Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is
E = qV or E = -1.6 x 10-19 (1.71) Joule,
since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives
E = 2.74 x 10-19 Joule