Adding weeks to the season, starting with the cheapest thing first

Where the heat actually goes

A backyard pool is a large, shallow, uninsulated tank of water with an enormous surface area relative to its volume, sitting outdoors. It gains heat from the sun during the day and loses it to the air overnight, and the surface is where nearly all of that exchange happens. Evaporation is the biggest single mechanism: turning water into vapor takes energy, and that energy comes out of the pool.

Which is why the interventions that work are the ones that address the surface, and why they are cheap. Stop evaporation overnight and you keep the heat the sun gave you for free during the day. Everything else on this page is secondary to that.

Step one: the cover, and using it properly

A bubble solar cover is a sheet of bubble material that floats on the surface. It reduces evaporation dramatically while it is on, and it lets sunlight through to warm the water beneath. It is the highest-return item in this entire hub and it is inexpensive.

Three things decide whether yours works. Orientation: bubbles face down, toward the water. This is the single most common mistake and it costs you most of the benefit. Coverage: covers are sold to be cut to size, so buy oversize and trim to your pool rather than accepting a gap round the edge. Thickness: published in mil, and higher mil lasts longer rather than heating better — 12 mil and 16 mil are the common options. Our solar cover roundup covers the choice.

Step two: the free adjustments

Placement, if you still have the choice. A pool in sun for most of the day runs meaningfully warmer than one in the shade of a fence or a tree, and this is decided once, on setup day. If you are still planning where the pool goes, our setup guide covers the other placement considerations at the same time.

Uncover early, cover before dusk. Take the cover off in the morning so the water gets the full day of sun, and put it back on before the evening drop rather than at bedtime. Most of the loss happens in the first hours after the air cools.

Run the pump in daylight. Circulating during the warm part of the day moves warmed surface water down through the pool rather than letting it sit and radiate. It is a small effect, but it costs nothing if you were going to run the pump anyway — our run-time guide covers scheduling.

Windbreak. Wind accelerates evaporation, which is the main heat loss. An existing fence or hedge on the prevailing side does real work here, and it is worth factoring in when you decide where the pool goes.

Step three, and only then: a heater

If the cover is on, the pool is in sun and you still want to swim in May and September, then a heater is a legitimate purchase — but it is a purchase with a running cost attached that dwarfs everything else in this hub, and the number is computable if the manufacturer publishes the power draw in kilowatts.

What about solar heaters and heat pumps?

Solar heating panels and heat pumps both exist for this category, and both change the arithmetic above in ways that depend heavily on your climate, your sun exposure and your electricity rate. We are not going to publish a payback period for either, because an honest one requires inputs we do not have for your yard.

What we will say is the ordering: a solar cover comes first regardless, because it reduces the heat you need to add in the first place. Adding heat to an uncovered pool is heating the sky. Whatever heating technology you eventually choose, the cover makes it cheaper to run.

Knowing when to stop

There is a point in autumn where extending the season stops being worth the running cost, and it arrives earlier than enthusiasm suggests. Once you are heating against genuinely cold nights, the heater is running most of the time against a losing surface, and the bill reflects it.

The more useful skill is closing at the right moment. Closing while the water is still warm leaves the pool biologically active under the cover for months, which is how people find a swamp in April. Our winterizing guide covers the sequence for pools that stay up, and storing a pool covers the ones that come down. The cover question either way is in solar cover versus winter cover.

None of this is measurable without a thermometer, which is the point our thermometer roundup makes: a solar cover is either gaining you degrees overnight or it is not, and that is a reading rather than an assumption.