Keeping a pool warm overnight
Fine at six, cold by morning. Here is where it goes and what stops it, ranked by effect per dollar.
Quick picks
Ranked by how much overnight loss each one prevents, with the practical alternatives for pools where a blanket will not happen.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No photo The fix Sun2Solar 1600 Series Ultimate Blue Solar Cover Thickest tier, built for daily on-and-off handling. This is the job covers exist for. | An inground pool you cover most nights, every season | |
| 2 | No photo Makes it a habit Blue Wave Solar Cover Reel Thirty seconds alone is a routine. Five minutes and a helper is not. | Any inground pool with a cover you are not using enough | |
| 3 | No photo If a blanket is not happening Solar Sun Rings Partial coverage, deployed one disc at a time. Better than nothing by a wide margin. | A pool you would otherwise leave uncovered because the blanket is too much work | |
| 4 | No photo For shapes no cover fits HeatSavr Liquid Solar Cover Slows evaporation with nothing to handle at all. | Odd-shaped pools and anyone who will never use a blanket |
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Where it goes between dusk and dawn
The overnight drop has a specific cause and it is not mysterious. During the day the pool is gaining heat from the sun and losing it at the same time, roughly balancing out. After dark the gain stops and the losses do not.
Three things run all night:
- Evaporation — about 70% of total pool heat loss, per DOE, and it does not care that it is dark. Warm water in cool air is in fact ideal conditions for it, which is why you sometimes see steam coming off a pool in the morning.
- Radiation to the night sky — worst on clear nights, when the pool is effectively radiating to deep space. A cloudy night genuinely costs you less.
- Convection — and DOE puts a 7 mph wind at a 300% increase in energy consumption. Evening breezes are common.
| 20,000 gal x 8.33 x 3°F drop | = 499,800 BTU |
| x 70% evaporation share | = 349,860 BTU |
| x $28.36 per million BTU (gas) | = $9.92 |
$9.92 a night on gas, $3.76 on a heat pump
Over a 150-night season that is between roughly five hundred and fifteen hundred dollars of heat, depending on your fuel — which is the number a hundred-dollar cover is defending.
What works, ranked
1. A cover, every night. Nothing else is close.
A cover attacks the dominant loss directly. DOE puts the heating-cost saving from covering a pool when it is not in use at 50 to 70%, and there is no other hundred-dollar purchase in this subject that does anything comparable.
Thickness matters here for durability rather than heat: a cover used every single night gets handled 150 times a season, and that is what wears covers out.
2. A reel, if the cover is a two-person job
This is the difference between a good intention and a habit. A cover that takes five minutes and a helper gets used in April and abandoned by June, and its saving for those months is exactly zero. Reels, ranked.
3. A windbreak
The 300% figure is doing enormous work here and it is the most under-used intervention in the subject. If your pool sits in an open yard, something on the prevailing side is worth more thermally than most equipment.
4. Turn the heater down at night, not off
A pool cools slowly, so a modest overnight setback saves the higher loss rate that goes with a higher temperature without asking the heater to do a full heat-up in the morning. Turning it fully off and back on generally costs more than it saves, because you pay the whole recovery.
5. Run the pump during the day instead
Circulating at 3am pushes water through cold pipes and equipment and back into a pool radiating to a clear sky. The same run at 2pm does the opposite. Same electricity, same filtration.
What does not work
- Running the heater harder overnight. You are paying to replace heat as it escapes. Stopping the escape is cheaper than winning a race against it.
- A partial cover. Evaporation continues at full rate through every square foot you did not cover, which is why rings deliver roughly what they cover and no more.
- Bubble wrap from a hardware store. Cheaper for a reason: packaging bubble wrap is not UV-stabilized and goes brittle within weeks in sun, and it is thinner than even an 8-mil cover.
Measure it before and after
Read the temperature at dusk and at dawn, a foot down and away from the returns. Do it once uncovered and once covered, on similar nights. The difference between those two drops is what the cover is doing for your pool, which is a better number than anything published.
Turn it into dollars here, or read the full heat-loss breakdown.
Every pick, in detail
What each one publishes, what is good and bad about it, and — the section this category usually omits — what we could not judge without owning one.
Sun2Solar 1600 Series Ultimate Blue Solar Cover
The cover to buy if you intend to keep using it in five years. 16 mil is the thickest tier the category sells, and thickness on a solar cover buys durability rather than heat — Bob Vila measured every cover in its hands-on group at the same 4 to 5 degrees, from 12-mil clear to 16-mil blue. Pay the extra for the one that survives being dragged over the coping twice a day, not for a temperature difference that did not show up in the only published one-hour test in this category.
Best for An inground pool you cover most nights, every season
| Thickness | 16 milThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Color | Blue | Retail listing |
| Stated lifespan | 5 to 10 years for the 16-mil tierSwim University's published figure for 16-mil covers generally, not a Sun2Solar warranty statement. | Retail listing |
| Bubble side | Bubbles face the waterCategory-wide instruction, not brand-specific. The bubbles are the insulating layer and they have to touch the water. | Published standard |
4 to 5 degrees F in one hour
Claimed by Bob Vila, measured across all covers in their group. Conditions: One hour under 2pm sun on 89 to 92 degree F days. Source
Their figure was the same for every cover they put on the water. Read our reconciliation of what a one-hour reading actually measures before you plan around this number.
- Thickest tier sold, which is where the durability difference actually is
- Blue absorbs more of the incoming energy at the surface than clear does
- Cuts oversize to your pool shape with household scissors
- Heavy and awkward at 16 mil — a large inground cover is genuinely a two-person job without a reel
- Costs meaningfully more than the 12-mil version for no measured heating advantage
- Whether the seams hold up over five seasons of daily handling
- How the bubble layer survives repeated dragging over rough coping
- Whether the stated lifespan tier holds for this specific product line
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Blue Wave Solar Cover Reel
A reel is the single highest-leverage purchase in this whole category and almost nobody frames it that way. Every DOE saving figure assumes the cover is on the pool when the pool is not in use. A cover that takes two people and five minutes comes off on Friday and goes back on in October. A reel turns that into thirty seconds alone, which is the difference between the savings existing and not.
Best for Any inground pool with a cover you are not using enough
| Type | Wheeled above-ground-style reel for inground and above-ground useThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Why it matters | Converts a two-person five-minute job into a one-person thirty-second jobOur framing, not a manufacturer claim. The published savings all assume the cover is actually on. | Our arithmetic |
- Turns the cover into something you will genuinely use daily
- Keeps the cover off the ground, which is where covers get damaged
- Wheels, so it moves out of the way
- Costs a real fraction of what the cover itself costs
- Takes up deck space permanently
- Assembly is fiddly on every reel in this category
- How the tube and hardware hold up outdoors across seasons
- Whether the width range covers your pool without a wobble
- Assembly quality and hardware fit
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Solar Sun Rings
Floating discs you drop on one at a time instead of wrestling a full blanket, sold individually. That convenience is the whole product, and it comes out of the physics: rings leave gaps, and evaporation carries on through every square foot you did not cover. Expect a fraction of a full cover's effect roughly in proportion to the surface you actually covered, and buy them because you will use them, not because they work better.
Best for A pool you would otherwise leave uncovered because the blanket is too much work
| Coverage | Partial — discs leave gaps between and at the edgesThe gap is not a defect, it is how the product works. It does mean the DOE cover-saving figures do not apply at full strength. | Published standard |
|---|---|---|
| Typical price | About $25 to $35 each, or $140 to $160 or more per setSwim University's published price range, read 2026-09-02. Check the live price before buying. | Retail listing |
- One person can put them on and take them off in a couple of minutes
- No reel, no folding, no dragging anything over the coping
- Buy a few at a time instead of committing to one large purchase
- Partial coverage means partial results — the gaps keep evaporating
- Per square foot of coverage they cost far more than a blanket
- Wind moves them around
- How much surface a given number of rings really covers on your pool shape
- How long the material lasts against a full blanket
- Whether the anchoring holds in a breeze at your site
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
HeatSavr Liquid Solar Cover
A monolayer chemical that spreads a molecule-thick film across the surface to slow evaporation. It is the only option here that does not require you to physically cover anything, which is exactly why people buy it — and it is also the reason to be careful with the claims: a film that thin is disrupted by swimmers, wind and the return jets, and it is doing nothing for the hours after that until it re-spreads.
Best for Odd-shaped pools and anyone who will never use a blanket
| How it works | Self-spreading monolayer that slows surface evaporation | Published standard |
|---|---|---|
| Typical price | About $10 to $30 or moreSwim University's published range for liquid covers generally, read 2026-09-02. | Retail listing |
| Effect on daytime gain | None to speak ofA monolayer is optically negligible. Any benefit is on the evaporation side, which is the overnight-retention mechanism, not daytime gain. | Our arithmetic |
- Works on any pool shape, including the ones no blanket fits
- Nothing to store, fold, drag or trip over
- Cheap enough to try
- Disrupted by swimming, wind and circulation, and takes time to re-form
- Nothing published that would let you quantify what it actually saves
- Does not warm the pool during the day the way a physical cover can
- How much evaporation it actually suppresses on a real pool
- How quickly the film re-forms after use
- Whether dosing frequency in practice matches the label
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Questions people actually ask
How do I stop my pool losing heat overnight?
Should I turn my pool heater off at night?
Does a solar cover help at night?
Can I use bubble wrap instead of a pool cover?
- Average retail price of electricity to US residential customers: 18.34 cents/kWh, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential natural gas price: $24.09 per thousand cubic feet, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential propane price: $2.674 per gallon, week ending 30 March 2026U.S. Energy Information Administration · read 2026-09-02
- Energy content conversion factors: 3,412 BTU per kWh, 1,036 BTU per cubic foot of natural gas, 91,452 BTU per gallon of propaneU.S. Energy Information Administration · read 2026-09-02
- U.S. Department of Energy pool-energy figures (evaporation share, BTU per pound evaporated, cover savings, recommended temperatures, wind effect)Recreonics, reproducing and attributing the U.S. Department of Energy · read 2026-09-02
- Pool heater roundup: the 8.33 BTU per gallon per °F figure, attributed on the page to Alicia Toedter of Leslie'sBob Vila · read 2026-09-02
Every URL above was fetched and returned content on the date shown. Where a figure is ours rather than somebody else’s, the arithmetic is on the page and on the methodology page. If you find an error in it, we would genuinely like to know.
Read next
How fast a pool loses heat
Evaporation, radiation, convection and conduction, with the arithmetic on what each night actually costs and a measurement you can take yourself.
Solar pool covers worth buying
Ranked on the specifications that actually differ between covers — thickness tier and published lifespan — because the only hands-on test in this category found no heating difference at all.
Solar cover reels
Why a reel is the highest-leverage buy in pool heating, and which ones to look at. Thirty seconds alone versus five minutes and a helper decides whether a cover saves anything.
Does a solar cover pay for itself?
The full payback derivation from one measurement you can take tonight, plus a sensitivity table for people whose pool is nothing like the example.
Stretching the swim season
What actually adds weeks at each end of the season, why the shoulder season is a different problem from midsummer, and the order to spend in.