The load output of the smaller Victron controllers is quite versatile. It can be adjusted via the
software to be always on, always off, on only at night, and on only if the battery voltage is above a certain level (this level is even user adjustable) along with other options. The power will be drawn from the battery if the solar output is inadequate. The function can be changed via Bluetooth so it can also be used as
remote switch, such as turning on a light without getting out of
bed. The power
consumption of the load output is reported separately on the app so it provides extra monitoring of the connected device.
Some other brands of controllers only have very limited options and some can be programmed to perform sophisticated functions, such as turning a
pump on for 5 minutes if the battery voltage is above a certain level.
Keep in mind that the load output is limited regarding the current it can supply (often the same as the overall current rating of the controller, but sometimes less). Be conservative with devices that have high start up current (such as fridges). A relay can be used to increase this amount, but unless it is a latching relay the power
consumption of the relay is a potential nuisance.
The load output is rarely used on
boats, but it can be useful in some installations such as simply turning an
anchor light on at night, or for more complicated functions. For example it could be used to turn on an auxiliary fridge used for cold drinks in the cockpit, but only providing the battery voltage is high. There are many other possibilities.
Keep in mind that a simple
electrical system tends to be more reliable in a
marine environment. Every time I think of a clever function that could be automated with the load function, I have ultimately decided that the complication is not justified, but I still think it could be usefully used in some situations.