Though not as hot as the infamous habañeros, serrano peppers deliver plenty of heat and are the easiest hot chiles to obtain. These dark green or red peppers are between 1 and 2 inches (2.5–5 cm) in length and about 1/2 inch (1.3cm) in diameter, with a slight taper and a rounded end. The larger variety, termed the Largo, can be twice this size.
Serrano chiles are extremely hot peppers that range in heat between 5,000 and 20,000 Scoville heat units. How high a chile pepper scores on the heat scale is determined by high-performance liquid chromatography measurement of how many parts per million of capsaicin it contains. (Capsaicin is the compound that gives chile peppers their fiery bite.) This figure is then converted into the historic Scoville heat units that signify how much dilution is necessary to drown out the chile’s heat. The heat level of a chile is given as a range because it varies with how and where the pepper was cultivated.