Water Heater Sizing & Tankless Payback Calculator

Because a tankless unit is rated at a temperature rise, not an output temperature. To deliver a 120°F shower from low-40s winter inlet water it must produce a roughly 78-degree rise, and flow capacity falls sharply as the required rise climbs. A unit rated 9 GPM on a manufacturer summer table can drop to around 4 GPM at a Philadelphia winter rise. Size to January or you will be disappointed every winter.

It depends on peak hour demand rather than household size alone. Broadly, one to two people need 30 to 40 gallons, three to four need 40 to 50, and five or more need 50 to 80 — adjusted upward for multiple bathrooms or a large soaking tub. The calculator above works from your actual fixture count.

It depends on how long you are staying and whether your gas line needs upsizing. Payback typically runs eight to fifteen years here. If you are staying put, use a lot of hot water and have adequate gas supply, the case is strong. If you are moving within five years, or the gas line needs $2,000 of work, a conventional tank or a heat pump unit is usually the better spend.

A conventional tank draws roughly 40,000 BTU. A whole-house gas tankless draws 150,000 to 199,000. Existing half-inch gas line is frequently inadequate and sometimes the meter is too. Upsizing typically adds $800 to $2,500, and any quote that has not verified gas capacity is not a real quote.

Often yes, if you have the space. They are very efficient but extract heat from surrounding air, so they want a garage or basement with real air volume rather than a small interior closet, and they cool that space slightly. In 2026 the federal tax credit is gone, but PECO and PGW run rebate programs that change year to year — check current incentives before you buy, since they can move the comparison substantially.