Most water heaters get replaced with whatever was there before. This tool sizes one for what your household actually does — and it accounts for New Hampshire’s cold incoming water, which is the single biggest reason tankless units disappoint people here.
A tankless heater doesn’t store hot water — it raises the temperature of water flowing through it. So its real output depends entirely on how cold the water arriving is.
New Hampshire well water sits in the low 40s°F for much of the year. Getting that to a usable 120°F is roughly an 80-degree rise. The same unit in a warm-climate installation might only be doing a 45-degree rise, and it’s rated on the friendlier number. That’s why a tankless heater that would be plenty in Virginia leaves people here short in January — which is exactly when they notice. This tool sizes tankless for a cold-inlet winter, not for the number on the box.
Tank sizing starts from household size and bathroom count, then adjusts for how compressed your morning is — four people showering in sequence at 6:30 need more capacity than four people spread across three hours. Tankless sizing works backward from how many fixtures might run at once, at winter inlet temperature. Installed cost ranges come from the benchmarks on our Lakes Region cost page, adjusted for fuel type and the extra work fuel changes and venting usually require.
Estimate only. The real number depends on your existing venting, gas line capacity, electrical service, and where the unit sits. We’ll confirm against your actual setup before quoting.
Send us what this told you and we’ll confirm it against your actual setup — venting, fuel, electrical, and where the unit lives.
Your sizing and estimate are carried into the notes below. We’ll confirm it against your actual setup.
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