How to heat a pool with solar panels

What this page is for

Whether solar collector panels are the right technology for your pool is a separate question, and we answer it in two places already: the solar roundup ranks the systems and states the trade, and solar against gas runs the widest comparison in the category.

This page assumes you have decided and answers the next question, which almost nobody covers properly: what does actually installing them involve? How much collector, where it goes, how it plumbs in, what your pump has to be able to do, and which parts are sold separately.

Step 1: how much collector you actually need

This is the step that decides whether the whole project works, and it is the step people under-buy. The rule of thumb in the trade is collector area equal to 50 to 100% of the pool’s surface area, toward the top of that range in cooler or cloudier climates.

The useful move is to convert that into a number of boxes. A SunHeater S601P is two 2 by 20 foot panels — 80 square feet of collector per system. So:

A smaller pool is a much friendlier problem. An 18-foot round above-ground pool has a surface of about 254 square feet, so the same rule puts it at 127 to 254 square feet of collector — two of these systems lands comfortably inside that range.

This page is the method. If you want the products, our inground panel roundup ranks the kits on collector area, header size and published warranty.

Step 2: where the panels go

Three mounting options, and the constraint is almost never the area itself:

• Roof. The usual answer, because the area exists and it is unshaded. It is also the most work, the most plumbing run, and the option where a leak is expensive. A high-wind kit is a separate purchase on most systems.
• Rack. A frame at ground level, angled toward the sun. Much easier to service and to drain, and much easier to get wrong on orientation because you are choosing the angle rather than inheriting it.
• Ground. Panels laid flat on a suitable surface. The cheapest and simplest, at some cost in efficiency, and it needs ground that is not going to be walked on.

Orientation: south-facing is best in the northern hemisphere. East or west works, with more area to compensate.

Shading is the binding constraint, not square footage. Collector area that is shaded between roughly 10am and 4pm is not collector area. Go and look at the intended spot at midday, in the season you care about, before you order anything — tree shade in June is not tree shade in April.

Step 3: how it plumbs in

The panels go inline after the filter, so that what reaches them is already clean, and before any chlorinator or chemical feeder. Water leaves the filter, goes up to the collectors, is warmed as it passes through them, and comes back down to the return.

The part that makes it usable day to day is a diverter valve. Without one, every drop of filtration flow goes through the panels whenever the pump runs, including at night, which is genuinely counterproductive — a collector on a roof after dark is a radiator pointed at the sky and it will move heat out of your pool. With a diverter you take the panels in and out of circuit without repiping anything.

Step 4: the pump specification that rules people out

Your existing filtration pump has to push water up to the collectors and back, and it was not specified with that in mind. This is where a lot of above-ground setups fall over.

SunQuest states a minimum of 3/4 HP for its panels. Check your pump’s data plate before ordering anything at all — it is a two-minute check that prevents the most annoying possible outcome, which is a roof full of panels and not enough pump to feed them.

Two related points worth knowing:

• Header diameter is a flow spec. SunQuest publishes 2 inch outside-diameter headers and describes the benefit as less back pressure. Wider headers mean the panels resist the flow less, which matters precisely because the pump is working harder than it was designed to.
• Height costs you head. A ground or rack mount asks far less of the pump than a two-story roof does. If your pump is marginal, that alone can decide the mounting question.

Step 5: controls, or doing it by hand

The manual version is a diverter valve you turn. Panels on in the morning, off at dusk. It works, it costs nothing beyond the valve that came in the kit, and it depends entirely on somebody remembering.

The automatic version is a solar controller with two temperature sensors — one at the panels, one in the water — driving a motorized valve. It sends water to the collectors only when they are warmer than the pool, which also solves the night problem by itself. It is more equipment and more money, and it is usually the right call if the panels are on a roof you cannot see from the house.

What to expect, and the gap in the claims

Here the category does something worth pointing at. These are published figures for solar collector panels, and the pattern is not subtle:

Read the SunQuest condition again, because it is the honest one: sized and installed correctly. That means collector area at 50 to 100% of the pool surface — the four-to-seven-system arithmetic from the top of this page. Most disappointed owners bought one system and got one system’s worth.

Nothing on this site states a temperature without naming which of three quantities it is, because conflating them is exactly why this category’s published numbers range so widely. The reconciliation is here.

Freeze protection and winterizing

Panels full of water in a hard freeze split, and this is not a marginal risk — it is the normal failure mode. In any climate that freezes, the collectors have to drain down for winter, every year.

This is why the direct-flow design SunHeater advertises matters more than it sounds: a system that drains cleanly is a system that actually gets drained. Check how a given set of panels winterizes before you buy, and put the drain-down on the same list as closing the pool rather than treating it as an optional autumn job.

What panels will not do

• Work at night. They collect solar energy, so after dark they do nothing — and worse than nothing if you keep circulating through them, which is the argument for the diverter or the controller.
• Rescue a cold cloudy week. That is the mechanism rather than a defect. Panels extend a warm season at the margins and are poor at holding a temperature through bad weather.
• Stop the overnight loss. The Department of Energy puts about 70% of a pool’s heat loss on evaporation, and no collector addresses that. On solar this matters more than on any other technology, because your daily gain is capped by the sun rather than by your budget — a cover is the only way to make a fixed daily gain go further.

If this is turning into a bigger project than you wanted

Two honest off-ramps. If the appeal was the zero running cost rather than the panels specifically, the home-built version is the same physics for less money and more of your own labor. If it was the heat rather than the free-ness, the ranked picks across every heater type set solar beside heat pumps and gas on a single cost-per-delivered-BTU basis, and for a lot of pools the answer turns out to be a heat pump and a cover rather than a roof full of collectors.