Transfer switches are electrical switches that alternate the load between two sources of power. Most commonly, this is between city power and a generator for when the power goes out. In places where storms are common, such as North Carolina, Many homes are built with permanently built-in power generators. This is usually wired directly into the house through a transfer switch.
There are two types of service switches: automatic and manual. An automatic switch will automatically switch to an alternate power source whereas a manual switch will require someone to switch to the alternate power source.
A subpanel is a waypoint between the main panel and other circuits that are located further down the line. In many cases, these are mandatory to bring a building up to code. Subpanels are often thought of as mini service panels which are not far from the truth. The basic structure is the same as that of the main panel.
Often times, subpanels are added to homes or buildings when the owner decides to add on to the structure. An addition can mean that there is how enough room on the existing panel for the new circuits. In this case, a subpanel will be installed.
Another reason where a subpanel may be used is if a certain area of the house is drawing more electricity than the rest due to its function. This is not uncommon in kitchens or entertainment spaces. In this case, a subpanel can be installed for the specific area.
Note: A subpanel does not provide any additional electricity to the property that it services.