What counts as the peak hour?

The single hour of the day when the household draws the most hot water, which for most homes is the hour everyone gets up and showers. It is not the daily total and not an average. A heater sized to the daily total would be enormous and would spend most of the day losing heat through its walls for no benefit at all.

How many gallons of hot water is a shower?

The DOE first hour rating worksheet uses 10 gallons of hot water per shower. That is hot water leaving the heater, not water leaving the shower head, because the shower mixes hot with cold. A long shower at a high flow rate uses considerably more, and a low flow head at a short duration uses less, so adjust the count if your household is at either extreme.

Should I count the dishwasher and clothes washer?

Only if they actually run during the peak hour. Most households run the dishwasher after dinner and the clothes washer at the weekend, neither of which overlaps the morning shower rush. If they do overlap, the worksheet gives 6 gallons for a dishwasher and 7 gallons for a clothes washer load, and running them outside the peak hour is the cheapest way to reduce the heater you need.

Does a low flow shower head change the answer?

Yes, and it is the cheapest change available. A 2.5 gallon per minute head running eight minutes draws roughly twice what a 1.75 gallon per minute head does over the same time. Two or three efficient heads can move a household down a tank size, which saves more over ten years than the heads cost.

Sizing calculator

Peak Hour Hot Water Demand Calculator

Quick answer

Peak hour demand is the hot water used in the busiest hour of the day. Using the DOE worksheet figures, a shower is 10 gallons, a bath is 20, a shampoo is 4 and a dishwasher run is 6, so a typical family morning lands between 40 and 70 gallons.

Result

Peak hour demand 72 gal Published standard

Activities counted 5

Tank that usually covers it 75 gal Convention

Count the busiest hour, not the day. A household that uses 180 gallons across a whole day but never more than 30 in any hour needs a far smaller heater than the daily total suggests.

The formula

Peak hour demand = (showers x 10) + (baths x 20) + (shampoos x 4) + (hand and face washes x 4) + (dishwasher x 6) + (clothes washer x 7) + (hand dishwashing x 4) + (food prep x 5) + (shaves x 2)

These are gallons of HOT water at the tap, from the DOE and Energy Star first hour rating worksheet. They are not mixed water at the shower head: a 10 gallon shower draws about 10 gallons of hot from the heater and makes considerably more than 10 gallons of warm water at the body.

The figures above are computed with these exact inputs: Showers 4, Baths 0, Hair shampoos 2, Hand and face washes 4, Shaves 1, Dishwasher runs 1, Hand dishwashing 0, Food preparation 0, Clothes washer loads 0.

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
Gallons per activity from the DOE and Energy Star first hour rating worksheet	Published standard	Gallons per activity from the DOE and Energy Star first hour rating worksheet

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.

Intermediate

Rheem

Rheem PROE50 M2 50 Gallon

$945.99

The professional line 50 gallon electric, which is the single most common residential water heater size in the country.

Best for: A straight 50 gallon electric replacement

Check price on Amazon

Beginner

GE 40 Gallon Lowboy

$522.00

A lowboy is short and wide, which is the answer when the tank has to fit under a stair or in a crawlspace and the tall version physically will not go in.

Best for: Height restricted closets and crawlspaces

Check price on Amazon

Beginner

GE 30 Gallon Electric

$576.00

Thirty gallons suits one or two people. Going smaller than the household needs is the cheapest mistake to make and the most annoying to live with.

Best for: One or two people, or an apartment

Check price on Amazon

Beginner

GE Smart 40 Gallon Short

$621.00

A ten year warranty usually means a thicker anode rod rather than a better tank, and the anode is a part you can replace yourself for about thirty dollars.

Best for: A short-height 40 gallon slot

Check price on Amazon

Intermediate

GE Smart 40 Gallon Short, 12 Year

$675.00

The twelve year version of the same tank. Compare the price difference against the cost of one anode rod before paying for it.

Best for: Hard water areas where the anode does real work

Check price on Amazon

Expert

Bradford White

Bradford White RE340T6 40 Gallon

$1,003.13

Bradford White sells through plumbing supply rather than retail, so this is the tank a plumber is most likely to already stock parts for.

Best for: Anyone who wants the plumber to have parts on the truck

Check price on Amazon

Frequently asked questions

What counts as the peak hour?: The single hour of the day when the household draws the most hot water, which for most homes is the hour everyone gets up and showers. It is not the daily total and not an average. A heater sized to the daily total would be enormous and would spend most of the day losing heat through its walls for no benefit at all.
How many gallons of hot water is a shower?: The DOE first hour rating worksheet uses 10 gallons of hot water per shower. That is hot water leaving the heater, not water leaving the shower head, because the shower mixes hot with cold. A long shower at a high flow rate uses considerably more, and a low flow head at a short duration uses less, so adjust the count if your household is at either extreme.
Should I count the dishwasher and clothes washer?: Only if they actually run during the peak hour. Most households run the dishwasher after dinner and the clothes washer at the weekend, neither of which overlaps the morning shower rush. If they do overlap, the worksheet gives 6 gallons for a dishwasher and 7 gallons for a clothes washer load, and running them outside the peak hour is the cheapest way to reduce the heater you need.
Does a low flow shower head change the answer?: Yes, and it is the cheapest change available. A 2.5 gallon per minute head running eight minutes draws roughly twice what a 1.75 gallon per minute head does over the same time. Two or three efficient heads can move a household down a tank size, which saves more over ten years than the heads cost.

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.