Some electrical devices consume energy more quickly than others even when the applied voltage is the same. The difference is apparent in the electrical current required by each device. The rate at which energy is consumed is known as power, measured in Watts (W), and it is related to voltage and current by the following equation:
When connecting devices to an Arduino (or any other microcontroller), it can be important to check the power requirements. Each I/O pin on the Arduino Uno can supply a maximum of 40mA. At 5V, this gives a maximum power rating of 0.2W. If a device require more power than this, it has to get it from somewhere else. The 5V power output pin on the Arduino is supplied directly from the voltage regulator which is rated for 1A, and so it can theoretically supply up to 5W; however, there are other limiting factors. A USB connection, for example, can supply a maximum of 500mA which limits the overall power consumption to 2.5W. The voltage regulator is also thermally limited so that when the temperature gets too high it will shut down temporarily.
For devices which require more than about 1W, an independent power supply is a good idea.
The total current for all I/O pins is 200mA (giving a maximum power of 1W). If an LED requires 20mA, you can power up to ten of them using the I/O pins without risking damage to the microcontroller. Because the 5V power output pin can supply more power than the digital pins, you can use transistors to get around this overall limit. A transistor requires a very small signal current on the base pin to connect a larger current between the collector and emitter. The arrangement is shown in the circuit diagram below.