I want to install an ECO 36 36kW electric water heater which requires 150 amps max. My house panel is 200 amp and is out of space. What are my best options?
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.
First, I would think about a load calculation for your house to make sure adding 150A of load does not justify adding additional service capacity. I can think of two facilities with 200A panels where it would be perfectly reasonable within their service. However, the result of this load calculation may be that you just need bigger service. If you can get 400 (320) amp service, that is typically handled by installing two 200A panels. You could retain your first panel and fit a second panel right next to it.
If the load calculation supports staying at 200A, then you have a simple matter that the "full panel" situation is absolutely intolerable. Again the answer is another 200A panel right next to it, but in this case you feed it as a subpanel, fitting a 60-100A breaker (60A breakers have the advantage of being $9) in the main panel. Then move a bunch of smaller branch circuits to the subpanel, freeing up slots in your main panel. You will need to free up 6 spaces to accommodate the three 2-pole breakers the water heater will require.
Make sure to lay out the subpanel so you could trivially convert the setup to 400A/dual main panels. That way you don't have to do any work if you go that way later.
Getting a water heater that is large enough is a Very Correct Thing To Do. I would even say that a 150A heater is not excessive, you will get about the same BTU as my gas unit.
Here is what is the mistake, however: shipping on-demand hot water all over a large house. For instance my friend has a sprawling ranch house, with bedrooms and those bathrooms clustered at one end of the house, and kitchen, laundry and the guest bathroom clustered at the other. Between them is a very long hot water pipe, which causes endless waiting for the water to heat up. This also wastes a lot of energy, because after filling the long pipe with hot water, that hot water is then abandoned in the pipe. Having one single huge tankless, in that house, would be a great design waste.
Getting rid of that long wait is often a design goal of switching to tankless. Fitting the tankless in the same place as the tanked heater will not solve it.
The beauty of electric hot water is it has no flue. It can be sited almost anywhere. The ideal place is 6 inches from the shower's hot water faucet, but I'll settle for under that bathroom's sink.
So don't carry the heat long distances in copper pipe. Carry it in copper wire. Heat carried via wire moves at the speed of light and has no wastage, no electrons abandoned in the wire at the end of the shower. In other words, that calls for smaller tankless units distributed around the house as needed.
This impacts load calculations, because you don't necessarily need to factor for all the units being on at the same time.
You need to vacate 2 slots in you CB panel or replace the panel with another 200amp panel with more slots. Do an amperage survey with an amp probe. Turn on everything you would use at the same time on any one breaker and see how much amperage is loading the circuit with your clamp on amp probe. Consolidate circuits if you can.