Sizing Your Portable Water Heater! When it comes to choosing the right tankless water heater, size truly matters (just ask Sasha after a chilly rinse).  The last thing you want is for your shower to turn arctic midway through a singalong. To figure out which size is best for you, start by considering two things:  Flow Rate: Add up the gallons per minute (GPM) required by all fixtures you plan to run at once (showers, sinks, even that never-ending soaker tub). Temperature Rise: Find the difference between the incoming water temperature and your desired hot water temp. In chillier climates, you'll need more power.     For example, a small apartment might be cozy with a 3-4 GPM unit, perfect for one shower and a sink. Meanwhile, a bustling household or retreat cabin—with friends popping in—could need a beefier 7+ GPM system to keep everyone (and their rubber ducks) happy.