what do you have right now?

There's a bit of disconnect on your question details. An electric water heater does involve a "hot water tank" unless it's a tankless unit. Is electric tankless the target of your question?

I’m sorry - I meant a tankless unit

Electric tankless requires a large electric circuit. More than a regular tank heater, more than an electric dryer, more than an electric range, usually more than an electric central heating system. Tankless must produce sufficient heat to raise the incoming cold water to the target temperature instantly as it flows through.

The lowest seasonal temperature of the incoming cold water is a factor (colder water requires more heating capacity), along with the highest desired output temperature, and how many concurrent hot water tasks (volume of water flow).

My whole-house tankless is on a 120 amp circuit, although that size unit may be more than you need for a MIL suite running (maybe?) one hot water task at a time (or two at most?).

Do you have the electrical capacity to handle it?

This question has been asked and answered numerous times in this forum. Please do a quick search on here first - then follow up with any questions.

I'd look for a different solution to an electric tankless heater. Expensive to install, expensive to run, limited capacity, etc, etc.