The number one question we get (by far) is, “how hot can solar pool heating get?” It’s obviously a good question. Mistruths from competing technology dealers can skew perceptions about solar heating performance. How much heat depends on a lot of factors. It is a weather dependent technology (but so are other heaters in different ways).
Before we get into how hot your pool can get, it’s important to understand how any pool heater works, whether solar, electric, or gas. Most people only run their pool circulation pump during the day. Therefore, regardless of heat source, your pool only gets heated during the day. At night when the pump is off, your pool cools down. The extent to which cooling occurs depends on the ambient temperature, wind, and other factors.
No matter what heat source you use, your pool will be cooler in the morning than in the afternoon. It takes time for all heaters to heat a massive body of water. Think about how long it takes a gallon of water to boil on the stove. It needs to be raised about 140 degrees to boil. That’s 140 gallon-degrees. A typical 12,000 gallon pool needs to go up about 10 degrees for comfortable swimming for much of the year. That’s 120,000 gallon-degrees, or about 850 times the energy needed to boil that pot of water. That’s a LOT of energy! We don’t really use gallon-degrees as a unit of measure, but BTUs (British Thermal Units) are the standard performance metric used.
But all you really care about is how hot the pool will get using a solar heater, right?
Electric heaters (pool heat pumps) are weather dependent in the respect that they work faster and more efficiently when the weather is relatively good. When it gets very cold, heat pumps don’t work at all. They simply can’t extract any heat from the ambient air. Both electric and gas heaters, like solar heaters, are in a constant battle against heat loss, mostly at the surface of the pool.
All heaters for pools are there simply to replace heat lost to the adjacent air.
So How Hot Will a Solar Pool Heater Get My Pool?
You may be asking this question as a new pool owner, so we have to step back and look at how an unheated pool acts. Unheated pools are not the same temperature year-round, and the pool temperature can be drastically cooler than ambient air temperatures. That’s because cool nights cause pools to lose heat. In fact, they lose heat to the cold night much faster than they recover in warm days. Cold nights make pools cold quickly, whether heated or unheated.
People are surprised that their pools are so cold sometimes. Today (December 19, 2016) we broke a record here in Fort Myers with daytime temperatures reaching 91ºF. However, unheated pools only reached 73-74ºF! That’s just the reality of pool temperatures this time of year.
Pools in the area reached 82-88ºF when heated with our solar panels today! The amount of heat depends on a lot of factors, most importantly coverage relative to your pool size, orientation, and shading.
Unheated swimming pools can get as cold as 66ºF in Southwest Florida. These cold snaps are usually short in duration, and you probably don’t want to swim on these days anyway, which are usually cold and pool weather. Solar heated pools during these cold snaps reach 74ºF – 78ºF typically. This is pretty cool for most people, but definitely swimmable.
In the spring and fall, unheated pools usually hover in the upper 70’s. Solar heated pools can be in the upper 80’s, and people start turning solar heaters off at times.
In the summer, solar heaters are rarely used, as unheated pools can reach into the upper 80’s naturally.
The short answer you have been waiting for is that solar heated pools with ideal systems usually reach 8-12 degrees above an unheated pool. It can be less or more depending on weather.
Unheated pools in Southwest Florida are very close in temperature to the nearby Gulf of Mexico surface temperature. The chart below takes NOAA data and plots the average monthly Gulf temperature.
The bottom line is that unheated pools only spend about 6 months above 80ºF and solar heated pools achieve this temperature about 10 months of the year. That greatly extends your swimming season, and temperatures will be well into the 80’s (or 90’s if desired), for several months of the year.
And it’s all free after the initial installation cost! You can’t say that about electric or gas heaters.
Now weather can be a funny thing. It was 91ºF today. In another year on this day it could have been 65ºF outside. We can’t tell you what temperature your heated (or unheated) pool will be on December 19 of any given year. We can tell you that a solar heated pool will be substantially warmer and you will have a much better chance of comfortable swimming.
Despite today’s 91ºF day, you can’t expect a pool to be 91ºF. It takes a lot of energy to recover nighttime heat loss this time of year. With unheated pools hovering around 74ºF, a solar heated pool in the mid-80’s is performing amazing feats!
Doing that with electricity or gas would be expensive, and it will all evaporate the next evening (quite literally).
Your experience with solar pool heating panels, or any pool heater, will be much better if you understand how pool heaters work, the limitations, the costs, and the realities of maintaining heat inside open bodies of water. Swimming pools are almost always outside. Therefore, we are at the mercy of the weather. If you are realistic about when you can enjoy your pool, a solar pool heater will be a great investment that will drastically increase your pool use and enjoyment.