What size tankless water heater for a family of 2?
Size to 3.5 GPM: one shower running while someone uses a bathroom sink. At that draw, one gas unit handles it anywhere in the country; the question is only which size class. The table below runs the number against every US region's winter water.
The size by region, at 3.5 GPM
Delivered heat is 500 × 3.5 GPM × the rise from your winter inlet water to 120°F. Gas classes assume 82% efficiency; "two units" means the draw is past a single 199,000 BTU residential unit.
| Region | Winter rise | Heat needed | Gas unit (input) |
|---|---|---|---|
| Far north (MN, ND, MT, ME, VT) | 78°F | 136,500 BTU/hr | 180,000 BTU |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 122,500 BTU/hr | 160,000 BTU |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 108,500 BTU/hr | 140,000 BTU |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 94,500 BTU/hr | 140,000 BTU |
| Gulf coast and Florida | 48°F | 84,000 BTU/hr | 140,000 BTU |
This is the easy end of tankless. At 3.5 GPM the draw sits inside a single unit's range even at a far-north 78°F rise, so the shopping decision is a size class, not an architecture. Buy the class from the table, not the biggest unit on the shelf: an oversized burner short-cycles on small draws like a bathroom sink.
Our scenario is an assumption, stated plainly: one shower running while someone uses a bathroom sink is the worst moment we consider realistic for 2 people. If your household's collisions are bigger (a garden tub plus two showers, say), run the calculator with your own moment; a tub filler alone is 4 GPM.
The one household size where electric tankless is a real answer
A family of two is the only scenario on this site whose worst moment is not two showers at once, and the gap is what puts a whole-house electric unit genuinely on the table. At 3.5 GPM the heat is small enough to make from a panel in the warm half of the country: a Gulf-coast 48°F rise needs about 24.6 kW, roughly 103 amps at 240 volts, and off-the-shelf whole-house electric units run to 36 kW. Move the same draw to a northern 70°F rise and it is 35.9 kW, about 150 amps, which is most of a 200-amp service with the range and the dryer still waiting. Electric tankless is a climate decision before it is a product decision.
The other thing about two people is how cheap the scenario is to shrink. The bathroom sink in it is 1 GPM and nobody is attached to it. Two people frequently never collide at all, and if your real worst moment is one shower and nothing else, that is 2.5 GPM and the unit drops a size class in every region in the table.
Questions people ask
How many GPM does a family of 2 need from a tankless?
About 3.5 GPM: one shower running while someone uses a bathroom sink, at 2.5 GPM per shower head. If your peaks are calmer than that, size to your real worst moment instead; the calculator takes any combination.
Does the brochure GPM rating apply to my house?
Only if your winter inlet water is 85°F, because ratings are quoted at a 35°F rise. Real winter inlets run 42 to 72°F depending on region, so real capacity is roughly half to three-quarters of the brochure number.