Pool Cover Savings Calculator

How much water does a pool lose to evaporation?

The rule of thumb is a quarter inch a day in summer, and it's a decent one — but "a day" hides a threefold spread. Evaporation is driven by the gap between the vapor pressure of your warm water and the moisture already in the air, multiplied by wind. Warm water in cool, dry, moving air evaporates fast; a pool sitting in still, muggy air barely loses anything. The same 15 × 30 pool (450 ft²), run through the equation the calculator uses:

Uncovered, breeze at the surface as noted, no swimmers. "Worth in °F" is how far that heat loss would drop a 16,000-gallon pool with nothing replacing it — sunshine puts a lot of it back by day, which is why the overnight drop is what you actually notice.

Notice the last row: a heated pool in the shoulder season is the worst case, because the water is warm and the air is cold and dry. That's also exactly the pool that is paying for its heat — the two facts about pool covers that matter most are the same fact.

Why evaporation is 70% of your pool's heat loss

Water can't evaporate without taking energy with it — about 1,048 BTU for every pound, which is what makes sweating work and what makes a wet swimmer shiver in a warm breeze. A pool is a very large wet swimmer. Every gallon that leaves the surface carries roughly 8,740 BTU out of the water — the same energy it takes to heat that gallon by a thousand degrees. Add up the gallons in the table above and evaporation accounts for around 70% of a typical outdoor pool's total heat loss (the US Department of Energy's figure); radiation to the night sky and convection make up the rest.

That's why the heater-sizing rule of thumb drops from 12 to about 5 BTU per ft² per °F when a cover goes on: the cover isn't insulating much, it's simply stopping the water from leaving. And it's why every one of those gallons costs far more in heat than it does in water:

How much does a pool cover save?

The DOE's headline numbers are 50–70% off heating costs and 30–50% off makeup water. Both are right, and both are averages that hide the thing that decides your number: how many hours a day the cover is actually on. A cover in the shed saves nothing. The calculator models that directly — a solar blanket stops about 90% of evaporation while it's on, so "on whenever nobody's swimming" (call it 20 hours) prevents about three-quarters of the season's loss, and "nights only" about half. Over a 5-month season, in the "typical summer" conditions from the table above:

Solar blanket on 20 h/day, 150-day season, $8/1,000 gal water, $1.50/therm gas at 82% efficiency, $0.17/kWh electricity at a heat pump COP of 5. Heating columns are the cost of replacing only the evaporative heat the cover kept in; a cover also trims radiant and convective loss, so the real number is a little higher. Your weather and rates go in the tool.

Read the heated columns against the price of the cover. An 80-dollar solar blanket on a gas-heated 15 × 30 pool has paid for itself in the first couple of weeks of the season and then earns its price again every week after that; even on a heat pump it's the best-returning thing you'll ever bolt to a pool. On an unheated pool the water savings alone take a few seasons to cover it — the honest return there is the right-hand column: the pool starts every day several degrees warmer than it otherwise would, which is most of what people mean when they say a solar cover "heats" the pool.

Solar blanket, solid cover, or liquid cover?

For evaporation, any physical cover that lies on the water works about the same: a bubble solar blanket, a solid winter-style cover, an automatic cover on tracks — all stop 90% and more of the loss while they're on. The bubbles on a solar blanket add a little insulation and let some sun through, but their real job is floating; a plain sheet of plastic would keep the water in nearly as well. Where covers differ is how often you'll actually deploy them, which is why a $150 reel often saves more than a $150 better blanket.

Liquid covers — the fatty-alcohol films you pour in weekly — are the exception. In still air the published figures run 30–40% reduction; a breeze pushes the film to one end of the pool and the benefit heads toward zero, and swimmers break it up. The calculator uses 30% for a liquid cover in all conditions, which is generous on a windy day. They're worth it where a physical cover genuinely won't happen — awkward shapes, a spa you use daily — but they're a supplement, not a substitute.

What the options cost, roughly, and how long they last — so the payback number above means something:

Cost ranges are 2025 US retail; the effectiveness figures for physical covers come from the DOE and cover manufacturers, and the liquid-cover range from independent tests rather than the label.

Does a solar cover actually heat the pool?

Mostly it stops the pool cooling, which from the swimmer's side is the same thing. Run the numbers in the tool with your pool volume filled in and you'll see the mechanism: uncovered, evaporation is pulling several degrees a day out of the water, and the sun is putting some back; cover it, and the sun's contribution stays. A clear bubble cover does pass some sunlight to the water and traps a little warm air in the bubbles, so it gains slightly more than a solid one — but the "raises temperature 10–15 °F" line on the box is describing the gap between a covered pool and an uncovered one after a few weeks, in a good climate, not something the cover does in an afternoon. In a cool, windy spot the honest expectation is that the pool stays swimmable more of the season; in a hot dry one it can push a pool from comfortable to too warm, which is why desert owners often cover only at night.

Other ways to cut evaporation — and how they rank

Every other lever moves one of the terms in the same equation, and none of them moves it as far as a sheet of plastic on the surface:

• Windbreak. Wind is the multiplier: 7 mph across the water roughly doubles the loss of a sheltered yard (compare the rows in the first table). A fence, hedge or screen that drops surface wind is the second-best investment after the cover, and it also keeps the cover from blowing off.
• Turn the thermostat down. Evaporation rises steeply with water temperature because vapor pressure does — every degree you don't heat is roughly 5% less evaporation and the heat that goes with it. Try 82 °F against 86 °F in the tool.
• Water features off when nobody's watching. Waterfalls, deck jets and fountains are evaporators by design — they turn a flat surface into spray. Put them on a timer for the hours people are actually there.
• Shade cuts the sun's warming of the surface and lowers daytime evaporation somewhat, but it also cuts the free solar gain — a trade, not a win, for a pool you'd like warm.
• Chemistry doesn't help. Nothing you dose changes the vapor pressure of water. Products that claim to "reduce evaporation" are liquid covers, with the caveats above.

What a cover does to your chemistry

Evaporation takes pure water and leaves everything dissolved behind, so an uncovered pool in a dry climate slowly concentrates its calcium hardness, CYA and salt — and the top-up water brings its own calcium in. A cover slows that creep, which for hard-water areas means fewer partial drains (the dilution calculator is where that ends up). Two more effects to know about: a covered pool aerates less, so pH climbs more slowly — expect to add acid less often — and an opaque cover shields chlorine from the sun, so free chlorine holds longer. Neither replaces testing; both mean your doses shrink after the cover arrives, so re-test before you repeat last month's numbers.

One caution: a pool under a cover with a heater running is the easiest way to overshoot temperature, and warm covered water with low chlorine is where algae is happiest. Keep the weekly test honest, and if the pool is heated, let the heater sizing calculator show you how much smaller a heater the covered pool really needs — that's the other half of this same math.