How To Size A Water Heater For Best Performance

A water heater in a garage that requires sizing.

The size of your water heater is important. A properly sized water heater will meet your household’s hot water needs while operating more efficiently. Therefore, before purchasing a water heater, make sure it’s the correct size. In this article, we’ll cover how to size a tankless or traditional gas or electric water heater for your home and provide some helpful hints about the installation process.

Sizing A Demand-Type Water Heater

A demand-type water heater senses when hot water is needed and heats it on demand. This type of water heater can be sized by considering the number of people in a household, their typical shower duration, how often they cook or do laundry, etc. 

For example, if you have two adults living at home with one teenager who takes long showers, then an 80-gallon gas/electric tankless unit would meet your need for hot water capacity. If you are renting out part of your house to students that take smaller showers than adults (typically), then a 50-60 gallon gas/electric tankless unit will provide adequate supply.

For a more precise calculation, use the following equation:

Q = (H x T) / 60

Where Q is gallons per minute, H is household occupants in a number of people, and T is typical shower duration. For example, for a three-person home with showers lasting 20 minutes, you would need 36 gallons per minute which can be served by either an 80 gallon.

Moreover, to determine the rise of temperature for the water entering a shower, use this equation:

R = (T x C) / D

Where R is temperature rise and T is the desired final temperature of hot tap water, for example, if your desire is for 115 degrees Fahrenheit, then set D at 20, which will give you an increase of 35 degrees Celsius.

To calculate the flow rate, use this equation:

Q = (C x L) / D

Where Q is gallons per minute, C is the desired output temperature and Lis length of pipe in feet, for example, if your desire is for 100 degrees Fahrenheit water at a flow rate of 40 gallons per minute, then set D at 300, which will give you an increase in the flow rate of 12 gallons per minute.

A faster flow rate sometimes can reduce the temperature of distant faucets, so check the temperature of your faucets before and after installation.

If you are installing a tankless water heater, make sure it’s sized correctly for both flow rate and desired output temperature. If not, this will reduce efficiency or cause an overheated condition which can lead to premature failure.

Tank Heat Pump water heaters are sized based on the total heating load required and should be installed with a professional. The first-hour rating is the number of Btuh that the water heater can deliver in an hour and must be greater than or equal to the total heating load.

In order to size your water heater, you need to determine how much hot water your household needs as well as the flow rate coming into it.

Sizing A Solar Water Tanker

A solar water tanker is a perfect solution for remote locations with limited access to water. It’s also a great way of supplementing your current water supply if you have an unusually high demand during certain times of day, such as an early morning or late evening when solar radiation typically peaks.

The recommended size of a solar water tanker is that the reservoir should be at least 40% larger than your hot water usage. If you have three people in your household, then, for example, this would mean that your storage capacity needs to be 60 gallons or more.

Sizing A Collector Area Of The Solar Water Tank

The collector area of the solar water tank is measured by surface area. To determine how much space you need, multiply the length of your roof by its width and divide that number in half. If your house has a square roof with dimensions 40 feet long on all sides, for example, then it would have an area of 1600 sq ft (40 x 40). The collector area should be at least 40% larger than the hot water usage. Moreover, the storage volume of the solar tank should be at least 100 gallons or more.

Factors That Determine The Size Of The Water Heater

The size of the water heater depends on how much hot water is needed and the type of household. If you live in an apartment, for example, then a 50-gallon tank would be more than enough to satisfy your needs.

Water heaters are rated by their storage capacity, which tells us the number of gallons they can hold at a time. The largest units have capacities of up to 300 gallons that last approximately 24 hours without needing any attention from homeowners.

However, these larger tanks cost significantly more money than smaller ones with half-size or smaller capacities that need less energy input because they cycle through them faster during use. A typical family demands about 60 gallons per day, so it’s best to purchase a unit with 120-gallon capacity, which is the most common size for households.

One of the easiest ways to determine if a water heater will be large enough, but not too big, is by calculating how many gallons you use on an average day (60) and multiplying it by four days in a week (240), then dividing that number by 30 days per month (720). 

This leaves us with about 120-gallons total or about 100 gallons less than the suggested amount. The estimated cost difference between this unit and one twice as large would only be $50-$75; so what’re another hundred bucks when we’re talking more convenience?

Some other considerations to take into account are the age of the tank, energy efficiency ratings, and venting capabilities. 


When determining your water heater size, plan for the future by considering various factors such as household occupancy, number of bathrooms, etc. Moreover, talk with a qualified heating professional to determine how many gallons per minute (GPM) you need to meet these needs.

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