Is that detached garage on its own service or fed off a home service. If off the home, you need the capacity to take an 80 amp hit when the house is running at peak. Not many people have that capability.
Depending on the incoming water temperature ( think wintertime temp) and the flow needed, you might possibly need 120 amps of capacity to achieve the desired temperature rise.
Many people find they are looking at several grand of electrical upgrades to install the electric tankless.
It is far easier to support the 30 amp draw of a tank type water heater.
If the apartment is empty for long periods, you could either put the tank type on a low temp setting or shut it down.