What Size Water Heater Do I Need? Tank and Tankless

Two numbers decide the size

The US Department of Energy's Energy Saver guidance treats the two main kinds of water heater differently, because they run out of hot water in different ways.

A tank runs short when a busy hour drains it faster than the burner or element can reheat it. A tankless heater never stores water, so it cannot run empty, but its heat output caps the flow: open too many taps, or feed it colder water, and the temperature drops. DOE notes that a properly sized heater meets the household's needs while operating more efficiently. Heat pump water heaters with a tank are sized like any other tank. For combination systems that heat both the house and the water, DOE says to consult a qualified contractor.

Tank heaters: estimate your peak-hour demand

Peak-hour demand is the most hot water your home uses in a single hour. DOE's method has two steps: decide which time of day (morning, noon or evening) is your busiest, keeping in mind how many people live in the home, then add up the hot water used in that hour with the figures from its worksheet:

Multiply each figure by the number of times it happens in your busiest hour and add the results. DOE notes that its estimates are averages from a variety of information published online, and that some manufacturers offer calculators that also take the length of each use into account. If your showers are short, or your household uses less hot water per use than these averages, your own number will be lower. The worksheet estimates one hour only; it is not a figure for daily hot water use. The volume converter turns gallons into liters for any other number in this guide.

Peak-hour demand for example households

The households below are example values, built from DOE's per-use figures, except the middle row, which is DOE's own worked example. The last column shows the usage bin a heater with that first-hour rating falls into under the federal test procedure; efficiency ratings are compared within these bins, as explained further down.

DOE's rule is to look for a model with a first-hour rating that at least matches your peak-hour demand: for the household in its example it prints a peak-hour demand of 66 gallons and a first-hour rating of 66 gallons or more. (The three uses it lists add up to 65; the table above shows the sum, and the gallon of difference changes nothing about the method.) A University of Florida extension publication adds that the rating should be within 1 to 2 gallons of the peak-hour demand, so the target is a close match rather than the biggest tank you can fit. A busier hour that moves you up a bin, as the laundry does for the family in the fourth row, is worth spreading out: running the washer at another time of day takes 25 gallons out of the busiest hour.

Reading the first-hour rating

The federal test procedure in 10 CFR 430 defines the first-hour rating as an estimate of the most hot water a heater can supply within an hour that starts with the heater fully heated. In the test, water is drawn at 3.0 gallons per minute for tanks of 20 gallons or more, from a supply held at 58 °F, with the heater set to deliver 125 °F.

• Where to find it. DOE says the yellow EnergyGuide label lists it in the top left corner as "Capacity (first hour rating)". The Federal Trade Commission requires that label on all new conventional storage water heaters, but not on heat pump water heaters; for those, and as a cross-check, the manufacturer's literature gives the rating.
• Why two 50 gallon tanks differ. DOE lists three things the rating depends on: the tank capacity, the heat source (burner or element) and the size of that burner or element. The tank covers the first part of the hour; the heat input decides how fast it refills with hot water.

The reheating part follows from the physics further down: every gallon warmed by 70 °F takes about 584 Btu. An electric element of an example 4,500 W puts 15,354 Btu per hour into the water, enough for about 26 gallons an hour. A heater that gets an example 30,000 Btu per hour of heat into the water reheats about 51 gallons an hour. The same tank with the faster heat source has the higher first-hour rating, which is why the rating, not the gallons on the tank, is the number to compare.

Tankless heaters: flow rate and temperature rise

DOE notes that tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate, and that they typically provide 2 to 5 gallons (7.6 to 15.2 liters) of hot water per minute, gas-fired models more than electric ones. Sizing one takes three steps from its guidance:

1. List the hot water devices you expect to use at the same time, for the whole house or for one remote spot such as a single bathroom.
2. Add up their flow rates in gallons per minute (gpm). DOE's example runs a 0.75 gpm faucet and a 2.5 gpm showerhead at once, so the heater needs at least 3.25 gpm (12.3 liters per minute). If you do not know a fixture's flow, DOE suggests holding a bucket under it for one minute and measuring what you catch.
3. Work out the temperature rise: the output temperature you want minus the temperature of the incoming water. DOE suggests assuming 50 °F (10 °C) for incoming water unless you know otherwise, or measuring it with a thermometer under a cold tap. For most uses it suggests 120 °F (49 °C), a rise of 70 °F (39 °C). For a dishwasher without its own heater you might want 140 °F, a 90 °F rise, but DOE cautions that 140 °F increases the risk of scalding.

The temperature converter turns any of these into Celsius. Note that a temperature difference converts without the 32 offset: a rise of 70 °F is a rise of 39 °C, not 21 °C.

Fixture flow is capped by law, which gives you upper limits when you do not want to measure. Federal standards in 10 CFR 430.32 limit showerheads made after January 1, 1994, to 2.5 gpm at 80 psi, and lavatory and kitchen faucets to 2.2 gpm at 60 psi. EPA's WaterSense label goes lower:

DOE's method adds the full fixture flows. At the tap, part of that flow is usually cold water mixed in, so the result errs on the generous side. DOE also points to low-flow fixtures as the way to reduce the flow a tankless heater has to cover.

How much heat a given flow needs

Behind every tankless rating is one piece of physics. EIA defines the Btu as the heat needed to raise one pound of water by 1 °F, and NIST's property data confirm that the specific heat of water stays within a fraction of a percent of that between 40 and 140 °F. NIST puts water at 8.337 lb per gallon at 60 °F. Warming one gallon by 1 °F therefore takes about 8.34 Btu, and a flow of one gallon per minute for an hour is 60 gallons.

The tables below are that formula worked out. They show the heat that has to reach the water. An electric heater turns practically all its input into heat in the water; a gas heater loses part of its fuel's heat up the vent, so its gas input, the Btu per hour on its rating plate, has to be higher than the figure here. The manufacturer's chart of flow against temperature rise for a specific model is the final word.

The figures match DOE's description of the market: it says a 70 °F rise is typically possible at about 5 gpm through gas-fired demand heaters and 2 gpm through electric ones. By the formula, that is 175,000 Btu per hour of heat into the water for the gas unit and 70,000 Btu per hour, 20.5 kW, for the electric one. The power converter switches between Btu per hour and kW for any other rating, and the energy converter does the same for Btu and kWh.

Fixtures at the same time: example flows

The fixture combinations below are examples; the flows come from the federal limits and WaterSense specifications above. Each row uses DOE's default rise of 70 °F. The last column is the current an electric tankless heater would draw at an example 240 V (amps = watts ÷ volts).

Two things stand out. Swapping a federal-limit shower and faucet for WaterSense ones cuts the heat a tankless unit must deliver for that pair by 26 percent. And for electric units the current is large. Whether your panel and service can carry it is not something to estimate from a table: DOE recommends a qualified plumbing and heating contractor, and the wiring is a job for a licensed electrician under your local code. DOE also notes that even the largest gas-fired model may not keep up with several simultaneous uses in a large household, and that two or more tankless heaters, or a separate one for an appliance, are an option at extra cost.

Colder incoming water lowers the flow

A tankless heater's heat output is fixed, so the flow it can deliver at 120 °F depends on how cold the water arrives. DOE notes that faster flow rates or cooler inlet temperatures can reduce the water temperature at the most distant faucet. The table takes DOE's two typical heaters, sized for 70 °F at 5 gpm (gas) and 2 gpm (electric), and divides their output by 500 × the rise for other inlet temperatures. The inlet temperatures are example values; measure your own at a cold tap in the coldest month if you can.

Size for the coldest water you expect, not the average. Some tankless models are thermostatically controlled and vary their output temperature with the flow rate and inlet temperature, according to DOE, so check how the model you are considering behaves at its limit. To see what heavy use costs on an electric unit, multiply kW by hours: the 20.5 kW heater at full output for 10 minutes uses 3.4 kWh, and the electricity cost calculator turns kWh into dollars at your rate. The kW vs kWh guide explains the difference between the two units.

Efficiency: the UEF on the label

Size comes first; then compare efficiency among the heaters that fit. DOE says a water heater's efficiency is given by its uniform energy factor (UEF): the higher it is, the more efficient the heater. The UEF test runs every heater through a 24-hour draw pattern chosen by its first-hour rating (tanks) or its maximum gpm rating (tankless), from 10 CFR 430 appendix E:

DOE warns that a UEF can only be compared with heaters in the same bin, and that a higher UEF does not always mean a lower operating cost when the heaters use different fuels. The EnergyGuide label also shows an estimated yearly energy cost, which already folds in the fuel. ENERGY STAR states its tankless criterion as gpm over a 67 °F rise, the gap between the federal test's 58 °F supply and 125 °F delivery. At DOE's 70 °F planning rise the same heater delivers a little less: a 4 gpm rating at 67 °F works out to 3.83 gpm at 70 °F.

Federal rules set a floor. The minimum UEF in 10 CFR 430.32 for heaters made before May 6, 2029, depends on fuel, type, bin and, for tanks, the rated volume. Worked out for example sizes:

An electric resistance element cannot deliver more heat than the electricity it uses, so a UEF above 1 points to a heat pump. Large electric tanks over 55 gallons already have to reach about 2. For heaters made on or after May 6, 2029, the rule sets 2.30 for most electric storage heaters of 20 to 55 gallons and 2.50 for 55 to 120 gallons, with a separate, lower standard for a defined class of small electric storage heaters of 20 to 35 gallons. ENERGY STAR's criteria sit above the federal floor: a UEF of at least 0.95 and 2.8 gpm for gas tankless heaters, and at least 0.81 (medium bin) or 0.86 (high bin) with a first-hour rating of 51 gallons for gas tanks up to 55 gallons.

Heat pump water heaters: sized like a tank

A heat pump water heater moves heat from the surrounding air into a storage tank instead of making it with an element, which DOE says makes it two to three times as efficient as a conventional electric resistance heater. For sizing it is a tank: DOE uses the same first-hour rating method, and ENERGY STAR suggests starting from the gallon capacity of your current tank and buying larger if you have added or plan to add people, or a fixture or appliance you use often. Three points from DOE and ENERGY STAR matter before you choose one:

• Rating. ENERGY STAR certified heat pump water heaters have a first-hour rating of at least 45 gallons per hour and a UEF of at least 3.30, or 2.20 for models on a 120 V, 15 A circuit and split systems. There is no EnergyGuide label requirement for them, so get the first-hour rating from the manufacturer.
• Space. DOE says they need a location that stays between 40 and 90 °F (4.4 to 32.2 °C) year-round with at least 1,000 cubic feet (28.3 m³) of air around the unit. They cool the space they stand in and do not work efficiently in a cold one.
• Cold conditions. ENERGY STAR notes that in cold surroundings with low incoming water temperatures the unit may switch temporarily to a backup standard electric mode, which uses more energy in those months.

Running costs belong to other guides. The steps that lower the water heating part of a power bill are in how to lower your electric bill, and heat pump vs gas furnace shows how to compare a heat pump's electricity with gas for space heating.

Before you buy: a short checklist

• Tank or heat pump tank: add up your busiest hour with DOE's per-use figures, and pick a first-hour rating that at least matches it, ideally within a gallon or two.
• Tankless: add the flows you run at once, take the rise from your coldest incoming water to 120 °F, and check the model's flow at that rise in its literature.
• Efficiency: compare UEF only within the same bin, and look at the yearly cost on the EnergyGuide label for heaters on different fuels.
• Fuel: if you are weighing gas or propane, the gas bill guide explains therms and prices and the propane tank guide covers how long a tank of fuel lasts.
• Installation: DOE recommends a qualified plumbing and heating contractor, especially for gas units, and checking that the installer obtains any local permit and knows your local codes. Venting, gas lines and electrical circuits are not do-it-yourself numbers.

Frequently asked questions

Sources

• US Department of Energy, Energy Saver: Sizing a New Water Heater (March 26, 2025; archived copy; the Energy Saver page is no longer on energy.gov): first-hour rating, peak hour demand worksheet and gallons per use, tankless flow rate and temperature rise, 50 °F inlet and 120 °F output defaults, 70 °F rise at 5 gpm gas and 2 gpm electric
• US Department of Energy, Energy Saver: Tankless or Demand-Type Water Heaters (March 10, 2025; archived copy; the Energy Saver page is no longer on energy.gov): 2 to 5 gpm typical output, simultaneous uses, installation by a qualified contractor and local codes
• US Department of Energy, Energy Saver: Heat Pump Water Heaters (March 27, 2025; archived copy; the Energy Saver page is no longer on energy.gov): two to three times as efficient as electric resistance, 40 to 90 °F space and 1,000 cubic feet of air
• US Department of Energy, Energy Saver: Estimating Costs and Efficiency of Storage, Demand, and Heat Pump Water Heaters (March 26, 2025; archived copy; the Energy Saver page is no longer on energy.gov): uniform energy factor, four usage bins, compare UEF only within a bin
• 10 CFR 430.32, Energy and water conservation standards: (d) minimum uniform energy factor of water heaters before and from May 6, 2029; (o) faucets 2.2 gpm; (p) showerheads 2.5 gpm (eCFR, up to date as of October 2026)
• 10 CFR 430, Appendix E to Subpart B: uniform test method for water heaters (first-hour rating definition, 3.0 gpm test draws, 58 °F supply and 125 °F delivery, draw pattern bins by first-hour rating and maximum GPM, 8.24 lb per gallon at 125 °F, 3,412 Btu per kWh)
• US Environmental Protection Agency, WaterSense: Showerheads (labeled models 2.0 gpm or less, standard showerheads 2.5 gpm)
• US Environmental Protection Agency, WaterSense: Bathroom Faucets (labeled faucets and accessories 1.5 gpm or less, standard flow 2.2 gpm)
• ENERGY STAR: Residential Water Heaters Key Product Criteria (heat pump UEF 3.30 or 2.20 and first-hour rating 45 gal/hr; gas storage UEF 0.81 or 0.86 and 51 gal/hr; gas tankless UEF 0.95 and 2.8 gpm over a 67 °F rise)
• ENERGY STAR: Heat Pump Water Heaters (sizing from the current tank, 40 to 90 °F installation space, backup electric mode in cold conditions)
• NIST Chemistry WebBook: thermophysical properties of water at 14.696 psia, 40 to 140 °F (density 62.367 lb per cubic foot at 60 °F, specific heat about 1.00 Btu per lb per °F)
• NIST Office of Weights and Measures: Liquified Petroleum Gas 20 Pound Cylinders, Measurement Matters, July 2021 (the US gallon as 231 cubic inches)
• US Energy Information Administration: British thermal units (Btu), 1 kWh = 3,412 Btu
• UF/IFAS Extension, Energy Efficient Homes: Water Heaters (FCS3277/FY1025): choose a first-hour rating within 1 to 2 gallons of peak-hour demand

Written by Tobias. Updated October 22, 2026.