When working with LED lighting, particularly when those LEDs are part of a battery-powered project, it may be important to calculate the power use of your LEDs in the circuit. This is a simple task with a multi-meter capable of measuring current, resistance and voltage, but if you lack one, it is possible to estimate the LED's power use by consulting the packaging and manufacturer's sheets that came with the LEDs. You only need to find the current and the voltage of your LEDs.
TL;DR (Too Long; Didn't Read)
Calculating the power use of LED lighting is a critical step for any battery-powered electronics project, and thankfully it is simple to do. To calculate LED power, you'll need to know your LED's current and voltage, which you can find either through the use of an electrical multi-meter or by consulting the packaging and manufacturer's materials. LED power is calculated by multiplying the LED's current by its voltage. Be careful when working with electrical circuits and currents, even when measuring them.
The first step to calculating an LED's power use is to determine the LED's voltage. If you do not have a multi-meter on hand, look at the manufacturer's data sheet and find the typical forward voltage of the LED unit, or measure it with your multi-meter while the LED is powered on. Alternatively, you can estimate the voltage based on LED color. White LEDs tend to have a voltage of 3.5, red has 1.8 volts, blue has 3.6 volts, and 2.1 volts for a green, orange or yellow LED.
Once you've taken note of your LED's voltage, you will need to determine the current. This can be measured directly with a multi-meter to determine the exact value, but the manufacturer's materials should offer a rough estimate of typical current. Once you have this value, you can very quickly and very easily calculate the power use of your LEDs.
Calculating LED Power
To calculate an LED's power use, simply multiply the LED's voltage (in volts) by the LED's current (in amperes). The result, measured in watts, is the amount of power your LEDs use. For example, if your LED has a voltage of 3.6 and a current of 20 milliamperes, it will use 72 milliwatts of power. Depending on the size and scale of your project, your voltage and current readings may measure in smaller or larger units than the base ampere or watt, and unit conversions may be necessary. When doing these calculations, remember that 1000 milliwatts is equal to one watt, and 1000 milliamperes is equal to one amp.