Measure R Value of Water Heater

If you would rather measure, than be given, the R value of your system, you will need to measure temperature. This method of using standby heat loss to arrive at the empirical R value can be applied to many different systems, but here a tank type water heater is measured.

Measuring resistance to heat transfer (R-value in the US, RSi in Si units) removes uncertainty that arises from theoretical calculations based on lab produced standard conditions, which may have little relation to your local conditions. For example, how much standby energy is really saved by the added insulation of that hot water heater blanket? Following is a method for end users to empirically determine specific R values. This method measures the tank system as a whole, including all nuances such as losses in connecting pipes and temperature pressure valves for example.

Tools required include:

  1. thermometer
  2. clock
  3. tape measure

A method to record readings and the dimensions of the tank to determine the system’s surface area and volume. Before we get to an empirical R-value, we have to determine the actual heat energy transferred from the system (water tank) to the surroundings (air).

Measure Standby Heat Loss Over Time

The concept here is to isolate our test system from energy inputs; so turn off the gas, or the breaker, or the sun.

Surroundings must be large enough that heat transferred from the tank does not measurably raise it’s surrounding’s temperature.

Wireless thermometers with a probe are useful to get next to the tank under the insulation, but the important thing is that you measure the system’s or tank’s temperature and not that of the surroundings. Also keep in mind, it is the change in temperature we want, so measurement technique must be consistent. Measure the temperature the same way each time.

Record the temperature of the system and it’s surroundings after disabling heating energy inputs. As the system cools record the two temperatures every hour for a few hours. Recording exactly on the hour is not critical, but your findings will be more precise. Likewise, a ten hour test will be more precise than a three hour. Overnight testing can be appropriate.

Calculate Standby Heat Loss

One equation to calculate standby heat loss or thermal transfer is:

Heat(power) = V * C * ΔT / t
Where V is volume, C is volumetric heat capacity, ΔT is temperature change, t is time

US units will have to be converted to Si (metric) units of meters, liters, and degrees Celcius. Numbers you provide:

  1. volume of the tank system in liters, or its diameter and height in meters
  2. minutes elapsed during the temp drop test
  3. °C of system temperature change during the test

diameter meters
height meters

Volume Liters
Time minutes
Temp. Drop °Celcius

Energy (kJ) kilojoules
Heat Loss (power) Watts
Heat Loss (power) BTU/h

Calculate R Value (RSi)

Now that we have measured the energy loss, we can calculate the coresponding R value using the following:

R = A (ΔT) / H
Where A is surface area, ΔT is temperature differential across an insulating layer, and H is heat power transferred

Numbers you provide:

  1. surface area of the tank system in square meters, or its diameter and height in meters
  2. Average temperature difference between ithe nside of the tank system and it’s surroundings in degrees Celcius
  3. Heat power transferred from the system to its surroundings in Watts


diameter meters
height meters

Surface Area meters2
ΔTemperature avg. Δ °C

Heat Loss (power) Watts (J/s)

RSi °C·m2 / W
R-value (US) °F·ft2·h / BTU