Stovetop espresso devices, sometimes called moka pots, produce coffee that has several differences from espresso produced by an electric espresso machine. Coffee from a stovetop machine is not technically espresso and falls somewhere between standard brewed coffee and espresso both in strength and taste.
Both stovetop and electric espresso machines use pressure to force water through finely ground coffee beans. Stovetop devices use steam from boiling water in order to generate this pressure, while electric machines use a variety of methods including steam, pistons and pumps. The main difference is the level of pressure that is generated, which is measured in bars. The pressure in stovetop machines is usually between one and two bars, while the standard for electric espresso machines is nine bars.
The higher pressure found in electric machines results in a fuller extraction from the grounds. The espresso that is produced has more flavorful oils and produces a layer of crema, a light brown foam emulsion that is a defining characteristic of espresso. Under the right conditions, stovetop machines can produce a small amount of crema, but it is not typical of the process. Although the coffee that they produce is not technically espresso, however, stovetop machines are significantly less expensive than electric machines.