SEER stands for Seasonal Energy Efficiency Ratio. This is the most commonly used metric to denote how energy-efficient HVAC units like air conditioners, heat pumps, and furnaces are. We are going to look at how to calculate the SEER rating.
The SEER rating is not as easily calculated as the EER rating (Energy Efficiency Ratio). EER is the basic metric that is calculated by dividing maximal output (BTUs) by maximal wattage (W) when an AC unit, for example, is running at 100% output. This is the EER formula:
Of course, we don’t run an air conditioner at 100% output all the time. We sometimes run it at 80%, 60%, 50%, and so on, depending on our cooling needs. To encapsulate the real-life usage of air conditioners and their energy efficiency, the SEER rating was introduced.
When we calculate the Seasonal Energy Efficiency Ratio rating, we don’t only take into account the energy efficiency at 100% cooling output. In fact, the SEER rating presumes that you run an air conditioner at 100% only for 1% of the time.
To capture the realistic operation of an AC unit, the SEER rating includes running a unit at parts loads (namely, at 25%, 50%, and 75% output, in addition to 100% output).
Here is what the equation for calculating the SEER rating looks like:
We are going to look at how we use this equation to calculate SEER rating from partial loads (namely, EER100%, EER75%, EER50%, EER25%):
SEER Rating Calculation
To calculate the SEER rating, we have to use the above weighted partial load equation. As we can see, the SEER formula presumes that we:
- Run an AC unit at 100% load for 1% of the time (EER100%).
- Run an AC unit at 75% load for 42% of the time (EER75%).
- Run an AC unit at 50% load for 45% of the time (EER50%).
- Run an AC unit at 25% load for 12% of the time (EER25%).
These partial loads make for a much more practical energy efficiency rating (pure EER rating is theoretical). This simply means that we have to measure partial EER ratings at 25%, 50%, and 75% to be able to calculate the SEER rating.
Once we get the partial EER ratings, we can insert them into the SEER rating equation, and calculate the SEER rating. To illustrate best how to do that, let’s look at an example:
How To Calculate SEER Rating Of A 2-Ton Mini Split? (Example)
Let’s say that we have 2-ton mini-split heat pumps. This unit can generate up to 24,000 BTU of cooling output. How do we calculate the SEER rating of such a unit?
First off, we have to measure the outputs at 25%, 50%, and 75% load. We already know that the output at 100% load is 24,000 BTU (since this is a 2-ton unit). Let’s presume that we measured the outputs found in the following table, and, based on those, calculated the partial EER ratings:
|HVAC Load:||Cooling Output:||EER Rating:|
|100%||24,000 BTU||12.0 (EER100%)|
|75%||21,300 BTU||14.2 (EER75%)|
|50%||16,700 BTU||16.7 (EER50%)|
|25%||10,950 BTU||14.6 (EER25%)|
Now that we have these partial EER ratings, we can insert them into SEER equation:
SEER = (1 × EER100% + 42 × EER75% + 45 × EER50% + 12 × EER25%)/100
Here is how the SEER rating is calculated:
SEER Rating = (1 × 12.0 + 42 × 14.2 + 45 × 16.7 + 12 × 14.6)/100 = 15.35
As you can see, this 2-ton mini split heat pump has a 15.35 SEER rating. It’s very interesting to see that the EER vs SEER rating relationship you can read more about here stands quite well. Namely, this units has:
- EER rating of 12.
- SEER rating of 15.35.
By calculating partial loads, you can calculate SEER rating yourself. With most HVAC units, you already get the calculated SEER rating on the specification sheets.
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