When you say on demand I assume you mean tankless water heaters.
There are different sizes of tankless water heaters. They should be sized specifically to your needs.
- The heaters are graded by number of bathrooms the unit can heat with water running at the same time.
- If your needs are not met by one tankless you can run two for five or more bathrooms.
I installed one in my house a little more than a year ago. It has saved us on average $75 dollars a month in natural gas. Basically cut our bill in half. We have two full baths and one half bath.
We never run out of hot water, ever!!!
We went with a Richmond brand, but there are many to choose from. I would recommend anyone who is replacing their water heater use a tankless. To never run out of hot water alone is enough for me to justify the 200 dollars in extra cost. The lower gas or electric bill is just a bonus.
For more information:
Read "What are the pros and cons of a tankless water heater? We are considering replacing our gas hot water tank." a Q&A answered by Danny Kelly.