What Size Water Heater Do You Really Need?

Choosing the right water heater size can feel be a difficult task. Too small, and you’ll be left with cold showers when multiple taps are running. Too large, and you’ll waste money heating water you don’t use. In this guide, we’ll break down the factors that determine the ideal size for both tank and tankless water heaters, plus a few tips to help you avoid common sizing mistakes.

Why Water Heater Size Matters

Your water heater has one job: deliver hot water when you need it. But an undersized unit will struggle during peak demand (think morning showers, laundry, and dishwashing all at once). On the flip side, an oversized heater continuously heats more water than necessary, which drives up energy bills.

Choosing the right size ensures:

• Consistent hot water for everyone in your home.

• Lower energy costs by reducing unnecessary heating.

• A longer lifespan because the system isn’t overworked.

Types of Water Heaters and How They’re Sized

Before diving into numbers, it’s important to know how different water heaters are measured:

1. Tank Storage Water Heaters

These units store hot water in a tank, ready to go when you turn on the tap. The size is measured in gallons of storage capacity. Key metrics include:

• Tank capacity (e.g., 30, 50, 75 gallons).

• First-hour rating (FHR): The amount of hot water a unit can deliver in one hour, starting with a full tank.

• Recovery rate: How quickly the heater can reheat water after use.
  

2. Tankless (On-Demand) Water Heaters

These don’t store hot water. Instead, they heat water as it flows through the system. Their size is measured by flow rate (in gallons per minute, or GPM) and the temperature rise they can achieve.

How to Calculate Your Water Heater Size

The perfect water heater size depends on your household’s peak hot water demand. It’s the maximum amount of hot water you’ll need at one time.

Step 1: Estimate Peak Hour Demand (For Tank Models)

Add up all the hot water you use during the busiest hour of your day. Here’s a quick guide:

• Shower: 20 gallons per shower.

• Bath: 30-40 gallons (depends on tub size).

• Dishwasher: 6-7 gallons.

• Clothes washer: 20-25 gallons (hot cycle).

• Handwashing, cleaning, etc.: 2-3 gallons per use.

Let’s suppose: If two people shower (40 gallons) while the dishwasher (7 gallons) and washing machine (20 gallons) are running, your peak demand is ~67 gallons. You’ll need a tank with an FHR of at least 67 gallons.

Step 2: Calculate Flow Rate (For Tankless Models)

Tankless systems require calculating total GPM. Add the flow rate of fixtures you might use simultaneously:

• Standard showerhead: 2.5 GPM.

• Bathroom faucet: 0.75 GPM.

• Kitchen sink: 1-2 GPM.

• Dishwasher: 1.5 GPM.

If you often run a shower (2.5 GPM) and kitchen sink (1.5 GPM) together, you’ll need at least a 4 GPM tankless unit.

Recommended Water Heater Sizes by Household

Here’s a general guideline for tank water heaters:

For tankless units, most small households do well with 4–6 GPM models, while larger families may require 8+ GPM or multiple units.

Other Factors to Consider

1. Recovery Rate

Even with the correct tank size, a low recovery rate can leave you waiting for hot water to replenish. If your household uses a lot of hot water in quick succession, choose a unit with a faster recovery rate.

2. Space & Installation

• Tank models require floor space (often 50-70 inches tall and 20-30 inches wide).

• Tankless units are compact and can be mounted on walls, saving space.

3. Climate & Temperature Rise

Colder incoming water (common in northern climates) means your water heater must work harder, especially for tankless units. Factor this into your sizing calculations.

4. Energy Efficiency

A perfectly sized unit is more efficient than a larger one. Look for Energy Star-rated water heaters to cut down utility bills.

Signs Your Current Water Heater Is the Wrong Size

• Frequent cold showers even after short use.

• High energy bills due to an oversized tank.

• Inconsistent water temperature when multiple fixtures run.
  

If your system is over 10-12 years old, struggles with recovery, or is the wrong size, consider upgrading. Modern units are not only more efficient but can also be sized more precisely based on your lifestyle.

At Voomi Supply, we offer a wide selection of top-quality water heaters designed to suit homes of all sizes. From small tank units to high-capacity tankless systems, you’ll find reliable options backed by expert knowledge and fast delivery.