Covers
A cover is worth what you actually use it
A pool cover directly suppresses evaporation, the largest loss term in this model's default season. Its actual return still depends on the price you pay, the cover type and how many hours it is on; the gap between "I have a cover" and "the cover is on fourteen hours a day" is where most of the disappointment lives.
The mechanism is evaporation, not insulation
On the heat-loss page the evaporation term is typically 86% of the total loss for this pool and season. A physical cover essentially eliminates it — there is no exposed water for vapour to leave from. Everything else the cover does is secondary: it floats near air temperature so it cuts the driving difference for convection and sky radiation, and if it is opaque it also blocks the sunlight you were getting for free.
That last point is why an opaque insulated cover left on through a sunny day can genuinely cost you more than it saves in a mild climate, while a clear bubble cover in the same position saves in both directions.
What a cover saves on your pool
Set the hours honestly. A cover that goes on at bedtime and comes off at breakfast is about ten hours, not twenty-four.
Season heat without the cover 196.18 million Btuwhat your heater is supplying now
Season heat with it 32.27 million Btu83.6% less
Saved per season, heat pump $1,368at 15.23 cents per kWh
Saved per season, gas $3,585at $1.79 per therm
Cover pays for itself in 0.4 seasonson the heat-pump saving
Water not evaporated 16,035 gallonsmake-up water you do not buy
Hours on is the whole variable
| Cover hours per day | Season heat needed | Reduction | Heat-pump cost | Saved |
|---|---|---|---|---|
| 0 h/day | 196.18 million Btu | 0.0% | $1,662 | $0 |
| 4 h/day | 136.90 million Btu | 30.2% | $1,180 | $482 |
| 8 h/day | 88.48 million Btu | 54.9% | $775 | $887 |
| 12 h/day | 45.66 million Btu | 76.7% | $411 | $1,251 |
| 14 h/day | 32.27 million Btu | 83.6% | $294 | $1,368 |
| 16 h/day | 21.57 million Btu | 89.0% | $197 | $1,465 |
| 20 h/day | 5.39 million Btu | 97.3% | $48 | $1,614 |
The relationship is close to linear in hours, which is a more useful fact than it sounds. It means the honest question to ask before buying is not "does a cover work" — it plainly does — but "will this cover get put on". A heavy manual cover on a large pool gets used for two weeks and then does not. A reel, or a powered cover, or simply a smaller lighter cover, converts an unused good idea into a real saving.
The cover types are not interchangeable
| Cover type | Evaporation stopped | Sunlight passed | Season reduction | Saved per season |
|---|---|---|---|---|
| Bubble / solar blanket | 95% | 75% | 83.6% | $1,368 |
| Insulated foam or vinyl cover | 98% | 10% | 84.2% | $1,381 |
| Liquid (chemical) cover | 35% | 98% | 35.5% | $565 |
An insulated opaque cover blocks more heat loss than a bubble cover and passes almost no sunlight. Whether that trade is worth it depends entirely on whether the hours it is on overlap with daylight. Overnight-only, insulated wins. Left on all day in a sunny climate, the bubble cover frequently wins because it is still collecting.
Liquid covers are modelled conservatively here. They are a molecular monolayer, wind disrupts them, and the published performance claims span a range so wide that treating them as equivalent to a physical cover would not be honest.
What a cover does not do
- It is not a safety cover. Bubble and insulated thermal covers will not hold a child or a pet and can be more dangerous than open water because they look solid. A safety cover is a different product to a different standard and it does not save meaningful heat.
- It does not fix an undersized heater. It lowers the load, sometimes enough to change the equipment class, but the heat-up requirement is a volume problem and a cover does nothing for it.
- It does not last. Ultraviolet light destroys bubble covers, faster in Phoenix than in Boston. Budget for replacement inside the payback window above, which is why the calculator asks how many seasons you expect rather than assuming.