Calculating the power used by light-emitting diodes, or LEDs, can be an important part of any electronics project -- particularly when deciding how many batteries you may need. To determine an LED's power requirements, you need to know how much current flows through the LED and its voltage. Once you have this information, you can multiply the current by the voltage to determine the LED's power requirement.
Determine the LED forward voltage. Look at the manufacturer's data sheet and find the specification for the typical forward voltage. This is a close approximation to the voltage across the diode. Alternatively, measure the voltage drop across the LED when it is lit with a voltmeter. Otherwise, just estimate the forward voltage based on the color of the LED. Use 1.8 volts for a red LED, 3.5 volts for a white LED, 3.6 volts for a blue LED and 2.1 volts for a green, orange or yellow LED.
Determine the electrical current flowing through the LED. Look at the manufacturer's data sheet and find the typical electrical current associated with the typical forward voltage. This electrical current is a close approximation to the current through the diode. Alternatively, measure the electrical current through the LED with an ammeter if you want an exact value. Otherwise, just roughly estimate that the current through the LED is around 20 milliamperes.
Calculate the LED power. Multiply the voltage across the LED by the current through the LED. For example, for an LED that has typical forward voltage of 2.1 volts with an associated typical current of 20 milliamperes, calculate that the LED power is 42 milliwatts, since 2.1 multiplied by 0.02 is 0.042. Remember that power in watts is equal to current in amperes multiplied by voltage in volts, and that 1000 milliwatts is equal to one watt and 1000 milliamperes is equal to 1 ampere.
TL;DR (Too Long; Didn't Read)
You can also calculate the current and voltage across the LED. This requires that you analyze the electrical circuit that the LED is a part of. If the circuit contains many other electronic components besides the LED, this can become a complex task.
Most electronic circuits with LEDs that have been designed for maximum reliability, have been designed such that the LED has a voltage across it that is near the typical forward voltage specification given by the manufacturer. Because the forward voltage across the LED sets the value of the electrical current through the LED, the power that the LED requires is approximately equal to the forward voltage multiplied by the current at the forward voltage.
When approximating the power, consider that the current through a LED will vary considerably with just a small change in voltage across the LED. Also consider that different LEDs, even though they produce the same color light, will have somewhat different levels of current for the same forward voltage.