Does a solar cover alone get you a longer swim season?

The short answer

Usually, yes. Across 3,206 cities, a solar (bubble) cover on its own adds at least one month above 78°F in 2,787 of them. It adds one month in 558, two in 1,940, three in 227 and four or more in 62. In 222 cities it is the best-value option outright, because no heater adds enough months to justify its price. So buy the cover first, swim a season, and only then decide whether you still want a heater.

Why a cover matters so much

Most of a pool's heat leaves through evaporation at the surface, and most of that happens at night. A cover stops it. The model credits a cover with about 6°F of extra water temperature in sunny months and halves surface losses for every heater option, which is why every heater on the city pages is priced with a cover: running a heater on an open pool means paying to heat the night sky.

Where a cover makes the biggest difference

City	Unheated months	With cover	Most possible
New York, NY	1	3	5
Los Angeles, CA	0	3	8
Chicago, IL	0	2	4
Houston, TX	4	6	9

Large cities where a cover alone adds two or more months.

Where a cover is not enough

City	Unheated months	With cover	Most possible
Phoenix, AZ	6	6	9
Seattle, WA	0	0	4
Memphis, TN	4	4	7

In 419 cities a cover adds nothing: either the water is already warm every month the air allows, or the sun is too weak for the extra few degrees to cross 78°F. If your city is one of them, check its page for which heater is worth pricing.