Solar pool heating, measured — not promised.

Our pool. Everything on this page comes from the pool and spa at our own working installation. Sensors log a reading about once a minute, around the clock, into our own database — and this page reads it directly, so what you see is the raw record. Logging began June 12, 2026; the record shown here starts July 15, once sensor placement and calibration were settled — earlier readings measure the shakedown, not the system.

The controls. Two nearby unheated pools carry the same sensors — one sits in full sun, the other in shade. They show what pool water does with no heating at all. Every “boost” figure is simply the pool or spa temperature minus the shaded control’s reading at the same moment: a direct measurement, not a model or a projection.

Why we measure against the shaded control. Our pool sits in shade for most of the day, so shaded water is what this pool would actually be doing without the solar system, so that’s what we compare it against. The full-sun control is still logged — it has its own chart under “More charts” — but it describes a sunnier pool than ours. Note which way this cuts: unheated water in full sun warms itself more than unheated water in shade, so measuring against the shaded control makes the boost figures larger than they would be against the full-sun one. Both readings are in the downloadable data if you want to run the comparison the other way.

Our heating limits. These are the limits we set on our own system, not a ceiling on what solar can do: the controller stops heating once our pool nears — °F or our spa nears — °F. Peak temperatures are capped on purpose — on hot, sunny days the water would climb past those lines without them.

The gas heater. The system also has a gas heater. A gas burn warms water far faster than sunshine can, which makes it easy to spot in the data. Days the heater runs are flagged automatically, and its rise is never counted in any solar statistic. Since July 15, 2026 it has run on 15 days, all of them for the spa; our pool hasn’t used it once. On those days only the solar preheat before it fired is credited. By the time it fired, the sun had typically done 72% of the heating.

No pool cover. A solar blanket is the easiest way to make a solar heating system look better than it is — it holds heat in overnight and stops the evaporation that cools a pool all day. We don’t use one, on the pool or the spa. Every figure on this page is what the panels alone did to open water, which is also what an uncovered pool of yours would see.

Data gaps. If the logger goes quiet for more than an hour, the gap is shaded on the chart and the day is marked on the calendar instead of being smoothed over — and the header says so while it lasts.

Definitions. “Swim hours” are noon to 10 PM. A day counts as swim-ready when the pool averages — °F or warmer during swim hours.

Heat and what it would cost. We count the heat our heater would have had to supply to keep the water as warm as the panels kept it. Hour by hour, we take how far above the shaded unheated pool our water sat and multiply it by how much heat this pool loses for each degree of that lead, a figure measured from the pool’s own overnight cooling, not assumed. Our heater burns propane, so that is what we price: the U.S. average of $2.67 a gallon (winter 2025–26) in an 84% efficient heater. On natural gas, at SoCalGas’s average residential rate in January 2026 ($2.08 a therm), the same heat would cost about $1,743. On days the heater ran, only the hours before it fired are counted.