Solar Pool Heating Panels Make Swimming Pools Hot

How Hot Can Solar Pool Heating Get?

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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.




  1. Southern California here. March 30, 2019. Night time temps 48-55. Day temps mid 70’s. A week ago our pool temp was 54 degrees. Bought a clear 12 mil solar blanket. Turned on our solar pool heater from 9am – 4pm. In a week, our pool temp has gone to 88 degrees from 54. We heat the pool from the bottom pop ups to better heat the water (warm water rises). With solar cover, and 50 degree nights, the pool temp drops approximately 5 degrees, and then during the day is 6 degrees warmer than the day before. Today I expect to go above 90 degrees pool temp. If you use a solar pool heater, get a solar cover. Ours cost 149.00, and it saves chemicals, and allows our pool not to evaporate/lose water/drop in temp significantly at night. We can get our attached spa to 100 degrees in this weather by isolating the return and suction to the pool running water to the roof solar panels. I run my pump at 1800 rpm (variable speed pump) which turns the pool over once per day and is optimal for the 4 gpm each of our solar pool panels requires to maximize heat gain, or delta T (temp difference between cooler input water, and solar output temp water).

  2. Why at night do I get very cold water coming in through my bottom drains when the pump and panels are off?

    1. Author

      Hi Ed,

      Your question is a bit perplexing, but if I am to read into it a bit, I have a potential answer… If your pool pump is off then no water should be flowing. I suspect you are talking about a very slow and gentle flow of water coming from the main drains. This could be a situation where the pool is virtually still, but warm water rises and it could be flowing into the pool jets and skimmer and cooling near the surface and returning to the main drains. I don’t see how else water could come IN through the main drains. Pools lose a LOT of heat on cool nights, so this could explain your phenomenon. Covering your pool would help.

      Other than that, I don’t have any definitive solutions for you.

      I’m not sure where you are located, but your IP address suggests Cape Coral. If that is the case, give my office a call and I would be happy to discuss or take a look.

  3. I am a new pool owner and my pool is heated with solar panels. I turned off the solar early April bc the pool was around 90 degrees. Currently the pool is about 94 degrees in the afternoon. Not refreshing. My pump runs between 10 am and 6 pm with very warm water coming out of the spickets. How do U get my water cooler? It was not this warm last year. Thank you.

    1. Author

      Hi Kathleen,

      Are you in Florida? If so, your pool is not getting to 94 without being heated by something. It sounds like your solar pool heater is still on. If you are in our area we would be happy to take a look at it. You can also send me a picture. I’ll email you with instructions.


  4. Hi Jason
    We are looking at adding a heater to our Cape Coral pool and need help. We are snowbirds and come back the day after Christmas and usually have family visiting at this same time. We would like to return to a warm comfortable pool. My better half……she will only swim when the water is close to 90 and that’s where we are stumped. Do we install an pool electric heater or electric heat pump. We realize the cost to run the on demand heater will be much higher during those couple months and would be shutdown once the solar system kicks in.

    1. Author

      Hi Thomas,

      There might be some confusion here. A “pool electric heater” and an “electric heap pump” are the same thing. A heat pump is an electric appliance that works like an air conditioner in reverse. It takes latent heat in the air and uses a compressor to heat a refrigerant that transfers that heat to the pool water with a heat exchanger.

      If you are talking about an electric resistance heater, those were used at one time in the distant past for heating spas. They are not powerful enough to heat a whole pool, and are wildly inefficient.

      If you are looking to heat your pool close to 90 degrees, you will only be able to do that about 320 days of the year. Heat pumps become ineffective when the ambient temperatures are very low. It is also very expensive to heat your pool when the ambient temperature is cool. What you desire is a tough ask. Expect to pay a very high electric bill to achieve your desired temperature in the months of January and February.

      One alternative is a gas heater. It is even more expensive to purchase gas, but it will reliably heat your pool in any weather. It also heats up faster so you can use it more as an on demand heater without having to heat your pool all the time or well in advance of use.

      With respect to a solar heater, they can run simultaneously with both heat pumps and gas heaters. A solar heater can achieve about 8-12 degrees above that of an unheated pool. So whatever auxiliary heater you choose, you will have that much less to heat your pool. We call this a hybrid system. Keep in mind that pools in Cape Coral get down to around 66 degrees in the coolest part of winter, so a solar heater alone will only achieve temperatures in the upper 70’s at best without another heat source. Still, that drastically reduces heating costs.

      Let us know if we can further help. Call our office at 239-491-8010 for an in-home or telephone consultation appointment.

  5. We are located in Ave Maria Fl and use our <600gal spa year round @100f. We have a common system new and wish to swim in <12,000g pool 10 months /year @85f. Min. We have a heat pump and multi speed circulating pump our exposure is ESE and SSW for the pool inside lanai.

    1. Author

      Hi John,

      If that’s what you are looking to do, you will need more than a solar pool heater. You will need to supplement it with your heat pump at least months of the year. See the chart in this article. Southwest Florida pools that are unheated only spend about 7 months of the year above 75ºF. Since solar pool heating adds around 10 degrees to an unheated pool on average, you will only achieve 85ºF for that period of time. Most people shoot for 80ºF and achieve 10 months above this temperature. That’s the reality of pool heating – either live with the free heat that solar provides or prepare to pay dearly for it.


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