Do I need to run 2 showers at the same time?

Most whole house units can handle 2 showers at a time. You need to get the product selection information on the model you want. They will walk you through several questions and help determine what is right for you.

If you have average low flow shower head than they have flow rate of about 2.2 gallons per minute, per head. If you use high flow shower head, like Speakman, than the flow may be up to 4 gln per minute.

Rinnai tankless heaters that we install come in several different flow ratings. One is 7.5 gln/m and next model up is 9.4gln per minute. Those are the two most popular models. There is about $128.00 difference in cost between those two units. I would not go to the lower than 7.5 gln/m.