I'm trying to figure out what size water heater I would need to meet both Heat Load and DHW load. This is for a new house and design heat load is ~36 000 BTU/hr. I am installing radiant heat in thin slab in the floors. The thing I don't know how to factor in is DHW load. If I do sizing for DHW I would need a standard dual element 5500 watt 60 Gal tank. My plan was to use a 5500W dual element 60 gal tank but change the elements to non flip flop operation. This would give me 11KW of power. Is that enough? BTW my only choice is electric.

No, it's not. You need an electric boiler. a 36,000 BTU/hr load is just about 11kw by itself.

Sorry if this is an inane question, but you can't get propane?

Rob is right.

It is not wise to get creative with approved appliances.

Electric boiler with reset and a Marathon water heater sized to the load and conditions e.g. off-peak.
Your radiant floor designer should help you with this.

Ok so I guess I'll rephrase my question. How would I go about sizing a tank water heater for combination use. It's done with propane tank heaters so if I size the water heater properly shouldn't it work the same. I can get a light industrial build to order anywhere in the 6-54 KW range (21 000- 190 000 BTU/hr). The reasoning for a single heat source was to add solar to it later on.

You will need a tank and heat exchanger for solar. Depending on your climate you can expect 50 duty load (saving about $25/month here in Minneapolis). The whole system must be designed as one before you start.

You probably don't want a tank if you're going to add a solar tank later.