FAQ: WHAT ARE THE POWER REQUIREMENTS?
Voltage
“Power sources SHOULD provide 5±0.25V”
“The newer Pi(3/2/B+) have a voltage monitor chip (APX803) which triggers at 4.63±0.07V. This controls the Red Power LED.”
Low Power
“China Power Supplies: They may appear to work OK for a lightly loaded Pi, but may not if many peripherals are connected.”
“If you use poor quality cables to connect to the Pi you will have problems.”
Current
“The Pi 3.3V rail is widely assumed to provide 50mA, but this is not officially documented for recent Pi models.”
“The regulator chip (which supplies both 3.3V and 1.8V) is rated at 1A.”
“Tests by a member indicate up to 800mA can be used – subject to an adequate power supply.”