What Size Tankless Water Heater Do I Need?

Something that has been on the rise especially as people become more conscious of the environment is the shift from a traditional water heater to a tankless water heater. And with comes the questions what size tankless water heater do I need. If you’ve ever run out of hot water during a shower, especially while the washing machine is running, you know how frustrating it is. Choosing the right size tankless water heater isn’t just a matter of comfort; it affects your energy usage, hot water supply, and even long-term energy costs.

Unlike traditional tank water heaters, tankless units provide hot water on demand. However to enjoy those benefits, you need to choose the correct size. This guide will help you understand sizing, features, costs, and everything else you need to make an informed decision let’s break it down together.

Understanding the Basics of a Tankless Water Heater

First thing first is actually recognize what a tankless water heater heats is. One of the core benefits of a tankless water heater is that water only when it’s needed. Instead of storing gallons of water, it rapidly warms the flow as it moves through the unit whether it’s a natural gas, electric, or tankless gas water heater.

The two most important sizing factors are:

• Maximum flow rate (GPM) — how much water your household uses at peak times.
• Required temperature rise — the difference between your incoming water temperature (often 40°F–60°F in northern states) and your desired temperature (usually around 120°F).

If multiple fixtures—like a shower head and washing machine—are used at the same time, your tankless system must be powerful enough to meet the total hot water demand. Failing to match your needs with the right size system means lukewarm or even cold water at peak times.

Sizing Your Electric Tankless Hot Water System

When choosing an electric tankless hot water unit, calculate your required flow rate. Here’s a breakdown of average usage:

• Shower head: 2.0–2.5 GPM
• Washing machine: 1.5–2.0 GPM
• Kitchen faucet: 1.0–1.5 GPM

A larger household or a home with a high number of fixtures might need a larger unit with higher capacity. Also consider the temperature of the water entering your home called the inlet water temperature or incoming cold water temperature. Colder climates require more heating power, which increases the required temperature rise.

By correctly calculating your water flow rate, desired hot water temperature, and household size, you’ll find the right tankless water heater size for reliable performance.

How Much Does a Tankless Heater Cost to Install?

Now to break down the tankless heater cost and run depends on the brand, fuel type, and your home’s infrastructure. Average breakdown:

• Electric models: $500–$1,500
• Tankless gas water heaters: $1,000–$2,500
• Installation: $1,000–$2,000
• Permits & materials: Varies by region

While upfront costs are higher than for traditional water heaters, you’ll see energy savings over time. Heating water only when needed significantly reduces energy costs, especially in homes with high hot water usage.

Rinnai vs. Rheem Tankless Water Heaters

Now to some comparison for a better understanding here are two leading brands in the United States as of 2025 are:

• Rinnai tankless water heater: Known for high British Thermal Units, reliable heating, and excellent lifespan. Best for large households or high demand.

Learn more about Rinnai tankless water heaters and what sets them apart:

• Rheem tankless water heater: Offers affordable and efficient electric models, great for moderate homes and homeowners seeking balance between price and performance.

Read more about Rheem tankless water heaters on their official website:

When shopping, consider the lifespan, warranty, energy savings, and user reviews. For small spaces, point-of-use models may be a good idea for serving a single bathroom or kitchen. Both brands offer models that can meet your hot water needs and are compatible with air conditioning and heating control systems. And remember — it’s always a good idea to ask you home expert for personalized advice, since your setup may require specific recommendations.

Contact us for a more personalized assessment of your home.

Tankless Hot Water Heater Installation

What should you expect when it comes to Tankless hot water heater installation takes 4–8 hours, depending on system type and location. Key factors include:

• Existing plumbing/electrical access
• Mounting space (wall vs. closet)
• Venting (for gas models)
• Permit requirements

A licensed pro will ensure proper water pressure and desired temperature settings. Be prepared for minor home adjustments, especially if switching from a traditional tank water heater.

Tankless Water Heater Cost

Hidden costs can include:

• Electrical upgrades (for electric tankless water heaters)
• Gas line extensions (for tankless gas models)
• Water softeners or filters
• Permit or inspection fees
• Labor for additional plumbing work

These add-ons impact your tankless water heater cost, so always request a detailed estimate that includes the amount of water needed at peak, climate-related adjustments (for a colder climate), and the size of your home.

Right Size Tankless Water Heater for Your Home

Here’s a quick checklist to find your right tankless water heater size:

• Count your number of people and number of bathrooms

• Estimate how much water you’ll need at peak times

• Calculate required flow rate and temperature rise

• Decide between electric vs. tankless gas based on fuel availability

• Choose a reputable brand and installer

Still unsure? Don’t worry — our team can help you find the right size model for your home’s hot water supply. Whether you’re replacing an old unit or planning a new water heater, getting the size right means better comfort, lower energy bills, and reliable performance year-round.

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