What is COP, and why does it matter for pool heating?
COP is the single number that tells you how efficient a pool heat pump is. Here is what it means and what a good one looks like.
Key Takeaways
- COP stands for Coefficient of Performance. It is the amount of heat a unit delivers for each unit of electricity it draws
- A COP of 16 means the unit returns 16 units of heat for every 1 unit of electricity, which is why heat pumps cost so much less to run than gas
- COP is not a fixed number. It falls as the air gets colder, so the headline figure is a peak, measured under warm conditions
- When comparing units, check the conditions the COP was measured at. Two units quoting the same number are not always comparable
Most pool heating specifications lead with a COP figure. It is usually the biggest number on the page, and it is rarely explained. This guide covers what it actually measures, what a good figure looks like, and the one thing to check before comparing two units on COP alone.
What COP means
COP stands for Coefficient of Performance. It is a ratio, and it answers one question: for every unit of electricity this unit draws, how many units of heat does it put into the water?
A COP of 16 means 16 units of heat out for 1 unit of electricity in.
That ratio sounds impossible if you think of a heat pump as a heater. It is not. A bar heater or a gas burner makes heat, and no matter how well built it is, it can never return more energy than it consumes. A heat pump does something different. It moves heat that already exists in the outside air and transfers it into your pool water. The electricity is not being converted into heat, it is running a compressor and a fan that relocate heat from one place to another.
That is the whole reason the number can be greater than one, and why a heat pump is the low-cost way to heat a pool.
The practical version is straightforward. Higher COP, lower running cost for the same amount of heat.
Why COP is not a single fixed number
This is the part most specification sheets leave out, and it is the part that matters most when you are comparing units.
COP changes with conditions. It is not a property of the unit in the way that weight or output is. It is a measurement taken under a specific set of circumstances, and if the circumstances change, the number changes.
Three things move it:
- Air temperature. A heat pump gathers heat from the air, so the more heat there is available, the less work it does to collect it. On a mild day the COP is high. On a cold morning the same unit will be working considerably harder, and its COP will be lower.
- Water temperature. The bigger the gap between the air and your target water temperature, the harder the unit works to bridge it.
- Humidity. Moist air carries more available heat energy than dry air, which is why COP figures are usually quoted alongside a humidity percentage.
So the headline COP on any brochure is a peak figure, recorded under favourable conditions. That is not a trick, it is the industry convention, and every manufacturer does it. But it does mean the number on its own tells you less than it appears to.
The question to ask when comparing units
Because COP moves with conditions, the only fair comparison between two units is one made at the same conditions.
Manufacturers usually state these alongside the figure, in a format like 27°C air / 26°C water / 80% humidity. If one unit quotes a COP measured at 27°C air and another quotes one measured at 15°C, the first number will look far better while telling you nothing useful about how the two compare.
When you are looking at a spec sheet, look for the test conditions printed next to the COP. If they are not stated, ask. A supplier who cannot tell you the conditions their headline figure was measured at is quoting a number they cannot stand behind.
The second question worth asking is what the unit does at lower temperatures, because that is when you will actually notice the difference. Peak COP describes the unit on its best day. Performance in the shoulder seasons describes it on the days you are deciding whether to swim.
COP across the AstroMax range
Every AstroMax unit publishes its COP alongside the conditions it was measured at, in the spec table on each product page. Check the figure there against the conditions it was measured at, and compare on a like-for-like basis.
Where COP sits among the things that matter
COP is worth understanding, and it is worth comparing. It is not worth choosing on alone.
A very efficient unit that is undersized for your pool will run constantly and still struggle to hold temperature, and its excellent COP will not save you money because it never gets to stop. A correctly sized unit with a slightly lower COP will reach temperature, then idle, and cost you less across a season.
Sizing comes first. What size heat pump do I need for my pool? covers how to match output to your pool volume. After that, COP is a good way to separate two correctly sized options.
The other things worth weighing are the warranty, and whether anyone will actually turn up when something goes wrong. Efficiency is a number on a page. Service is what you experience for the next ten years.
In short
COP tells you how much heat you get per unit of electricity, and it is the reason a heat pump costs a fraction of gas to run. Treat the headline figure as a best-case measurement, check the conditions it was taken at, and compare like with like. Then size the unit properly, because a well-sized unit at a good COP beats an efficient one that is too small.
If you would like help working out which unit suits your pool, get in touch with our team or see choosing your heating.
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