Energy Efficiency Ratio Calculator: How to Use It

This calculator works out the Energy Efficiency Ratio (EER) of an air conditioner or heat pump: you enter the cooling capacity in British thermal units per hour (BTU/h) and the electrical power input in watts, and it returns the EER value. EER is a simple number that tells you how much cooling you get for each watt of electricity, so a higher EER means a more efficient unit. The calculator also shows the result in kilowatts of cooling per kilowatt of input, which is the same ratio expressed in SI units.

Formula

The Energy Efficiency Ratio is defined as the cooling output (in BTU per hour) divided by the electrical power input (in watts). The formula is:

EER = Cooling Capacity (BTU/h) / Power Input (W)

Where:

• Cooling Capacity is the amount of heat the unit can remove from a room in one hour, measured in British Thermal Units per hour (BTU/h). In metric, this is often given in kilowatts (kW) of cooling.
• Power Input is the electrical power the unit draws from the wall, measured in watts (W).

The result is a dimensionless number, but it is commonly written as BTU/(W·h). For example, an EER of 10 means that for every watt of electricity, the unit removes 10 BTU of heat per hour.

To convert to the Coefficient of Performance (COP), which is the ratio of cooling output to power input in the same units, divide the BTU/h by 3.412 (since 1 W = 3.412 BTU/h). The calculator does this for you in the second output.

Worked Example

Using the default values from the calculator: a window air conditioner with a cooling capacity of 12,000 BTU/h and a power input of 1,200 W.

1. Write down the formula: EER = Cooling Capacity / Power Input.
2. Plug in the numbers: EER = 12,000 BTU/h / 1,200 W.
3. Divide: 12,000 / 1,200 = 10.
4. So the EER is 10.0.

The calculator also shows the approximate COP: (12,000 / 3.412) / 1,200 = 3,517 / 1,200 ≈ 2.93. This means the unit produces about 2.93 units of cooling for each unit of electricity, which is typical for a small air conditioner.

When to Use EER

EER is most useful when comparing the efficiency of air conditioners and heat pumps that operate under the same conditions. The U.S. Department of Energy uses EER as a standard rating for window units and through-the-wall units. For central systems, you will often see SEER (Seasonal Energy Efficiency Ratio) instead, which is a seasonal average. EER is a snapshot at a specific outdoor temperature (usually 95°F / 35°C), while SEER is an average over a cooling season.

Use EER when:

• You are comparing two window or portable air conditioners.
• You want to estimate the operating cost of a unit at peak load.
• You are checking whether a unit meets a minimum efficiency standard (many regions require a minimum EER for new units).

For a quick estimate of yearly energy use, you would need the unit’s SEER rating and the number of cooling hours in your region. EER alone does not tell you how much the unit will cost to run over a season because it does not account for part-load operation.

How to Read the Result

A higher EER means better efficiency. For example, an EER of 12 is better than an EER of 10. The difference is not linear in cost: a unit with EER 12 uses 20% less electricity than one with EER 10 for the same cooling output (1/10 – 1/12 = 0.0167, or about 17% less).

Typical ranges:

• Window units: EER from 8 to 12.
• Portable units: EER from 6 to 10 (they tend to be less efficient because they also exhaust heat through a hose).
• Central air conditioners: EER from 8 to 14, with SEER ratings from 14 to 25.

When you compare two units, make sure they have the same cooling capacity. A 10,000 BTU unit with EER 10 uses 1,000 W, while a 12,000 BTU unit with EER 10 uses 1,200 W. The bigger unit costs more to run, but it cools a larger room. The EER only tells you how efficiently each converts electricity into cooling, not which one is cheaper to operate for your space.

Common Mistakes and Misconceptions

One frequent error is mixing up units. If the cooling capacity is given in kilowatts (kW) instead of BTU/h, you must convert before using the formula. 1 kW = 3,412 BTU/h. For example, a unit with 3.5 kW cooling and 1.2 kW input has an EER of (3.5 × 3,412) / 1,200 = 11,942 / 1,200 ≈ 9.95.

Another mistake is confusing EER with SEER. SEER is always higher than EER for the same unit because it averages over a season with milder temperatures, where the unit runs more efficiently. Do not compare an EER number to a SEER number directly.

Also, the power input is the electrical power the unit draws, not the cooling power. Some people mistakenly use the “cooling capacity” in watts (which is the thermal output) as the input. That would give an EER of 1, which is meaningless.

Finally, the EER rating is measured at a specific outdoor temperature (95°F / 35°C) and indoor temperature (80°F / 27°C). Real-world efficiency varies with temperature. On a hotter day, the unit works harder and the EER drops; on a cooler day, it rises. The rated EER is a standardized comparison, not a guarantee of your actual efficiency.

Limitations of the EER Formula

The EER formula is a simple ratio, but it has limits:

• It does not account for part-load operation. Most air conditioners cycle on and off, and they are most efficient when running at full capacity. SEER is a better measure for seasonal energy use.
• It does not include the energy used by fans or controls if those are not part of the rated power input. The rated power input usually includes the compressor and the indoor/outdoor fans, but check the spec sheet.
• It does not consider the climate. In a humid area, a unit may need to run longer to remove moisture, so the same EER may result in higher energy use.
• It is not a measure of cooling speed or comfort. A unit with a high EER may still have a weak fan or poor temperature control.

For a more complete picture, look at the SEER rating, the Energy Star label (if applicable), and the unit’s power consumption at different fan speeds. Also consider the size of the room: an oversized unit will short-cycle and waste energy, while an undersized unit will run constantly.

How to Choose an Efficient Unit

When shopping for an air conditioner, follow these steps:

1. Determine the cooling capacity you need based on room size, insulation, windows, and sun exposure. A common rule of thumb is 20 BTU per square foot of living space, but use a load calculation for accuracy.
2. Compare units with similar capacity. Look at the EER or SEER rating on the EnergyGuide label.
3. Calculate the annual operating cost: (Capacity in BTU/h / EER) × hours of use per year × electricity rate per kWh / 1000. For example, a 12,000 BTU unit with EER 10 running 1,000 hours at $0.15/kWh costs (12,000/10) × 1,000 × 0.15 / 1000 = 1,200 × 0.15 = $180 per year.
4. Consider the installation type: window units are usually less efficient than central systems, but they also cool a smaller space.
5. Check for variable-speed compressors, which improve part-load efficiency and SEER, even if the EER is similar.

Remember that a higher EER often comes with a higher purchase price. Calculate the payback period: if a unit with EER 12 costs $100 more than one with EER 10, and it saves $30 per year in electricity, the payback is about 3.3 years.

Frequently Asked Questions

What is a good EER rating?
For window units, an EER of 10 or higher is considered good. Portable units often have lower EERs, around 8 to 9, because they lose efficiency due to the exhaust hose. For central air conditioners, look for EER above 11 or SEER above 15.

How do I convert EER to SEER?
There is no direct conversion because SEER is a seasonal average that depends on climate and how often the unit cycles. In general, SEER is about 1.2 to 1.5 times the EER for the same unit, but this is only a rough estimate. Use the official ratings from the manufacturer.

Can I use EER for a heat pump in heating mode?
No. For heating, the efficiency is measured by the Heating Seasonal Performance Factor (HSPF) or the Coefficient of Performance (COP). EER only applies to cooling. Some heat pumps have a separate EER for cooling and a COP for heating.

Why is my air conditioner’s EER lower than the label?
The label EER is measured under standard conditions (95°F outdoor, 80°F indoor). If it is hotter outside, the compressor works harder and draws more power, lowering the EER. Also, a dirty filter or low refrigerant can reduce efficiency. Clean the filter and have the unit serviced if you notice a drop.

Does a higher EER always mean lower electricity bills?
Not necessarily. If the unit is too large for the room, it will short-cycle and waste energy. Also, a higher EER unit may have a higher upfront cost. But for the same cooling capacity and usage hours, a higher EER will use less electricity.

What is the difference between EER and COP?
EER uses mixed units (BTU per hour per watt), while COP uses the same units for both output and input (both in watts or both in BTU/h). To convert EER to COP, divide by 3.412. A COP of 3 is roughly equivalent to an EER of 10.2.

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in c | Cooling capacity | BTU/h | 12000 | 0
in p | Power input | W | 1200 | 0
out eer | EER | BTU/(W·h) | c / p | 1
out cop | COP (approx.) | - | (c / 3.412) / p | 2