Where the 50 to 100 per cent rule comes from
Solar pool sizing is usually quoted as a fraction of the pool surface, and the fraction is not a tradition. It is what you get when you divide one number by another.
Collector area = daily heat loss ÷ (insolation × efficiency)
On the defaults: a 512 square foot pool losing at 5 BTU per hour per square foot per degree, sitting 6 degrees above ambient, loses 15,360 BTU an hour, or 368,640 BTU over a day. A collector seeing 1,600 BTU per square foot per day at 60 per cent efficiency delivers 960. Divide and you need 384 square feet, which is 75 per cent of the pool surface — squarely inside the rule of thumb.
Now change one input. Move to a windier site where the loss coefficient is 7 rather than 5 and the answer becomes 538 square feet, or 105 per cent of the pool. Ask for 10 degrees instead of 6 and it is 640 square feet, 125 per cent. Put a cover on for 12 hours a day and it drops to 236, or 46 per cent. The rule of thumb is one point on a curve, and the curve is steep.
| Change from the defaults | Collector area | As % of the pool |
|---|---|---|
| Defaults: 6 deg F, k = 5, no cover | 384 sq ft | 75% |
| Windy site, k = 7 | 538 sq ft | 105% |
| Hold 10 deg F instead of 6 | 640 sq ft | 125% |
| Cover on 12 h a day, 70% effective | 250 sq ft | 49% |
| Weaker sun, 1,200 BTU/sq ft/day | 512 sq ft | 100% |
The teaching point: solar pool heating works because the target is cold
A solar collector loses heat to the air in proportion to how much hotter the fluid inside it is than the surroundings. That single fact decides which solar thermal applications pay and which do not.
Domestic hot water wants 120 or 130 degrees. A collector running at that temperature on a 60 degree day is 60 or 70 degrees above ambient, bleeding heat continuously, which is why those collectors need glazing, insulation and sometimes a vacuum, and why they still run at 40 to 50 per cent.
Pool water wants 82. On the same day the collector is running maybe 20 degrees above ambient, so the losses are small, and an unglazed rubber or polypropylene mat with no glass and no insulation runs at 60 to 75 per cent. It is also the cheapest solar thermal hardware there is, because it is a mat with tubes in it.
That is the whole economic case, and it is why solar pool heating is the one solar thermal application with a payback measured in a few seasons. The flip side is the same physics running backwards: on a cool windy day an uncontrolled collector loop is a radiator, and water circulated through it comes back colder than it left. Every solar pool system has a differential controller for exactly that reason, and it is not an optional accessory.
Insolation, and how to get a number for your site
The input here is the solar energy landing on a square foot of collector over a day, in BTU. Most published data is in kilowatt hours per square metre per day, so:
BTU per sq ft per day = kWh/m²/day × 317
The 317 is 3,412 BTU per kWh divided by 10.764 square feet per square metre. A US summer figure of 5 kWh/m²/day is about 1,585 BTU per square foot per day; 6.5 in a desert summer is 2,060; 3.5 in a cloudy shoulder month is 1,110. The figure that matters is the one for the plane your collectors actually sit in, at the time of year you actually swim, and the tilt and orientation change it — the solar tilt angle calculator covers that side, and the solar array sizing calculator is the photovoltaic equivalent of this page for electricity rather than heat.
What the collectors cost you in pumping
Collectors sit above the pool, and getting water up to them adds head to the circulation loop. On a single-speed pump that is a modest increase in draw. On a variable-speed pump it can be worse than it looks, because the extra head forces a higher speed and power goes with the cube of speed — the pool pump run time calculator works that relationship out properly.
The pump energy is small against the heat captured on any reasonable installation, but it is not zero, and it is the reason the payback line here nets it out. It also is not the same all day: a collector loop that runs while the sun is on it and shuts down when it is not costs far less than one tied to the filtration schedule.
What this page will not tell you
It will not tell you what your pool will reach. The rise you enter is a target you set, and whether a given array holds it depends on weather over the season rather than on one representative day. The honest way to read the output is as a comparison: this much collector balances this much loss under these conditions, and here is how fast that moves when the conditions change.
The loss coefficient is the weakest input and it is deliberately exposed. A single lumped figure covering evaporation, convection and radiation is a simplification, and evaporation alone can be two thirds of the loss on a windy day. The pool heat loss calculator splits it out from your own air temperature, humidity and wind, and its number is a better input to this page than the default is.
Anything electrical within reach of pool water is licensed work, and the reason is that people are standing in the circuit. Bonding, grounding, receptacle placement, clearances and equipment disconnects are set by the code your jurisdiction has adopted and enforced by its inspector, and none of it appears on this page. If a pump, light, heater or panel needs more than plugging into a working receptacle, that is a job for a licensed electrician.
Questions people ask
How many square feet of solar collector does a pool need?
Enough to replace what the pool loses on the day you care about, which for a typical uncovered residential pool in reasonable sun works out at 50 to 100 per cent of the water surface area. The arithmetic is heat loss per day divided by what a square foot of collector delivers per day. A 512 square foot pool held 6 degrees above ambient with a moderate loss coefficient needs about 384 square feet of collector at 1,600 BTU per square foot per day and 60 per cent efficiency. Wind, a bigger temperature target or weaker sun push it up quickly, and a cover pulls it down harder than any of them push.
Why are pool collectors unglazed when other solar collectors have glass?
Because the water is barely warmer than the air, so there is very little heat to lose and nothing to insulate against. Glazing and insulation exist to stop a hot collector shedding heat to a cold sky, and they cost money, weight and some of the light they are protecting. A pool collector running 20 degrees above ambient does not need them, and without them it is a mat with tubes in it — cheap, light and roughly as efficient in that temperature range as an expensive glazed panel would be. Run the same mat at domestic hot water temperatures and it would be nearly useless, which is exactly the trade being made.
Can a solar pool heater cool the pool?
Yes, and that is not a fault. The same collector that gains heat when the sun is on it loses heat when it is not, and water circulated through a cold wet mat on a windy evening comes back cooler than it left. Every solar pool system therefore has a differential controller that compares collector temperature with water temperature and only diverts flow when the collector is genuinely warmer. Some owners use the effect deliberately in a hot climate, running the loop at night to shed heat from a pool that has become uncomfortably warm. Left uncontrolled it just costs you what you gained during the day.
How long does a solar pool heater take to pay for itself?
That depends entirely on what it displaces, which is why the calculator asks. Against a gas heater at a real therm price, in a climate with a long season, paybacks in the low single-digit years are common because the fuel being avoided is expensive and the hardware is cheap. Against a modern heat pump running at a COP near 5 on off-peak electricity, the displaced cost per BTU is far smaller and the payback stretches. Against nothing at all — a pool that was simply unheated and unusable in May — there is no payback to compute, only extra weeks of swimming, and that is a value judgement rather than an arithmetic one.
Do solar collectors need their own pump?
Usually not a separate pump, but they do need the existing one to work harder, because getting water onto a roof adds static lift and pipe friction to the loop. Some installations use a booster pump where the roof is high or the run is long. The calculator takes the extra input watts and the hours as inputs and nets the cost against the heat captured, which is the comparison that matters. On a variable-speed pump the added head has an outsized effect, since pump power goes with the cube of speed and the extra resistance forces the speed up. Whether any of that is worth it is a question the season-long numbers answer, not a single day.