with this as usually you can only connect small loads to the "lightbulb" terminals. So you may need a relay for large loads like a fridge.
The better regulators have adjustable "lightbulb" terminals that can be programmed to do smart things like the above and many more. Some are very clever and can say run a pump for 10mins only if the battery voltage is above a certain level. Some can be wired to make use of the excess potential power by automatically diverting it to something useful like fuel
polishing. Very smart, but most boat owners just ignore these "lightbulb" terminals.
The "lightbulb" terminals are just labeled "load" on some regulators.