These tend to be small, about the size of a credit card. They are battery-powered. This metronome is, however, excellent as a portable device. Cost is usually much less than the mechanical or electric types. Usually these metroomes can go below and above the standard metronome speed range.
Some electronic metronomes have lights that flash or bells that ding in addition to the peep.
Some produce more than one sort of peep. Most electronic keyboards have a built-in metronome. When all is said and done, I still go with the old-fashioned mechanical metronome. It's not without drawbacks, though. Mechanical metronomes are fragile.