COP To EER Converter
Convert heat pump or air-conditioning efficiency between COP and EER, then estimate input power, output capacity and electricity cost from the same rating.
Efficiency Rating
Converted Efficiency
COP and EER are only comparable when they refer to the same operating condition. Do not compare ratings measured at different indoor or outdoor temperatures as if they were identical tests.
What COP And EER Mean
COP, or coefficient of performance, is a ratio of useful heating or cooling output to electrical input. A COP of 3.50 means the equipment delivers 3.50 kWh of heat or cooling for each 1 kWh of electricity used under the rating test. EER, or energy efficiency ratio, expresses cooling efficiency as Btu/h of cooling per watt of electrical input. It is common on air-conditioning and heat pump specification sheets, especially where imperial units are used.
The two ratings are linked by the conversion between watts and Btu per hour. One watt is equal to 3.412141633 Btu/h. That means an idealised COP value can be multiplied by 3.412141633 to give the matching EER value for the same test condition. The calculator also works backwards by dividing EER by the same factor.
Formula
EER = COP x 3.412141633COP = EER / 3.412141633Input power in kW = useful output kW / COPRun cost = input kW x hours x electricity price per kWhThe formulas treat COP as a pure ratio and EER as Btu/h per watt. They do not add seasonal weighting, fan power rules, part-load performance or defrost losses. Use the same rating condition when comparing two products.
When The Conversion Is Useful
This converter is helpful when a UK installer, building services report or heat pump datasheet uses COP, but an air-conditioning document uses EER. It is also useful when checking whether an old imperial specification broadly matches a newer metric one. The cost estimate can show the link between efficiency and electricity use: for the same 5 kW output, COP 4 uses less input power than COP 2.5.
For purchasing decisions, do not stop at the converted number. Look at the rated capacity, outdoor temperature condition, flow temperature for heating, indoor air condition for cooling, sound levels, refrigerant, controls, service support and seasonal performance. A high COP at a mild test point may not reflect cold-weather heating performance.
Quick COP And EER Table
| COP | EER | Meaning at the same test point |
|---|---|---|
| 2.00 | 6.82 | Useful output is twice the electrical input. |
| 2.50 | 8.53 | Typical of a lower-efficiency or difficult operating point. |
| 3.00 | 10.24 | Often used as a simple benchmark in quick comparisons. |
| 3.50 | 11.94 | A common mid-range example for heat pump discussions. |
| 4.00 | 13.65 | Higher output per unit of electricity at that test condition. |
| 5.00 | 17.06 | Strong test-point efficiency, not automatically seasonal performance. |
Worked Examples
COP 3.5 Datasheet
A COP of 3.5 converts to EER 11.94. If the equipment delivers 5 kW, the estimated electrical input is 1.43 kW at that operating point.
EER 12 Air Conditioner
An EER of 12 converts to COP 3.52. That does not mean the product will achieve COP 3.52 in every room, temperature and fan setting.
Cost Check
A 5 kW output at COP 4 needs about 1.25 kW input. Four hours at 28 p/kWh costs about GBP 1.40 in electricity.
Ratings That Should Not Be Mixed
- SEER and SCOP are seasonal ratings, not simple COP or EER values.
- HSPF uses a different seasonal method and should not be converted by this formula.
- Heating COP at a low outdoor temperature should not be compared with cooling EER at a mild indoor test.
- Whole-system performance can include pumps, fans, controls and distribution losses.
- Manufacturer “maximum” values can differ from rated values used for design.
When reading a datasheet, look for the small print near the rating table. It may state indoor dry-bulb temperature, outdoor air temperature, water flow temperature, fan speed, compressor speed or test standard. If two ratings use different conditions, convert the unit first, then keep a note of the condition beside the number. That habit prevents a neat converted value from being mistaken for a fair product comparison.
Using The Cost Line
The cost line is deliberately simple. It assumes the equipment runs at the entered output and COP for the whole run time. Real equipment cycles, modulates, defrosts, changes fan speed and reacts to room temperature. Use the cost result for quick comparisons between ratings, not for a final bill forecast. For a fuller estimate, combine seasonal performance data with expected load hours and the actual electricity tariff.
FAQs
What is the COP to EER conversion factor?
Multiply COP by 3.412141633 to convert to EER in Btu/h per watt for the same test condition.
Can I convert EER back to COP?
Yes. Divide EER by 3.412141633 to get COP.
Is EER the same as SEER?
No. EER is a test-point rating, while SEER is a seasonal rating. They should not be swapped without the proper seasonal method.
Does a higher COP always mean lower bills?
For the same delivered output and operating condition, a higher COP uses less electricity. Actual bills also depend on weather, controls, building heat loss and run time.
Why does capacity matter?
Efficiency is a ratio. Capacity tells you how many kW are delivered, which lets the tool estimate input power and running cost.
Sources
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units (SI), Special Publication 811. NIST. https://www.nist.gov/pml/special-publication-811
- American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2024). ASHRAE Handbook: HVAC Systems and Equipment. ASHRAE.
- U.S. Department of Energy. (2026). Central air conditioning and heat pump efficiency terms. Energy Saver. https://www.energy.gov/energysaver/central-air-conditioning
- Bureau International des Poids et Mesures. (2019). The International System of Units (SI), 9th edition. BIPM. https://www.bipm.org/en/publications/si-brochure