Aaron: In addition to what Tim said, the reason you might need to upgrade your service depends on whether you’re adding new electrical load or not. If you’re replacing an electric resistance water heater with a HPWH that has the same power, you’re fine. If you’re replacing a gas water heater with a 240V HPWH with strip heat, you’re adding a significant load.

Higher voltage implies lower amperage only if wattage is held constant. IIRC, wattage for 240V water heaters is 4500w.

Located anywhere is slightly misleading. Basements are great but a small room or closet like someone might try in the south without a basement would probably not be able to dissipate the cold air discharge.

Nobody has a circuit in their house that supplies 120Vac at 100 amps.

Cindi…I’ve had one of the very first “add on” HWHP’s since 2009, it finally died last summer. It only drew 5 amps of power on it’s 120V outlet. That’s efficient to begin with! The other “inefficiency” of course is that they are SLOW to recover, but the trade off of electric savings far outweighs it. I’m in the Great Lakes region, my tank is in the basement, an almost consistent ambient air temp of 58-70 degrees between winter and summer, respectively. It works fine. And as I understand it, at least hope too, since I’ve got a new “all contained” 120V plug in tank on the way, is that they are getting better at efficiency, i.e. better recovery times. It’s just me and the wife, so we don’t notice much problems. Get one!
P.S.: I’m lucky too, in that we’re on solar as well as a 1.4Kw Bluetti “off grid” solar generator, that I’ll be able to go back to running the HWHP from that, making my hot water totally free, rather than suck it off the main large solar array at least during the day. Best of both worlds.