Tankless vs Tank Water Heater: Sizing, Cost, and Endless Hot Water
Tankless vs storage tank water heaters — GPM vs first-hour sizing, efficiency class, lifespan, gas line and venting demands, and which fits your home.
Updated August 22, 2026
A storage water heater and a tankless water heater answer the same question — hot water at the tap — with opposite strategies, and the sizing math follows. A tank keeps a heated reservoir ready and is sized by its first hour rating (FHR): the gallons it can deliver in the busiest hour, a function of both tank volume and burner or element recovery. A tankless unit stores nothing and is sized by flow: the gallons per minute it can heat continuously at the temperature rise your groundwater demands.
That difference decides most of the trade. The tank pays a standby tax — heat leaking from the reservoir around the clock — but shrugs off any burst of demand that fits inside its first hour. The tankless unit wastes nothing between draws and never "runs out," but its output is a hard flow ceiling: per DOE Energy Saver, typical units deliver about 2–5 gpm, and even large gas-fired models can be stretched by a shower and a dishwasher running at once.
Every planning value in the table below — gallons per use, fixture flows, the 120 °F delivery basis, and the typical gas and electric unit capacities — is mapped in code from the same DOE worksheet constants that drive the water heater sizing calculator, and a cross-check test holds each cell equal to those verified exports.
Side by Side
| Spec | Storage tank | Tankless (on-demand) |
|---|---|---|
| Sizing basisBoth methods follow the DOE Energy Saver sizing worksheet encoded in the water heater sizing calculator. | First hour rating (FHR): pick a model whose FHR meets the peak-hour demand in gallons — FHR is a function of tank volume and recovery rate | Peak flow at temperature rise: sum the gpm of fixtures running simultaneously and check it against the unit's output at your rise to 120 °F delivery |
| Fixture demand planning values (DOE worksheet)Tank values are DOE-published worksheet figures; tankless flows are representative planning values, not code figures. | Gallons per use during the peak hour — e.g. a shower draws 20 gal, an automatic dishwasher 7 gal, a top-load clothes washer 25 gal | Simultaneous flow per fixture — e.g. a shower counts 2.0 gpm, a kitchen sink 1.5 gpm, a dishwasher 1.5 gpm |
| Delivery temperature planning basisCapacity of a tankless unit falls as the required rise grows: the same burner heats fewer gpm through a bigger temperature step. | Reservoir held at the thermostat setpoint; peak-hour demand is tallied in gallons of hot water delivered | Required rise = 120 °F delivery minus groundwater temperature — from 50 °F of rise on 70 °F southern groundwater to 80 °F on 40 °F northern groundwater |
| Typical unit flow class | Flow limited by the plumbing, not the heater — a single-family tank offers a ready 20–80 gallon reservoir (DOE Energy Saver) | A typical gas unit manages about 5 gpm and a typical electric unit about 2 gpm at a 70 °F rise (DOE); gas-fired models produce higher flow rates than electric ones |
| Efficiency metric & class behaviorUEF floors are ENERGY STAR certification criteria (category level), not averages of what is sold; a standing pilot light can claw back part of the tankless standby advantage (DOE). | Rated by Uniform Energy Factor (UEF) per the 10 CFR 430 test procedure; standby heat loss from the reservoir is the class handicap — ENERGY STAR V5.0 asks UEF ≥ 0.81–0.86 of gas storage models, by tank volume and draw pattern | Same UEF metric, no standby loss to pay — ENERGY STAR V5.0 asks UEF ≥ 0.95 of gas instantaneous models; per DOE, demand heaters run 24–34% more efficient than storage in low-use homes (≤41 gal/day) and 8–14% in high-use homes (~86 gal/day) |
| Installation demands | Drop-in replacement in most homes: existing vent, gas line or circuit, and floor space were sized for it; gas storage category is defined at ≤ 75,000 Btu/h nameplate input (ENERGY STAR V5.0 / 10 CFR 430 definitions) | The burner is far larger because it heats in real time — gas instantaneous units run up to 200,000 Btu/h input under the same definitions — so retrofits routinely need a larger gas line, new venting, or a heavy electrical feed; DOE advises a qualified installer for fuel, venting, and code reasons |
| Space & placement | Floor-standing tank claims a closet, garage, or utility-room footprint sized to its 20–80 gallon reservoir | Compact wall-hung cabinet; DOE also lists point-of-use roles a tank cannot play — remote bathrooms, appliance boosters, solar-system backup |
| Service life expectation | Storage water heaters last about 10–15 years (DOE Energy Saver) | Most tankless units have a life expectancy of more than 20 years, with easily replaceable parts that may extend it further (DOE Energy Saver) |
| Operating quirks | Drain the reservoir past its first hour rating and you wait through recovery; oversizing to avoid that raises purchase price and standby losses (DOE) | Never runs out, but output caps the flow: simultaneous heavy draws can exceed the unit, and DOE’s remedy — a second unit or dedicated point-of-use heaters — costs more; each unit also needs its manufacturer’s minimum activation flow to fire |
Which One for the Job
Family of five with back-to-back morning showers
Storage tank
Clustered sequential demand is exactly what first-hour-rating sizing is built for: a properly sized tank delivers its whole peak hour of showers from the reservoir plus recovery. A single tankless unit serves the same house only if the family never stacks draws — per DOE, even the largest gas-fired models can be stretched by simultaneous uses, and adding a second unit erodes the operating-cost case.
Electric-only condo replacing an aging water heater
Storage tank
A typical electric tankless unit manages only about 2 gpm at a 70 °F rise (DOE) — roughly one shower, nothing else — and whole-home electric on-demand capacity means a very large real-time electrical load. An electric storage tank, or a heat-pump water heater where the space allows one (ENERGY STAR certifies integrated HPWHs at UEF ≥ 3.30), spreads the heating over time on an ordinary circuit.
Vacation home that sits empty most weeks
Tankless (on-demand)
A storage tank pays its standby losses every unoccupied day; a tankless unit consumes fuel only when a tap runs, which is precisely the low-use profile where DOE puts the demand-heater advantage at its 24–34% maximum. Confirm the model’s freeze-protection provisions for an intermittently heated building with the manufacturer.
Gas-heated home chasing the lowest lifetime water-heating cost
Either
The tankless unit costs more up front but, per DOE, typically lasts past 20 years against a tank’s 10–15 and runs more efficiently — while the tank is cheaper to buy and may reuse existing venting and gas piping that a high-input tankless retrofit would have to replace. Which side wins depends on your hot-water volume, fuel price, and what the gas line and vent upgrade actually quote at; run both through the sizing calculator first.
Frequently Asked Questions
Does a tankless water heater really deliver endless hot water?
Endless in duration, capped in flow. Because it heats water as it passes, a tankless unit never depletes a reservoir — but DOE puts typical output at 2–5 gpm, and every extra fixture running at once subtracts from that budget. A long shower is effortless; a shower, dishwasher, and washing machine together can outrun even a large gas unit. Storage tanks flip the failure mode: any flow the plumbing allows, until the tank runs cool.
How much energy does switching from a tank to tankless actually save?
DOE Energy Saver puts demand water heaters 24–34% more energy efficient than conventional storage tanks for homes using 41 gallons or less of hot water daily, falling to 8–14% for high-use homes around 86 gallons per day — the advantage is the standby loss a tank pays whether or not anyone draws water. Two caveats: a standing pilot light on a gas tankless model gives part of that back, and the dollar savings ride on local fuel prices.
Can a tankless unit reuse the gas line and vent from my old tank heater?
Often not, and this is the retrofit cost that surprises people. A gas storage heater is a category defined at 75,000 Btu/h of input or less, while residential gas tankless units run up to 200,000 Btu/h (ENERGY STAR V5.0 / 10 CFR 430 definitions) because they do an hour’s heating in real time. That input jump frequently demands a larger gas line and different venting — size the run with a gas pipe sizing calculation and have a qualified installer confirm, as DOE advises.
Why does groundwater temperature matter so much more for tankless sizing?
A tankless unit’s rated flow always comes with a temperature rise attached, and the rise is set by your groundwater: heating 40 °F northern well water to 120 °F delivery is an 80 °F climb, while 70 °F southern water needs only 50 °F. The same burner pushes fewer gallons per minute through a bigger climb, so an identical unit is meaningfully smaller in Minnesota than in Florida. A storage tank hides this — colder inlet water just stretches its recovery time rather than throttling the tap.
Try the Calculators
Sources
- DOE Energy Saver, "Tankless or Demand-Type Water Heaters" — flow rates, efficiency percentages, standby losses, lifespan, simultaneous-use limits (read from Wayback snapshot; the live page was reorganized away)
- DOE Energy Saver, "Storage Water Heaters" — 20–80 gallon reservoirs, standby heat loss, first hour rating selection, oversizing caution (read from Wayback snapshot)
- ENERGY STAR Program Requirements for Residential Water Heaters, Version 5.0 — Tables 1–3 UEF criteria and Section 1 category definitions (75,000 / 200,000 Btu/h input bounds), read from the published specification
- 10 CFR Part 430, Subpart B, Appendix E — Uniform Energy Factor, first-hour rating, and max-GPM test procedure (cited as the metric definitions only)
- DOE Energy Saver "Sizing a New Water Heater" worksheet, as encoded and provenance-documented in the water heater sizing calculator