I have a late 2015 macbook pro and 2 external monitors. The monitors have usb-c input. Is it possible to use my monitor to power my laptop with a usb-c to thunderbolt 2 cable? If not, what is the most efficient way to power 2 external displays?
Your MBP is a 2015 model, which means it only uses the MagSafe 2 connector for power. It does not have USB-C and can't take power in over Thunderbolt from the monitors (assuming that they can supply power out via USB-C, which is not known at this point).
However, your question is how to power the displays. You'll need to read the manual to see what power inputs they have. I think it's unlikely that they take power in over USB.
If you want to know how to connect both displays to the Mac: your Mac seems to support 2 displays via Thunderbolt only. So don't use the HDMI port: get a Thunderbolt 2 to USB-C cable/adaptor (or other port on your display).