FAQFAQ
Questions people actually ask
Short answers, each with the arithmetic behind it and a link to where it is worked through properly.
Last updated
Twenty-one questions, grouped roughly in the order people arrive at them: covers first, then heating, then cost and temperature, then how this site works. Every answer links to the page that does the full job.
Everything, briefly
How many degrees does a solar cover actually raise a pool?
It depends entirely on which of three questions you are asking, and almost nobody says which. Across one sunny afternoon, physics caps a full-size pool at roughly 0.66°F per hour. Overnight, a cover prevents loss rather than adding gain — it addresses the roughly 70% of loss the Department of Energy attributes to evaporation. Over a week of continuous use the pool settles warmer, which is where the 10-to-15-degree claims come from. The full reconciliation.
Why do different websites give completely different figures?
Because they are measuring three different physical quantities and calling all of them the same thing. A 4-degree figure and a 15-degree figure can both be accurate about different things. Every temperature claim on this site names which of the three it is.
Bubbles up or bubbles down?
Down, against the water. The bubbles trap air and hold the film against the surface, and they can only do either in contact with the water. Why.
What thickness solar cover should I buy?
12 mil for most people, 16 mil if you have a reel. Thickness buys service life rather than heat: published lifespan tiers run about 1 to 2 years at 8 mil, 3 to 4 at 12 mil and 5 to 10 at 16 mil, while the only published hands-on comparison in this category measured the same temperature change across every cover in its group. The comparison.
Clear or blue?
It matters less than the marketing suggests. Clear transmits more light into the water, blue absorbs more at the surface, and the only test that compared both measured them the same. Choose on thickness, on UV blocking if chemical cost matters to you, and on whether you have had algae trouble. The comparison.
Should the cover be on during the day?
Yes, unless you are swimming or the pool is already warmer than you want. A cover transmits energy into the water with the cover on, and it is simultaneously suppressing evaporation — which runs hardest on exactly the warm breezy afternoons when you might be tempted to pull it off. Details.
Is a solar cover reel worth it?
On any pool where deploying the cover is a two-person job, it may be the highest-value purchase available. Every published saving assumes the cover is on the pool when nobody is swimming, and a cover that takes five minutes and a helper stops going on within weeks. Reels, ranked.
What is the cheapest way to heat a pool?
Cheapest to buy is a cover, at around a hundred dollars, attacking the roughly 70% of heat loss caused by evaporation. Cheapest to run is solar panels at zero fuel cost, then a heat pump at about $10.75 per million BTU delivered. Both answers.
Is a heat pump really cheaper to run than gas?
At US average residential prices, yes — about $10.75 per million BTU delivered against $28.36 for natural gas. A heat pump moves heat rather than making it, delivering several units for every one it draws, which is why electricity goes from the worst fuel to the best depending on the machine. The head-to-head.
Should I avoid electric pool heaters?
On a full-size pool, yes. An electric resistance heater costs about $53.75 per million BTU delivered — roughly five times a heat pump on identical electricity. It is genuinely defensible on a spa or a very small above-ground pool, where the absolute numbers stay small.
How long does it take to heat a pool?
Energy needed divided by delivered output. A 20,000-gallon pool needs 1,666,000 BTU for a 10-degree rise, which is about 5 hours on a 400,000 BTU gas heater and about 12 on a 140,000 BTU heat pump. Work it out for your pool.
What size heater do I need?
Divide your energy requirement by how many hours you will wait. And watch the rating trap: gas heaters are rated by fuel input and deliver roughly 82% of it, while heat pumps are rated by the heat they deliver. The sizing guide.
How much does it cost to heat a pool per month?
Mostly it depends on how much heat escapes, not how much you add. For a 20,000-gallon pool losing 3°F overnight, replacing that heat runs about $161.19 a month with a heat pump and about $425.19 with natural gas at US average prices. The full model.
What is the ideal pool temperature?
The Department of Energy publishes 78 to 80°F for active swimming and 82 to 84°F for recreational use. Every degree above your actual need costs 10 to 30% more on the heating bill, per DOE — the setpoint is the biggest lever you own. Choosing yours.
Why does my pool lose so much heat overnight?
Mostly evaporation, which DOE puts at about 70% of a pool’s total heat loss. Every pound of 80-degree water that evaporates carries away 1,048 BTU — roughly 8,730 BTU per gallon. Wind makes it far worse: DOE puts a 7 mph breeze at a 300% increase in energy consumption. The breakdown.
Does a solar cover pay for itself?
On a heated pool, usually within weeks. For a 20,000-gallon pool losing 3°F overnight, a roughly $130 cover pays back in about two weeks on natural gas and about five on a heat pump. Run it on your pool.
Can I heat a pool without a heater?
Partly, and further than most people expect. A cover, a windbreak, a lower setpoint and running the pump during the warm part of the day each have a published figure behind them. None of it produces a warm pool out of a cold cloudy week. How far it goes.
Do you actually test any of these products?
No, and we say so on every page that recommends anything. There is no lab, no test pool and no owned units. What we do instead is compile the published specifications, apply the Department of Energy’s pool-energy physics, and print the arithmetic where you can check it. The full method.
Why is there no product photography on some pages?
Because we will not put a stock photograph of some other pool cover under a named model — that is a picture pretending to be evidence about a specific product. Where we have no honest photograph of an exact model, the page says so.
How does this site make money?
Affiliate commission, mostly from Amazon. You pay the same price either way, and it never changes the ranking — our top cover pick is the cheaper 12-mil one, and we tell people to buy a hundred-dollar cover before a three-thousand-dollar heater. The full disclosure.
I think one of your numbers is wrong. What do I do?
Please tell us — the arithmetic is published so it can be checked. The contact page has a form, and corrections are fixed and date-stamped where you can see them rather than edited quietly.
Still stuck?
If your pool is cold right now and you want a decision rather than an essay, start here — four measurements that tell you which of four different problems you actually have.
If your question is about a number rather than a product, the methodology page lists every constant we use with the source it came from. And if you think one of them is wrong, tell us.