Tankless water heaters are gaining popularity for their energy efficiency and ability to provide hot water on demand. But choosing the right size is essential to ensure you never run out of hot water during a shower or while doing dishes.
A unit that’s too small will leave you with cold water, while one that’s too large could waste energy and money. In this guide, we’ll walk you through everything you need to know about sizing your tankless water heater to meet your home’s hot water demands.
Why Size Matters in Tankless Water Heaters?
Tankless water heaters don’t store hot water; they heat it instantly when needed. That means the size of your unit determines how many appliances or fixtures can get hot water simultaneously.
If the unit is too small, it won’t meet your household’s peak demand. On the other hand, an oversized unit may cost more upfront and consume unnecessary energy. Proper sizing ensures energy efficiency, consistent hot water, and longer appliance lifespan.
Key Factors That Determine the Right Size
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Flow Rate (Gallons per Minute – GPM)
Flow rate is the amount of hot water your household needs during peak usage, measured in gallons per minute (GPM). Here’s a rough breakdown of typical GPM usage:
- Shower: 2.5 GPM
- Bathroom sink: 0.5 GPM
- Kitchen faucet: 1.5 GPM
- Dishwasher: 1.5 GPM
- Washing machine: 2.0 GPM
If your household uses a shower and washing machine at the same time, you’ll need at least 4.5 GPM.
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Temperature Rise
This is the difference between the incoming groundwater temperature and your desired output temperature (usually 120°F). In San Antonio, groundwater temperature averages about 60°F. That means you’ll need a temperature rise of around 60°F.
Not all heaters can provide high GPM at a high temperature rise. That’s why this factor is critical in sizing.
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Household Size
| Household Size | Recommended Flow Rate |
| 1–2 people | 6-8 GPM |
| 2–4 people | 8-10 GPM |
| 4+ people | 10+ GPM |
If you’re unsure about the right flow rate for your hot water taps, consider your household size to estimate what you need from a tankless water heater. For a small household of two, a unit with a flow rate of 6 to 8 GPM is typically sufficient. Larger families of four may need a flow rate of 8 to 10 GPM. Don’t forget to factor in the temperature rise—the difference between incoming water temperature and your desired hot water temperature—when choosing the right size for your home.
Estimating Your Household’s Hot Water Demand
Calculate your household’s peak hot water demand by listing which fixtures might be used at the same time. Here’s a quick example:
- 1 shower (2.5 GPM)
- 1 kitchen faucet (1.5 GPM)
- 1 dishwasher (1.5 GPM)
Total: 5.5 GPM
This means you need a tankless water heater that can deliver at least 5.5 GPM at a 60°F temperature rise.
Read, How Do Tankless Water Heaters Work?
Recommended Tankless Water Heater Sizes by Home Size
Here’s a quick guide to choosing a water heater unit based on home size and number of occupants:
| Home Size | Typical GPM Need | Suggested Unit Capacity |
| 1–2 people (1 bath) | 3–5 GPM | 6–8 GPM unit |
| 3–4 people (2 baths) | 5–7 GPM | 8–10 GPM unit |
| 5+ people (3+ baths) | 7–10+ GPM | 10–12+ GPM unit |
These estimates help you avoid guesswork and plan your installation with confidence.
Gas vs. Electric Tankless Water Heaters – Sizing Differences
Gas-powered tankless water heaters can produce a higher flow rate and handle larger temperature rises compared to electric units. If you have a high hot water demand or live in a cooler climate, gas units may be more suitable.
Electric units are better for smaller homes or single-use applications like an additional bathroom or wet bar.
Also read, Electric vs. Gas Tankless Water Heater: Which One is Right for You?
Mistakes to Avoid When Choosing a Tankless Water Heater
- Underestimating flow rate: Don’t just guess how many fixtures will be used simultaneously.
- Ignoring temperature rise: Your region’s groundwater temperature affects sizing.
- Overlooking fuel type: Know the difference in capacity between gas and electric models.
- Skipping professional consultation: Incorrect installation or poor sizing can lead to long-term issues.
When to Call a Professional
While online calculators and guides help, nothing beats the insight of an experienced plumber. At PlumbSmart, we specialize in tankless water heater sizing, installation, and maintenance in San Antonio.
Our licensed technicians assess your home’s water usage, climate, and fuel availability to recommend the perfect unit. We ensure a smooth installation and consistent hot water performance.
Wrapping Up…
Choosing the right size tankless water heater ensures comfort, energy savings, and a smooth household routine. By calculating your GPM needs and temperature rise, and considering the size of your household, you can confidently select a unit that meets your demands.
Ready to switch to a tankless water heater or need help determining the right size? Contact PlumbSmart today for expert advice and installation services in San Antonio.
