Most Prius-es are simple hybrids; the car they're testing with is a Prius Prime, a plug-in hybrid with ~25mi of all-electric range from an 8.8kWh battery, that'

Most Prius-es are simple hybrids; the car they're testing with is a Prius Prime, a plug-in hybrid with ~25mi of all-electric range from an 8.8kWh battery, that's what they're trying to add range to. If that battery is exhausted you just drive on the gas engine as usual (I have a 2017 Prime). So this would just be a small boost, though to be fair I often can go a few days on a charge (but I don't drive a lot).