The Tankless Sizing Guide: How to Calculate GPM vs. Temperature Rise

The single most expensive mistake you can make when buying an electric tankless water heater is choosing the wrong size. If it’s too small, you’ll suffer from lukewarm showers. If it’s too large, you’ve overpaid for power you don’t need.

Unlike a tank heater (where size is just “gallons”), a tankless heater’s size is determined by two factors: Gallons Per Minute (GPM) and Temperature Rise (ΔT). Here is the simple, 3-step guide to getting it right.

Step 1: Calculate Your Peak GPM Demand

First, figure out the maximum amount of hot water you might use at one time. You don’t need to count every tap, just the ones that might run simultaneously.

• Modern Showerhead: 2.0 - 2.5 GPM
• Kitchen/Bathroom Sink: 1.0 - 1.5 GPM
• Dishwasher: 1.0 GPM
• Washing Machine: 1.5 - 2.0 GPM

Example: Your “worst-case scenario” is one shower (2.5 GPM) and a kitchen sink (1.0 GPM) running at the same time. * Your Peak GPM Demand = 3.5 GPM

Step 2: Calculate Your Required Temperature Rise (ΔT)

This is the most critical and most overlooked step. “Temperature Rise” (or ΔT) is the amount of work the heater has to do.

Formula: (Desired Temp) - (Inlet Water Temp) = Your ΔT

• Desired Temp: Most people set their shower to 105°F - 110°F. Let’s use 105°F.
• Inlet Water Temp: This is the temperature of the groundwater coming into your house. This dramatically changes by season and location. (Tip: Search online for a “groundwater temperature map” for your state).

Climate Scenarios: * Scenario A (Warm Climate - e.g., Florida): Your inlet water is 70°F.
* Calculation: 105°F - 70°F = 35°F ΔT (a low workload) * Scenario B (Cold Climate - e.g., Minnesota): Your inlet water is 40°F.
* Calculation: 105°F - 40°F = 65°F ΔT (a massive workload)

Step 3: Read the Manufacturer’s Spec Sheet

Now you can shop. Manufacturers rate their heaters by how many GPM they can produce at a specific ΔT.

Let’s use the thermomate ET240 (24kW) as our example. Its spec sheet says: * 4.68 GPM @ 35°F Rise

Let’s analyze this: * In Florida (35°F ΔT): You need 3.5 GPM. This heater provides 4.68 GPM. It’s a perfect fit and can easily handle your shower and sink. * In Minnesota (65°F ΔT): Your workload (65°F) is almost double the 35°F rating. This 24kW heater’s flow rate will be cut roughly in half. It will only produce about 2.5 GPM.

This means in Minnesota, the same 24kW heater can only support one shower at a time. This isn’t a flaw; it’s physics. Sizing correctly means matching the heater’s capability to your climate and your demand.