What Size Tankless Water Heater Do I Need?

What is a Tankless Water Heater?

A tankless water heater, also called as an on-demand water heater, is a revolutionary device that heats the water directly without the use of the storage tank. When you turn on the hot water tap, the cold water travels in the unit through a pipe where either a gas burner or an electrical element heat the water immediately. This design eliminates the need to maintain a reservoir of hot water, significantly improving energy efficiency and providing a constant supply of hot water.

Unlike traditional systems that might run out of hot water, tankless models continue producing hot water as long as there’s a demand. That means no more cold showers because someone else used up all the hot water earlier. This convenience makes them especially attractive for families and households that use water throughout the day.

Plus, their compact size allows them to be installed in tighter spaces — think closets, under sinks, or even outside on a wall — which is a game-changer for smaller homes or condos.

How Does It Differ from Traditional Water Heaters?

Traditional water heaters store and preheat a specific amount of water (usually 30 to 80 gallons) in a large tank. Once that hot water is used up, the tank needs time to refill and heat the new water. During this recovery time, you may find yourself waiting for another hot shower or wash cycle.

Tankless water heaters, on the other hand, operate on-demand. They heat water only when needed and shut off when the tap is turned off. This not only saves energy but also extends the appliance’s lifespan. Most tankless units last 20 years or more with proper maintenance, compared to 10–15 years for traditional heaters.

In essence, tankless systems are more efficient, space-saving, and tailored to modern households that value both performance and savings.

READ MORE: How to Test a Water Heater Element

Why Size Matters in a Tankless Water Heater

The Role of Flow Rate

Flow rate, measured in gallons per minute (GPM), is the most crucial factor when sizing a tankless water heater. It tells you how much hot water the unit can produce at any given time. If your household often uses multiple water fixtures at once — say, a shower and a dishwasher — you’ll need a unit with a higher GPM capacity to meet demand without sacrificing water pressure or temperature.

For example, a standard shower uses about 2.5 GPM, while a kitchen faucet might need 1.5 GPM. Add those together, and your water heater must handle at least 4 GPM to meet the demand without running cold. If your unit is undersized, it will struggle to keep up, leaving you with inconsistent hot water.

Larger homes with multiple bathrooms or families with simultaneous water usage will naturally require a more robust system. It’s not just about the number of people in the house; it’s about how and when they use hot water.

Importance of Temperature Rise

Temperature rise refers to the difference between the incoming water temperature and the desired output temperature. Let’s say the groundwater temperature in your area is 50°F, and you want to heat it to 120°F — that’s a 70°F rise.

The colder your incoming water, the harder your tankless system has to work. That means if you live in a colder region, you’ll need a larger unit compared to someone in a warmer climate, even if your usage habits are identical. Most manufacturers include sizing charts that factor in average regional groundwater temperatures to help you determine the right model.

Getting the temperature rise right is critical. Undersizing will result in lukewarm water, while oversizing can lead to wasted energy and unnecessary expenses. Accurate measurements are key to making the best choice.

Calculating the Right Size Tankless Water Heater

Step-by-Step Guide to Sizing

Figuring out what size tankless water heater you need might sound complicated, but it boils down to two key things: how much water you need at once and how much you need it heated.

Here’s a basic guide:

1. Calculate Your Flow Rate Needs (GPM): Add up the GPMs for every fixture or appliance you might use at the same time.
   
2. Determine Your Required Temperature Rise: Subtract the temperature of your incoming groundwater from your desired hot water output temperature.
   
3. Match These Numbers to a Heater: Find a unit that can handle both your total flow rate and the temperature rise.
   

Let’s say you’re running a shower (2.5 GPM), a bathroom sink (1.0 GPM), and a dishwasher (1.5 GPM) all at once. Your required flow rate would be 5.0 GPM. If your groundwater is 50°F and you want 120°F, your temperature rise is 70°F. You’ll need a tankless unit that can heat 5.0 GPM by 70°F.

Determining Your Peak Water Demand

Peak demand is the highest amount of hot water you’ll use at any one time. This usually happens in the morning when everyone’s showering, or in the evening when laundry, dishes, and showers all happen at once. It’s critical to size your heater for this moment, not the average usage throughout the day.

Here’s a quick reference table for common appliance flow rates:

Add up the GPMs for all the fixtures you might use together, and you’ve got your peak demand.

Types of Tankless Water Heaters

Whole-House vs. Point-of-Use Units

When it comes to selecting the right type of tankless water heater, one of the first decisions you’ll face is choosing between a whole-house unit or point-of-use units. Each option comes with its own pros and cons, and your choice largely depends on your home’s layout and hot water usage habits.

Whole-house tankless water heaters deliver on-demand hot water to every fixture in your home without the need for a storage tank. These units are larger and more powerful, often handling the demands of multiple fixtures operating at the same time. If you live in a house with two or more bathrooms and regularly use several appliances simultaneously — like showers, dishwashers, and washing machines — a whole-house system is typically the better option.

On the other hand, point-of-use (POU) water heaters are smaller units installed near individual fixtures. These are ideal for isolated locations far from the main water heater, such as a guest house, detached garage, or even a bathroom on another floor. POU systems offer immediate hot water with minimal wait time, which reduces water waste and increases convenience.

Here’s a quick comparison:

Combining both types can also be an effective strategy — using a main tankless heater for the majority of your home and smaller POU units for areas that need faster access to hot water.

READ MORE: How Much Does It Cost to Replace a Water Heater?

Gas vs. Electric Tankless Water Heaters

Another key decision is choosing between a gas-powered or electric tankless water heater. Both have their benefits, but they function differently and vary in terms of installation, cost, and performance.

Gas tankless water heaters use either natural gas or propane to heat water. They’re known for their high output capacity and are typically better suited for larger households. Gas models can handle higher flow rates and work well in colder climates where a greater temperature rise is needed. However, they require proper ventilation and often need a larger upfront investment for installation and gas line modification.

Electric tankless water heaters, on the other hand, are generally more compact and easier to install. They’re ideal for smaller homes, condos, or POU applications. Electric units are also highly energy-efficient since they don’t experience heat loss from venting, but they may not keep up with multiple simultaneous demands in larger homes. Additionally, homes with older electrical systems may require panel upgrades to accommodate them.

Here’s how they stack up:

Ultimately, your home’s existing utilities and your household’s hot water needs will determine which type is best for you.

Factors Influencing the Right Size Choice

Household Size and Number of Bathrooms

When sizing a tankless water heater, your household size and the number of bathrooms are critical. More people typically mean more hot water usage, especially during peak hours. Likewise, multiple bathrooms can lead to simultaneous demands, requiring a higher flow rate capacity.

• For 1–2 people in a small home or apartment, a unit with 3–5 GPM is often sufficient.
  
• For 3–4 people with 2 bathrooms, a model capable of 6–8 GPM is more appropriate.
  
• Larger households with 3+ bathrooms may need 9 GPM or more, especially if appliances like washing machines and dishwashers are running during peak usage.
  

Keep in mind that your water usage habits can also affect this decision. For example, if multiple family members shower in the morning while the dishwasher is running, you’ll need more capacity than a household that staggers their water usage.

Here’s a general guideline:

Simultaneous Usage Needs

Your peak simultaneous usage — when several water fixtures are in use at once — is another vital consideration. It’s not just about how much hot water you use in a day, but how much you use at once. This is where flow rate becomes more important than overall water volume.

Let’s say two people are showering (2.5 GPM each), and someone is doing dishes (1.5 GPM). That’s a total of 6.5 GPM. If your tankless water heater can only handle 5 GPM at your local temperature rise, someone’s shower is going to run cold.

To ensure uninterrupted service, always size your unit for the maximum number of fixtures you expect to run at the same time. Consider weekends or mornings when everyone is home — this is your “worst-case” usage scenario.

Climate and Regional Temperature Differences

Where you live plays a big role in what size unit you’ll need due to the temperature rise requirement. The colder your groundwater, the more energy is needed to heat it to your desired temperature.

For example:

• In warmer climates (e.g., Florida, Southern California), groundwater might be around 70°F, so heating it to 120°F only requires a 50°F rise. A smaller unit can easily manage that.
  
• In colder regions (e.g., Midwest, Northeast), groundwater can be 40°F or colder. To reach 120°F, your heater needs to handle an 80°F+ rise, which requires more power and a higher GPM capacity.
  

Most manufacturers provide regional maps and charts to help you determine your local groundwater temperature. This ensures that the model you choose is capable of meeting your needs year-round — not just during the summer months.

Sizing Examples and Scenarios

Small Apartment or Condo (1–2 People)

If you’re living in a small apartment or condo, especially with just one or two people, your hot water needs are likely to be modest. You’re probably looking at using one fixture at a time — maybe a shower followed by washing dishes, rather than both at once.

In these scenarios, a tankless water heater rated for 3 to 5 gallons per minute (GPM) should be more than sufficient. A typical shower uses about 2.5 GPM, and a faucet uses about 1.0–1.5 GPM. Even if you occasionally run two fixtures simultaneously, your total demand likely won’t exceed 5 GPM.

Here’s a sample usage profile:

• Shower: 2.5 GPM
  
• Sink (kitchen or bathroom): 1.5 GPM
  
• Washer or Dishwasher (not used simultaneously): 1.5 GPM
  

Total peak usage: around 4.0–5.0 GPM

Most people in this living situation opt for a compact electric tankless water heater, which is cost-effective, easy to install, and provides enough hot water without the complexity of a gas line. Plus, electric models are better for condos and apartments because they typically don’t require ventilation.

Still, be sure to calculate your temperature rise based on your local climate. Even a smaller household in a cold region may require a slightly higher-capacity unit to ensure a comfortable hot water experience.

Medium Family Home (3–4 People)

For a typical family of three to four people living in a standard home with one to two bathrooms, the hot water demand increases significantly. You’re likely to experience multiple fixtures running simultaneously, especially during mornings or evenings.

In this case, a tankless water heater with a 6 to 8 GPM flow rate is generally ideal. Let’s take a real-world example of a peak usage scenario:

• Two showers running: 2.5 GPM x 2 = 5.0 GPM
  
• Washing machine in use: 2.0 GPM
  
• Bathroom sink or kitchen faucet: 1.5 GPM
  

Total peak usage: 8.5 GPM

You’ll want to round up slightly for safety and ensure the unit can handle your temperature rise. A gas-powered unit is often the best option here due to the higher heating capacity and cost efficiency in terms of fuel usage. Electric models are still viable but may require significant upgrades to your electrical panel, especially if you’re aiming for 8 GPM or more.

When sizing, consider habits like back-to-back showers, running laundry while cooking, or using multiple bathrooms. Always plan for peak demand — not average use — to ensure you won’t run short on hot water when it matters most.

READ MORE: How to Flush a Tankless Water Heater

Large Household or Multi-Bathroom Homes

If you have a larger home with 4 or more bedrooms, 3+ bathrooms, and a bustling household of 5 or more people, your hot water needs are considerable. In these cases, undersizing your tankless water heater can quickly lead to frustration — cold showers, slow dishwashing cycles, and general inefficiency.

For large households, you’ll need a system with a minimum of 9 GPM, and in some cases, up to 11 or more GPM. Let’s run through a possible usage scenario during peak hours:

• Three showers in use: 2.5 GPM x 3 = 7.5 GPM
  
• Kitchen faucet running: 1.5 GPM
  
• Dishwasher running: 1.5 GPM
  
• Laundry machine in use: 2.5 GPM
  

Total peak usage: 13 GPM

At this point, it may be worth considering installing two smaller tankless units in parallel or a combination of a whole-house unit and point-of-use heaters. This not only increases your hot water capacity but can improve redundancy and efficiency.

Also, ensure your gas supply or electrical system can support a larger unit. Installation will likely require professional help to ensure proper venting, fuel supply, and water line management.

Remember, the bigger the family and the more bathrooms in use at once, the more essential it is to have a properly sized system to avoid cold water cutoffs.

Mistakes to Avoid When Sizing a Tankless Water Heater

Underestimating Flow Rate

One of the most common mistakes homeowners make is underestimating how much hot water they’ll actually use at one time. Maybe you think you’ll never run two showers at once — until you do. Or you don’t count the kitchen faucet running while the dishwasher is on.

Even something as simple as using low-flow showerheads can mislead you into thinking your GPM requirement is lower than it really is. But appliances don’t always run at full flow rate, and peak times will surprise you.

Play it safe: add up all possible simultaneous demands and round up. It’s better to slightly oversize your unit than end up shivering mid-shower.

Ignoring Seasonal Water Temperature Variations

Another major oversight? Not accounting for seasonal changes in groundwater temperature. Even in moderate climates, groundwater can drop 10–20°F during the winter months — and that drastically increases your required temperature rise.

A heater that works fine in summer might struggle in winter, leading to lukewarm water when you need it most. Always size your unit based on worst-case conditions, not average or ideal ones.

For example:

• Groundwater in July: 65°F → Needed rise: 55°F
  
• Groundwater in January: 45°F → Needed rise: 75°F
  

That’s a significant difference and can push your system past its limits.

FAQs

Q. Can a tankless water heater be too big?

Yes, oversizing a unit can lead to inefficiencies. While it won’t harm your system directly, it may cycle on and off too frequently, which can reduce its lifespan and waste energy. Choose a model that matches your actual demand for optimal performance.

Q. What happens if the unit is too small?

An undersized tankless water heater simply cannot meet the warm water needs of your household. This can result in lukewarm or cold water, especially when there is high usage. It overloads the system, leading to early wear and maintenance issues more frequently.

Q. Can I install multiple smaller units instead of one large one?

Absolutely. Installing multiple point-of-use or smaller units in parallel is a great strategy for larger homes or those with unique layouts. It can increase capacity, improve energy efficiency, and reduce wait times for hot water.

Q. How long does a tankless water heater last?

With proper maintenance, a tankless water heater can last 20 years or more, significantly longer than a traditional tank model. Regular descaling and filter cleaning are essential for maximizing lifespan.

Q. Are rebates available for energy-efficient models?

Yes. Many ENERGY STAR-rated tankless heaters qualify for local, state, and federal rebates or tax credits. Always check with your local utility provider and government incentive programs before purchasing.

Conclusion

Choosing the right size tankless water heater is not a shot in the dark. If you understand the application of your home peak water, the needs of rising temperature and the difference between gas and electric models, you can choose a unit that meets your needs with certainty.

Whether you live alone in a cozy apartment or have a bustling household with multiple bathrooms, the key is to size for peak demand — not just average use. Use tools like manufacturer charts and online calculators, consider your climate, and always factor in future growth or renovations.

The correct size not only provides you with continuous hot water, but also maximizes energy efficiency, reduces wear and tears, and saves you ultimately on the lifespan of the unit. So, spend a little time to do some calculations of your true requirements in consultation with a professional if necessary, and enjoy your boundless hot water on your own terms.