What is UEF on a water heater?

UEF is the DOE Uniform Energy Factor, the ratio of hot water energy delivered to energy consumed, measured over a standard daily draw pattern. Because the test runs a whole day it already includes standby loss through the tank walls. A higher UEF is better, and units are only comparable within the same draw pattern size, which is also printed on the label.

Why does a heat pump water heater have a UEF above 1?

Because it does not create heat, it moves heat. A heat pump water heater takes warmth out of the surrounding air and puts it into the water, so it delivers more heat energy than the electricity it consumes. Typical units land between 3.0 and 4.0, meaning roughly a quarter to a third of the running cost of a resistance element for the same hot water.

Which is cheaper to run, gas or electric?

At typical prices, natural gas is usually cheaper to run than an electric resistance tank, and a heat pump water heater is usually cheaper than both. The comparison turns entirely on local fuel prices, so put your own numbers in rather than trusting a general answer. Propane is frequently the most expensive of the four on a per BTU basis.

Does a bigger tank cost more to run?

A little, through standby loss, but far less than people expect on a modern foam insulated tank. The energy to heat the water you actually use dominates the bill, and that is the same whatever size the tank is. Standby loss is the reason not to buy a much larger tank than you need, but it is not usually a large number on its own.

Running cost calculator

Water Heater Operating Cost Calculator

Quick answer

Annual cost is delivered energy divided by UEF, times the fuel price. Heating 55 gallons a day through a 70 F rise at a 0.92 UEF and 16 cents per kilowatt hour costs about $560 a year, while the same use on a 3.5 UEF heat pump costs about $147.

Result

Cost per year $597 Published standard

Cost per day $1.63 Published standard

kWhs per year 3,729 Published standard

Delivered heat per day 32,071 BTU Published standard

Change the gallons per day first if the figure looks wrong. Household hot water use varies more than any other input here, and 55 gallons a day is a planning convention rather than a measurement of your house.

The formula

Delivered BTU per day = gallons x 8.33 x rise. Input BTU = delivered / UEF. Annual cost = (input BTU per day / BTU per fuel unit) x 365 x unit price.

UEF is the DOE Uniform Energy Factor, measured over a standard daily draw pattern, and it already includes standby loss. That is why a heat pump water heater shows a UEF above 1: it moves more heat than it consumes as electricity, so the number behaves like a coefficient of performance rather than an efficiency.

The figures above are computed with these exact inputs: Heater type electricResistance, UEF from the label 0.92, Fuel price per unit 0.16 $, Hot water per day 55 gal, Temperature rise 70 F.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
DOE Uniform Energy Factor	Published standard	DOE Uniform Energy Factor, measured over a standard daily draw pattern and including standby loss
Doe Daily Draw: 55	Convention	Daily hot water use assumed for the annual cost figure. Actual household use varies widely and this is the input to change first
Water Weight: 8.33	Published standard	Published density of water at 60 F, 8.33 lb per gallon
Btu Per Kwh: 3412.14	Published standard	Exact conversion between the international table BTU and the kilowatt hour
Btu Per Therm: 100000	Published standard	A therm is defined as 100,000 BTU
Btu Per Gal Propane: 91500	Published standard	Published higher heating value of liquid propane, 91,500 BTU per gallon

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Beginner

Senville

Senville Hybrid Heat Pump 50 Gallon

Under $1,500

The lowest price of entry into a heat pump water heater in this catalog, and Energy Star listed, which means its efficiency figure was measured against the DOE test procedure rather than claimed.

Best for: A first heat pump water heater in a warm garage or basement

Check price on Amazon

Intermediate

Rinnai

Rinnai REHP50 50 Gallon

Under $2,000

A 50 gallon heat pump tank from a manufacturer with an established residential service channel, which is the part that decides what year eight looks like.

Best for: The standard 50 gallon replacement, done properly

Check price on Amazon

Intermediate

ACiQ

ACiQ 50 Gallon Hybrid

Under $2,000

A hybrid runs its compressor most of the time and falls back to resistance elements on a heavy draw, which is why a hybrid never simply fails to make hot water.

Best for: Households unwilling to risk a slow recovery

Check price on Amazon

Expert

Rinnai

Rinnai REHP65 65 Gallon

Under $2,500

Going up a tank size is the standard fix for a heat pump water heater, because a larger tank lets the slower compressor do more of the work and the elements less.

Best for: Four people who do not want the elements running

Check price on Amazon

Expert

Rinnai

Rinnai REHP80 80 Gallon

Under $2,500

Eighty gallons is tall. Measure the ceiling height and the 700 cubic feet of air a heat pump water heater needs around it before this is on the list at all.

Best for: Large households with the room and the air volume

Check price on Amazon

Size it, choose it, budget it, once

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Frequently asked questions

What is UEF on a water heater?: UEF is the DOE Uniform Energy Factor, the ratio of hot water energy delivered to energy consumed, measured over a standard daily draw pattern. Because the test runs a whole day it already includes standby loss through the tank walls. A higher UEF is better, and units are only comparable within the same draw pattern size, which is also printed on the label.
Why does a heat pump water heater have a UEF above 1?: Because it does not create heat, it moves heat. A heat pump water heater takes warmth out of the surrounding air and puts it into the water, so it delivers more heat energy than the electricity it consumes. Typical units land between 3.0 and 4.0, meaning roughly a quarter to a third of the running cost of a resistance element for the same hot water.
Which is cheaper to run, gas or electric?: At typical prices, natural gas is usually cheaper to run than an electric resistance tank, and a heat pump water heater is usually cheaper than both. The comparison turns entirely on local fuel prices, so put your own numbers in rather than trusting a general answer. Propane is frequently the most expensive of the four on a per BTU basis.
Does a bigger tank cost more to run?: A little, through standby loss, but far less than people expect on a modern foam insulated tank. The energy to heat the water you actually use dominates the bill, and that is the same whatever size the tank is. Standby loss is the reason not to buy a much larger tank than you need, but it is not usually a large number on its own.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.