The micro:bit is capable of supplying 3V power but only up to a maximum current of 90mA. Exceeding this threshold will cause excessive heating of and potentially damage to the onboard voltage regulator.
The onboard KL26 regulator is rated at a maximum of 120mA when powered from USB and a budget of 30mA is allowed for the nRF51822 and peripherals that make up the micro:bit itself.
The onboard KL26 regulator is not used when powered from battery.
For more information see: http://tech.microbit.org/hardware/powersupply/