IEC size, you get 2 common sizes that are 9V, PP3 and PM9, with PM10 being the less common version. Basically small, medium and large. The PM9 and PM 10 are mostly sold for use in portable radio's as a relatively long lasting power source. Though I have bought a few of the radios I always made up a mains adaptor, as the PM9 batteries actually cost more ($3) than the radio cost ($2), and I needed some radios for staff at work. Cheap radios, cheaply built ( 10V capacitor throughout, and they probably regard Capxcon as a premium brand) but they do work well enough, having at least not terribly bad audio.