The vehicle will use whatever power it determines that it needs. If you jump in and stomp on the accelerator, even with 27 miles of electric and a full hybrid battery, it will run on gas motor. If you drive gently, it might burn up all 27 miles of plug-in battery and 50% or more of the hybrid power before it starts the gas engine.
One of my other cars, also a PHEV, will let me conserve the battery for city use (some cities restrict ICE cars in certain areas or heavily taxed on a daily basis). I am not sure if the Niro can do this. Perhaps sport mode? My Nirowill use up battery miles first which is the way I like it. BEV range is usually at least 30 miles with a/c on, even though it indicates 24. The power of regen braking!
Yes, on the PHEV there is a button just left of the shifter that keeps the car in EV mode. I've left it on for the three weeks I've had mine and the ICE has only kicked in twice: once on my way home after driving 49 miles around town and once when I gunned it to clear another car in a merge situation.
I own the 2017 EX standard hybrid, NOT plug in. Don't forget however, that even if you deplete your 27 miles of all electric range, you will still be recharging with regen braking,,,,,,, so your vehicle will still run partial EV, ICE, and true HYBRID mode (ICE & EV), it will just run like mine, standard hybrid, still giving you great mpg's......... previous poster is correct, it would be best to charge up everyday, to top off your battery so you will gain from its full potential.