Does anyone know how much power out we can get from the USB port in a DSLR camera. Is it possible to get an output like 3.7v 1500mAh battery can supply from a DSLR?
The cameras often have a "type-B" USB connector (the "peripheral" side) and these aren't supposed to provide current (they are the side that normally draws current).
On addition the batteries in a camera aren't that big, you couldn't draw significant current from them without impacting the battery life.
Most cameras have historically operated in USB Device mode (which can, but is not required to, receive power from the host), not USB Host mode (which can, but is not required to, supply power to the connected device).
There is a third mode, USB OTG (on-the-go), which was created to allow for devices to act as both hosts (to power peripherals) and devices (to act as a peripheral to hosts controlling them). USB OTG is not strictly a software feature — the USB controller (or in the case of system-on-a-chip, the SOC chip) must support that mode of operation.
If I had to guess, most cameras introduced in the last couple years probably have the ability (in the chipset) to operate in USB OTG (on-the-go), meaning they can be either a host or a device. But that doesn't mean that manufacturers expose that functionality, or even turn it on, for anything but 1st-party devices. And to emphasize, that would still be a tiny minority of the cameras currently owned and in use by photographers.
It may be possible to reverse-engineer the chipset to figure out how to enable it, if it is even present. But that is far beyond the scope of Photo-SE. The camera industry is particularly guarded with their specs and interfaces, rarely releasing such information to the public. Probably the most reverse-engineered DSLRs are Canons. You can check out the CHDK — Canon Hack Developer Kit project. They have the most knowledge and information about Canon internals.