When looking at possibilities for electrically-powered pumps, I encountered several options that I want to remember in case I decide to add an electric pump to my rain water system. There are several types of pumps, but I haven't explored what is presently popular for the actual part which moves water.
What I did take note of is that one type of pump assembly would be convenient for many situations: A demand pump or on-demand pump combines a pump with a pressure cutoff switch. The pump will automatically turn off when its output side has more than a certain pressure, such as 30 or 40 PSI (pounds per square inch). When connected to a hose or watering system, this allows the pump to turn off when water is no longer needed or when the pump is moving water faster than it can drain out. A diaphragm tank (because there is a flexible diaphragm which uses pressurized air to push water out of the tank) can hold some pressurized water, so a more constant flow of water can be provided.
However, a pressure cutoff switch only controls what is happening on the output side of the pump and does not protect against what is happening on the input side. A low water cutoff switch or float switch can turn off the pump when the water level in the rain barrel gets too low. That protects the pump from trying to pump air, or pumping dry. A float switch can also be used to stop a pump when a destination barrel is full.
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