Why evaporation is nearly the whole story
Turning a pound of liquid water into vapour at pool temperature takes about 1,050 BTU. That heat comes out of the water that stays behind, so evaporation cools the pool far more effectively than conduction to the air ever could. On a typical warm evening, evaporation is somewhere between two thirds and nine tenths of the total surface loss, and the remainder is convection to the air plus radiation to the sky.
The driver is not temperature difference. It is the difference between the vapour pressure at the water surface, which is the saturation pressure at the water temperature, and the vapour pressure in the air above it, which is the saturation pressure at the air temperature multiplied by the relative humidity. Warm water under cool dry air has a large difference and evaporates fast even though nothing about it feels hot. Warm water under warm saturated air barely evaporates at all.
The other term is wind. The correlation used here has a still-air coefficient of 0.089 and a wind coefficient of 0.0782 per mile per hour, which means a 2 mph breeze at the surface nearly triples the still-air rate and 5 mph is over five times it. That single relationship explains most of the difference between two identical pools in the same town.
Reading the numbers the page gives you
| Output | What it is |
|---|---|
| Driving difference (inHg) | Water surface vapour pressure minus air vapour pressure. Zero or below means no net evaporation. |
| Evaporation (lb/h) | Area times the pressure difference times the wind term. Convert to gallons at 8.34 lb each. |
| Inches per day | The depth drop you would see with no rain, no splash and no leak. Compare it to what your pool actually does. |
| Convection and radiation | A single lumped coefficient times area times water-minus-air degrees. It is deliberately crude because it is the small term. |
The inches per day figure is the one worth checking against reality, because it is the only output you can measure with a ruler. If your pool is losing considerably more than the model predicts and the weather does not account for it, look for a leak rather than reaching for a bigger heater. The standard separation is the bucket test: set a bucket of pool water on a step so it sits at the same temperature and in the same air, mark both levels, and compare the drop after a day. Equal drops mean evaporation. A pool that drops further has a leak.
What a cover is actually doing
A cover works by putting a physical barrier between the water surface and the air, which stops the vapour transport. Everything else it does — cutting radiation to the night sky, reducing convection, adding a little solar gain if it is a clear bubble type — is secondary to that. This is why covers work at night and in cool weather rather than only in sun, and why an opaque insulating cover and a clear bubble cover both cut losses substantially despite looking like completely different products.
The reduction figure here is an input because real covers vary and real coverage varies more. A cover pulled tight over the whole surface is not the same as one that leaves a foot of open water down one side. Liquid surface products behave differently again and are disturbed by swimmers and wind. Ninety percent is a reasonable figure for a well fitted solid or bubble cover; if yours is partial, scale it by how much of the surface it really covers.
The hours matter as much as the percentage. A cover that goes on overnight and comes off in the morning stops most of the loss, because the hours from dusk to dawn are long, still and cool. A cover only on when nobody is home saves considerably less. The daily numbers on this page scale linearly with those hours, so the field is worth setting honestly.
Weighing that against the heating bill
Converting BTU to money depends on how the heat is replaced, and the page handles the three cases separately. Gas is priced per therm at 100,000 BTU, divided by the heater efficiency. Electricity is priced per kWh at 3,412 BTU, divided by the heat pump COP expressed as a percentage — a COP of 5 goes in as 500, because it delivers five units of heat per unit of electricity. An unheated pool replaces nothing, so the only cost is water and the only benefit of a cover is the water and the temperature it holds.
The water saving is real money in places where water is metered and it is a substantial volume anywhere. A quarter inch a day off 500 square feet is roughly 78 gallons, which is over 2,000 gallons in a month. That is water you paid for, warmed, and balanced, and every gallon that leaves takes its heat with it and gets replaced by cold makeup water that has to be warmed again. To size the heater that has to do that warming, the pool heater sizing calculator takes it from the other end.
Questions people ask
How much water does a pool lose to evaporation?
Commonly a quarter to a half inch a day for an outdoor pool in warm weather, but the range is wide and the drivers are humidity and wind rather than temperature. A sheltered pool on a humid still day can lose under a tenth of an inch. The same pool on a dry breezy day can lose well over half an inch. On 500 square feet, a quarter inch is about 78 gallons and half an inch about 156. Track it across several days rather than one, because a single windy afternoon distorts the average.
How do I tell evaporation from a leak?
The bucket test, which controls for the weather instead of guessing about it. Fill a bucket with pool water, set it on a pool step so it sits at the same temperature and in the same air, and mark the water level inside the bucket and the pool level outside it. Come back after 24 hours with the pump running as normal and compare. If both dropped the same amount, it is evaporation. If the pool dropped more, the difference is the leak rate. Repeat with the pump off to narrow down whether the loss is in the plumbing or the shell, since a pressure-side leak only leaks while the system runs.
Does a solar cover heat the pool or just stop it cooling?
Mostly the second, and the second is the larger effect. A clear bubble cover does pass sunlight and does add some gain, but a pool already absorbs most of the sun that reaches it, so the cover adds a modest amount. What it changes dramatically is the loss side, by stopping evaporation. That is why a cover raises pool temperature even when it is on overnight and off all day, and why an opaque insulating cover works nearly as well as a clear one despite blocking the sun entirely.
Why does my pool lose heat overnight when the air is warm?
Because evaporation does not require cool air. It requires the air above the water to be drier than saturated at the water temperature, and warm air at 50 or 60 percent humidity is exactly that. The pool surface is trying to saturate the air above it, and any breeze replaces that air with new dry air, so the process runs continuously. Add radiation to a clear night sky, which is much colder than the air, and you have a pool cooling steadily while a thermometer says the night is mild.
Is a liquid pool cover worth using instead of a physical one?
The mechanism is the same idea, a barrier at the surface, but the barrier is a molecular film that is far easier to disturb. Wind, swimmers, high flow returns and the skimmer all break it up, and it has to be re-dosed. Published reductions for these products are considerably lower than for a fitted physical cover and vary a great deal with conditions. If you use one, set the cover effectiveness on this page well down from 90 to reflect that. The one advantage they have is that nobody has to drag anything across the pool, which is why some pools that would never use a physical cover actually get some benefit.