ECO 36 on 200 amp panel
I have installed similar water heaters and because of the large amp draw you will need a larger service a sub panel alone is not enough. Several homes I was able to use a 30/40 service panel 300 amps continuous / 400 peak I think some call these 320 amp services. In one large all electric home I had to install a 400 amp panel. From either of these you can feed your existing main that would become a sub panel (some rewiring may be needed to isolate the neutral from ground) a 2nd sub could be added to power your water heater and any additional loads for future use. Because of the size of this load it is not as simple as just adding a sub from your existing main panel. The service drop and usually the meter base will also required to be changed. I would first check with the power company to verify your service can be upgraded without you having to purchase a larger transformer. Most customers making this change have been ok with the costs and results but I had 1 customer that after spending +10k or more had the tankless removed because he liked the 80 gallon electric better.