How the two differ in kind
An outdoor pool loses heat four ways: it evaporates, it radiates to the sky, it convects to the air, and it conducts into the ground. The first of those dominates the other three combined, and it is not close.
The reason is latent heat. Turning water into vapour takes a very large amount of energy per unit mass — far more than raising the same water's temperature by any ordinary amount — and every kilogram that leaves the surface as vapour takes that energy with it. A pool is a large, warm, open water surface exposed to moving air, which is close to an optimal evaporator. Wind increases it, dry air increases it, and a warmer pool increases it faster still, which is why heating a pool makes it lose heat more quickly than it did before.
That makes a COVER the intervention with the best return available, because it acts on the loss rather than replacing it. A cover on the water suppresses evaporation directly, and it does so at every hour it is on, without consuming anything. It also reduces radiant loss to a clear night sky, which is the second-largest term on a still night, and it keeps debris out.
A HEATER does something a cover cannot: it raises the temperature to whatever you want, on demand, regardless of sunshine. If you need a pool at a specified temperature in April, a cover alone will not deliver it. But a heater sized for an uncovered pool is sized to replace losses that need not happen, so it is bigger, runs longer and costs more, every season, for the life of the pool.
The factors that actually differ
Show
| Pool cover | Pool heater | |
|---|---|---|
| What it does | Stops heat leaving — mostly by suppressing evaporation, which is the largest loss. | Puts heat in, at a rate you choose, whenever you want it. |
| Running cost | Nothing. It consumes no energy at all. | The dominant operating cost of a heated pool, and it scales with how much heat is being lost. |
| Effect on the other one | Reduces the heater's required size and its run time substantially. The two are complements, not alternatives. | No effect on losses at all. Heating a pool warmer actually increases the rate at which it loses heat. |
| Temperature on demand | No. A cover retains what is there and lets a solar cover add a little; it cannot deliver a target on a cold week. | Yes. This is the thing only a heater does. |
| Water loss | Reduces it markedly, along with the top-up water and the chemicals that go with it. | None, and a warmer pool evaporates faster, so a heater increases water and chemical consumption. |
| Effort | Real. It has to go on and come off, and a cover that lives rolled up at the side does nothing. A reel changes the calculation. | None beyond setting a temperature. |
| Capital cost | Modest — the cover, and a reel if the pool is large enough to need one. | Substantial, plus installation, plus gas supply or electrical capacity. |
| Safety | A solar or thermal cover is NOT a safety cover and must never be treated as one — it can conceal a person beneath it. A safety cover is a different, certified product. | Not a safety consideration either way. |
| Debris and chemicals | Keeps leaves and dirt out, reduces chemical loss to sunlight, and cuts the top-up water that dilutes the chemistry. | No effect. |
| Where each is indispensable | Any outdoor pool, in any climate, at any time — there is no case where covering it is not the first move. | Where a defined temperature is needed at a defined time, or the season must be extended well beyond what the sun supports. |
Which one, and when
Choose pool cover when…
- The pool is outdoors and uncovered — this is the first and highest-return thing to do, before anything else is considered.
- Running costs are the concern, since a cover reduces them without consuming anything.
- The pool is in a windy or dry location, where evaporation is at its most aggressive.
- Water is metered, scarce or expensive, where reduced evaporation is a saving in its own right.
Choose pool heater when…
- A specific temperature is required on a specific date, which retention alone cannot deliver.
- The season is being extended well beyond what the local climate supports unaided.
- The pool is indoors or already covered, where the loss side has been addressed and the remaining question is input.
- The pool is used commercially or therapeutically, where temperature is a requirement rather than a preference.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does evaporation matter so much?
- Because vaporising water takes far more energy than warming it does. Raising water's temperature by a degree takes a certain amount of heat; turning that same water into vapour takes hundreds of times more per unit mass. So every kilogram that leaves the pool surface as vapour carries away an outsized quantity of heat, and an open pool is close to an ideal evaporator — a large, warm, exposed water surface with air moving over it. That single mechanism accounts for the majority of an uncovered outdoor pool's heat loss, more than radiation, convection and conduction combined. It also explains two things that surprise people: that a warmer pool loses heat faster than a cooler one, because evaporation accelerates with temperature; and that a windy day costs far more than a still one at the same air temperature.
- Do I still need a heater if I use a cover?
- It depends entirely on what you want the pool to do, and the cover changes the answer rather than removing the question. If the objective is a pool that is pleasant during the local swimming season, a cover alone frequently gets there in a warm climate, because it retains the heat the sun has already supplied and a solar cover adds a little on top. If the objective is a defined temperature on a defined date — a pool at a set temperature in early spring, or usable through a cool summer — no amount of retention creates heat that was never there, and a heater is the only thing that will do it. What the cover does in that second case is make the heater much cheaper to run and allow a smaller one, because it is no longer replacing losses that need not happen.
- Does a solar cover actually heat the water?
- A little, and its retention is worth far more than its gain. A solar cover is translucent and usually bubbled, so some sunlight passes through and warms the water beneath while the air cells insulate the surface. The measurable benefit comes overwhelmingly from the second effect: evaporation is suppressed while it is on, and radiant loss to a clear night sky is reduced. The consequence for how it is used is that it earns most of its keep OVERNIGHT and on cool days, which is the opposite of the intuition that it should be on during sunshine to collect heat. A cover left off overnight because it was on all day has been used in the least effective way available.
- How much heater do I need?
- It depends on the surface area, the temperature rise wanted above ambient, how exposed the site is, and — decisively — whether the pool is covered. Surface area is the driver rather than volume, because all the significant losses happen at the surface: a large shallow pool loses more than a small deep one of the same volume. Wind exposure matters as much as air temperature, which is why a windbreak is a genuine part of pool heating. And a cover reduces the required output substantially, which is why the sizing question should be asked after the covering decision rather than before it. Sizing a heater for an uncovered pool and then adding a cover leaves an oversized heater; doing it the other way round gives a smaller unit that costs less to buy and less to run.
- Is a solar cover a safety cover?
- No, and it is important not to blur them. A solar or thermal cover floats on the water to retain heat; it will not support weight, and a child or an animal that gets onto it or under it can be trapped beneath a sheet that then closes over them. It is a heat-retention product, nothing more. A safety cover is a different, certified product designed and tested to support a specified load and to be secured at the edges so that nothing can get underneath it, and it is fitted to a standard rather than simply laid on. If child safety is a concern — and around any pool it should be — the safety measures are barriers, alarms and a proper safety cover, and the solar cover sits alongside them rather than counting as one.
- What about liquid pool covers?
- They form a very thin molecular film on the surface that reduces evaporation, and they are a genuine option where a physical cover is impractical — an awkward shape, a pool in constant use, somewhere nobody will handle a cover. They are considerably less effective than a physical cover, because the film is disrupted by swimming, by wind and by the circulation system, and it needs continuous dosing to be maintained. They also do nothing about debris, nothing about radiant loss to the sky, and nothing about chemical loss to sunlight. The honest position is that a liquid cover is better than no cover and clearly worse than a real one, and it is best understood as the option for pools where the physical cover was never going to be used.
- Does covering the pool save water as well?
- Substantially, and it is an under-appreciated part of the case. The water that evaporates has to be replaced, and on a large uncovered pool in a warm, dry or windy location the top-up volume over a season is considerable. That has three costs beyond the water itself: the make-up water is cold, so it takes heat to bring up to temperature; it dilutes the pool chemistry, so chemicals have to be re-dosed; and where water is metered or restricted it is simply expensive or unavailable. A cover reduces all of that at once. Where water is scarce, the water saving alone can justify a cover before any heating argument is made.
- When should the cover be on?
- Whenever nobody is swimming, and overnight above all. Night is when the pool has no solar input, when the air is coolest, and when a clear sky pulls radiant heat away from the surface — so it is when an uncovered pool loses most, and when a cover returns most. Cool, windy and dry conditions during the day are the next priority, since wind and low humidity both accelerate evaporation regardless of sunshine. The practical obstacle is handling: a cover that requires two people and ten minutes will not be used, so on anything but a small pool a reel is not a luxury but the thing that determines whether the cover is used at all. A cover that spends the season rolled up beside the pool delivers none of the benefits described here.