Umair Gaines
Choosing the right size of electric water heater is important to ensure it meets your household's hot water needs while operating efficiently. The size of the water heater you need depends on the number of people in your household and the flow rate of water. Electric tank water heaters range from 20 to 80-gallon capacities, with a 40- to 50-gallon tank generally sufficient for 2 to 3 people, increasing by 10 gallons per additional person. Tankless water heaters are rated by the maximum temperature rise possible at a given flow rate, so you need to determine the flow rate and temperature rise required for your household. The flow rate through an electric demand water heater is typically 2 gallons per minute, and the temperature rise is calculated by subtracting the incoming water temperature from the desired output temperature.
| Characteristics | Values |
|---|---|
| Fuel Type | Electric, gas or a combination of both |
| Use | How you plan on using a water heater determines how much hot water it needs to supply, as well as whether you need a single unit or individual units at several points in the house |
| Size and Profile | Measure your space and take note of any structural features that will affect where the water heater can be installed. Water heaters are available in compact, medium, and standard sizes |
| Capacity | Standard residential tank water heaters range from 20 to 80-gallon capacities. A home with two or fewer people can be served by a 23- to 36-gallon tank. A tank size of 36- to 46-gallons is best for homes of up to four people. A home with five people can be served by a 46- to 56-gallon tank. For six or more people, look at larger tanks with a 60-gallon or greater capacity |
| First Hour Rating (FHR) | Describes how much hot water a given tank can produce in a single hour when it’s fully heated. The higher the figure, the greater the amount of hot water that's available |
| Hot Water Recovery Rating | Shows how quickly the water heater will reheat after extended use |
| Flow Rate | The flow rate through an electric demand water heater should be at least 2 gallons per minute. The flow rate through a gas-fired demand water heater should be at least 5 gallons per minute |
| Temperature Rise | The difference between the desired temperature setting of the water heater and the inlet water temperature. For residential use, the water temperature should be set to 120°F. Subtract the incoming water temperature (usually 50°F) from the desired output temperature to determine the temperature rise |
| Number of Users | The number of users is key for tank-style water heaters because the output is more or less fixed |
| Type of Water Heater | Tank storage heaters and on-demand tankless systems are the two main types of water heaters |
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What You'll Learn
- Flow rate: Measure water flow rate in gallons per minute
- Temperature rise: Subtract incoming water temperature from desired output
- Tank size: Consider household size and hot water needs
- First hour rating: Check how much hot water is supplied in an hour
- Energy efficiency: Compare electricity costs and heater efficiency
Flow rate: Measure water flow rate in gallons per minute
To determine the correct size of an electric water heater, you need to consider the flow rate and the temperature rise you'll need for your application. The flow rate is the volume of water passing through a pipe at any given time, typically measured in gallons per minute (GPM).
Measuring the water flow rate in gallons per minute can be done in a few different ways. Here is a step-by-step guide:
- Prepare a one-gallon container. If you don't have a one-gallon container, you can use any other container, as long as you know its volume and can do the proper conversion.
- Turn on the water and start timing how long it takes to fill the container using a stopwatch or your phone.
- Divide 60 by the number of seconds it took to fill your container. For example, if it took 30 seconds to fill a one-gallon container, the calculation would be 60 / 30 = 2 gallons per minute. This is your flow rate.
- If you plan to run multiple applications simultaneously, such as two showers at once, you will need to measure each application's flow rate and then add them together.
It's important to note that the flow rate through an electric demand water heater should be at least 3.25 gallons (12.3 liters) per minute. Additionally, a 70°F (39°C) water temperature rise is possible at a flow rate of 2 gallons per minute through electric heaters.
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Temperature rise: Subtract incoming water temperature from desired output
When choosing an electric water heater, it is important to consider the temperature rise, which is the difference between the incoming water temperature and the desired output temperature. This is a crucial factor in ensuring that your water heater meets your household's hot water needs while operating efficiently.
To determine the temperature rise, simply subtract the incoming water temperature from the desired output temperature. For example, if you want your water heated to 120ºF (49ºC) and the incoming water temperature is 50ºF (10ºC), you would need a water heater that can produce a temperature rise of 70ºF (39ºC).
It is important to note that the temperature rise may vary depending on the flow rate of water. Electric demand water heaters are typically rated to provide a temperature rise of 70ºF (39ºC) at a flow rate of 2 gallons per minute. However, faster flow rates or cooler inlet temperatures can sometimes result in a lower temperature at the farthest faucet. Therefore, it is important to consider both the flow rate and the temperature rise when selecting an electric water heater.
Additionally, the size of your household can impact the temperature rise requirements. A larger household with more people will require a higher flow rate and, consequently, a higher temperature rise to ensure that everyone's hot water needs are met. Conversely, a smaller household may require a lower temperature rise and can opt for a more compact water heater.
By considering the temperature rise and other factors such as flow rate and household size, you can choose an electric water heater that efficiently meets the hot water demands of your household. Remember to refer to product specifications and seek professional guidance when making your purchase decision to ensure you select the most suitable water heater for your needs.
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Tank size: Consider household size and hot water needs
When choosing a new water heater, it's important to select one that's the correct size for your needs. If a water heater is too small, you will experience cold water during peak hours. If it's too big, your energy costs will be higher. The two main types of water heaters are tank storage heaters and on-demand tankless systems.
Tank water heaters are sized based on the number of gallons the tank can store. This limits how much water a tank-style water heater can provide. A home with two or fewer people can be served by a 23- to 36-gallon tank. A tank size of 36- to 46-gallons is best for homes of up to four people. A home with five people can be served by a 46- to 56-gallon tank. For six or more people, look at larger tanks with a 60-gallon or greater capacity. In general, a 40- to 50-gallon tank is sufficient for an average household of 2 to 3 people, increasing by 10 gallons per additional person in the home.
Tankless heaters heat water as you use it and do not have a storage tank. They are measured by hot water flow rate in GPM (gallons per minute). The flow rate through an electric tankless water heater is 2 gallons per minute. The maximum flow rate is determined by the number of fixtures (e.g. washing machine, dishwasher, shower) that will be used at the same time, and the flow rate of each. For example, if your home would use a dishwasher (2-3 GPM), a shower (2-3 GPM) and a washing machine (3-5 GPM) all at once, that would be a maximum flow rate of 7-11 GPM.
Temperature rise is the other critical factor. This is the difference between the desired temperature setting of the water heater and the inlet water temperature. For example, if the hot water temperature is set to 120°F and the incoming water temperature is 55°F, the resulting temperature rise is 65°F. To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50°F (10°C). For most uses, you'll want your water heated to 120°F (49°C). In this example, you'd need a demand water heater that produces a temperature rise of 70°F (39°C) for most uses.
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First hour rating: Check how much hot water is supplied in an hour
The first-hour rating of a water heater is an estimate of the maximum volume of hot water that a storage-type water heater can supply within an hour, starting with the water heater fully heated. It is an essential piece of information that can usually be found on the rating card or label attached to your water heater.
The first-hour rating is calculated by multiplying the tank's total capacity by 0.70 (70%) and adding the heater's recovery rate (how quickly the water in the tank heats back up). The total will be the first-hour rating, measured by gallons per hour. For example, a 40-gallon water heater with a recovery rate of 22.5 gallons per hour at an 80-degree rise in temperature would have a first-hour rating of 50.5 gallons.
The amount of water in the tank is multiplied by 70% because, as water is being used, new cold water enters the tank and dilutes some of the heated water. The thermostat senses the cold water introduced into the tank and begins the heating process again. Therefore, the first-hour rating is an estimate of the water heater's capacity during peak hours when there is a high demand for hot water.
To determine the correct size of an electric water heater for your household, you should consider the number of people in your household and the amount of water used by each person or at the busiest times of the day. A 23 to 36-gallon water heater is sufficient for a couple with no kids, while a household of five or more people would require a 56-gallon water heater. You can add 10 gallons per person in the household to estimate the required tank capacity.
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Energy efficiency: Compare electricity costs and heater efficiency
When selecting an electric water heater, it's important to consider its energy efficiency to save on electricity costs. A water heater with a higher energy factor (EF) will be more energy efficient and cost less to operate. EF is based on the amount of hot water produced per unit of fuel consumed over a typical day.
The uniform energy factor (UEF) is another metric to consider, which is determined by assigning water heaters into one of four different categories of hot water usage and then evaluating their performance based on that usage. A higher UEF means a water heater is more energy-efficient and will cost less to operate compared to other water heaters in the same category. However, it's important to note that UEF can only be compared within the same category, and a higher UEF may not always result in lower annual operating costs, especially when comparing fuel sources.
To estimate the annual operating costs of an electric water heater, you can refer to the yellow Energy Guide label, which shows the estimated energy efficiency and costs. By comparing these estimates with other models, you can determine the potential savings and payback period of investing in a more efficient model, which may have a higher upfront cost.
Additionally, consider the size of the water heater and its first-hour rating (FHR). FHR is the number of gallons of hot water the heater can supply per hour, and it should match your household's peak hot water usage. A properly sized water heater will meet your hot water needs while operating more efficiently. If you have multiple fuel sources available, compare their fuel costs as well, as this can impact your overall energy costs.
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Frequently asked questions
If your water heater is too small, you will often run out of hot water, especially in winter. If it’s too big, your energy costs will be higher.
The two main types of water heaters are tank storage heaters and on-demand tankless systems. The size of a tank storage heater is determined by the number of gallons it can store, whereas tankless heaters are measured by flow rate in gallons per minute.
The number of users is key for tank-style water heaters because the output is more or less fixed. A home with two or fewer people can be served by a 23- to 36-gallon tank, and you can increase the tank size by 10 gallons for each additional person in the home.
The size of a tankless water heater depends on the maximum flow rate required by the hot water system, measured in gallons per minute. The maximum flow rate is determined by the number of fixtures that will be used at the same time, and the flow rate of each.
List all the hot water faucets and appliances you use at the same time and add up their flow rates. For example, if you use a dishwasher (2-3 GPM), a shower (2-3 GPM) and a washing machine (3-5 GPM) at the same time, that would be a maximum flow rate of 7-11 GPM.
