I seem to recall that building code lets you use smaller diameter vent piping if “powered” venting is used — but then, powered venting may be required anyway if a multistory vent is used. Have never dealt with it.
The space that became available after replacing my old tank with the tankless heater got used real quick for a solar power backup system for the house (which I installed). Battery banks outside, inverter inside. Around 800 amp hours.
The extreme cold in my area also presents another hazard to the tankless heat exchanger. Absent careful dampers, you can have cold air come down the vent pipe and freeze the heat exchanger during a period of non-use. That ruins the device and exposes you to a flood as well. Too many down sides to the technology in my area. It would have been fine in San Diego.
BTW, I'm approaching 14,000 miles on the Harley that I purchased last June with 7507 on the odometer. The 60 mile round trip to work each day is more enjoyable and affordable on the bike. A new tire went on the back at 13,000 miles. The Versys sits in the garage at home with 9500 miles on the odometer.