If you could remember, in electricity, power is energy consumed over time. There have been so many argument about what risks would it take an Android charger or generally chargers connected to the electric socket without really being connected to phones.
Apparently, without your phone connected to an electric socket or outlet, no work is being done right? But, there is a negligible work done when your charger is only connected to electrical socket.
There is a little work in keeping your charger (some electrical components like transformer, diodes, etc) awake ready for your device to be plugged in. Sometimes, they are indicated by LED light in some chargers showing that they are on.
ALSO READ: Best Android chargers you must have
For people that use metered electricity, leaving your Android charger connected to an electric socket wastes some watts. Although, these are not very noticeable but it happens. As long as you charger is plugged in, the electric current activates those working electrical components in your charger.
In some chargers, there are led indicators that show when the charger is correctly plugged in. So, in other to maintain both the led and some components in your charger, insignificant amount of energy is used.
Electricity can be quite a bitch. You cannot really know good the connections can be. I used to experience some weird electrical magic in my apartment. I always feel electrical shock whenever i feel my chargers. Sometimes, the bulb sparks even when the switch is turned off.
We may not know what may happen to a charger whose main purpose of design is to have something connected to it.
Have you ever thought whenever your charging completes, it pops up something like “Charging completed/battery full, please unplug charger from socket”? It is a very good indication that leaving charger plugged when not in use is a bad practice. It is an official caution from phone manufacturers.
In summary, if you are really concerned with safer living with electricity, the cost of purchasing another quality charger and some manageability in electrical bill which is somewhat not obvious, then it is not good to leave your charger plugged and unused.