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Sizing Your Solar Pool Heater

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What is an appropriately sized solar heater and what can you expect it to really do in terms of pool temperature and season? That's been the toughest question to answer this industry has ever faced. We've made enormous efforts over the last 2 decades in collaboration with many gov't and research groups to be able to give you an accurate answer. We've achieved that goal and we urge you to read this page and study the graphs. We urge all those we've worked with especially the government of Canada who financed a lot of this work (we didn't get any money) to take notice of where we've taken this body of work since the funding dried up and put some more money into it. There are still a lot of uncertainies.

Heat loss and gain to the pool is all based on the top surface area of the pool. There is an enormous amount of energy lost each day from a pool. Evaporation off the top surface is a huge factor. Direct gain to the pool by the sun is a big part of the heat gain. Wind loss, shade, are all proportional to the top surface area of the pool. The depth only affects the temperature fluctuation daily not the amount we "bump" the pool temperature up.

Measure top surface area (average length times average width) and enter it here in square feet.

  
The first chart below shows a typical solar heated pool in San Diego, California with a properly sized system. Note that if you are going to use a cover we're normally recommending 75% of pool area in collector area assuming the collectors can face south and there are no unusual shade or wind issues. Note the "bump" in pool temperature this provides. This kind of sizing will raise the pool temperature 10-15 degrees. As you can see that is enough to provide a fully useable pool for a longer season than you would normally achieve using a gas heater

Similar charts for your area can be found under "What to Expect in Your Region" on the navigation bar to the left or below. It doesn't matter much where you are, the proportion of pool area in collector area that makes sense is usually the same because the "bump" in pool temperature you're after is probably the same. You can adjust this factor up or down proportionately depending on what you want to achieve. For example if you are only looking for a 5 degree bump instead of 10-15 then you can drop this factor from 1 or 0.75 down to 0.5 or 0.33. Power is proportional to the surface area exposed to the sun and the bump in pool temperature is proportional to the power. Note that without a cover our standard base sizing is 100% (enter 1 in the box). Look at the season of pool use you can achieve at no heating cost and without the hassle or esthetics or safety issues of a cover! That shows why solar pool heating is the only way to go for so many pool owners. You wouldn't heat a pool with gas and leave it uncovered in the fringe season months. You would with solar.

Enter starting factor of pool area 0.75 or 1.00.
Don't use percent symbols, please.

  • Similar Sizing Charts For Your Region

    • Direction of Roof Factor

    The direction the collectors face is an important factor too of course. If the collectors are facing north they are facing away from the sun. Don't even go there. Keep the collectors facing somewhere between true east and west. In the sun and facing the sun are not the same thing.

    The best way to determine the roof orientation is to look at your house on google earth or a map. The orientation is all relative to true south/north not magnetic south. A compass on the other hand shows magnetic directions which are different. The middle of the solar day is when the sun is directly true south. Don't even consider roof spaces facing north of the true east/west line. You'll be disappointed.

    		  
    ROOF DIRECTION CORRECTION FACTORS
     
      Shallow pitch
    1 to 3 in 12    		 Normal pitch
    4 to 7 in 12    		 Moderate pitch
    8 to 11 in 12    		 Steep pitch
    12 in 12 (45 degrees)
    True South Facing 1.00 1.00 1.00 1.00
    True SW or SE 1.00 1.05 1.07 1.10
    True ESE or WSW 1.05 1.15 1.40 1.70
    True E or W 1.10 1.25 1.75 2.00

    Using the table to the left enter an appropriate adjustment factor for roof direction here:

     

    Wind Effect

    If you're right on the ocean you will have a higher heating requirement because the pool gets cooled so much by the wind. Solar collectors also have a wind cooling effect so look at where they are being located. Normally wind is not a factor we have to consider (enter 1) so just beware of usually windy situations. Flat roofs protect the solar collectors from wind loss. Use 1 for no wind factor and 1.2 if you think the collectors will see cool wind during the early and late season when you really need the power. Enter 1.5 if you're right on the ocean worst case.The Enerpool program that we used to come up with all of our sizing quantifications and charts for your region was verified and wind effects studied but never in enough detail to really be able to evaluate this effect in more extreme cases. If the collectors are wind protected, your solar heater will perform noticeably better if the air temperature is significantly colder than the pool temperature. Its mostly a big deal when there is direct ocean exposure

    The design of this type of fin tube style collector affords some natural wind protection as depicted in these examples. There are other reasons to use different profiles but for best performance next to the ocean we swear by the fin tube design all our products employ.On tile roofs where Powerstrip follows the wave of the tile, we also benefit from the fact the collector surface is hidden from the wind cooling effect.

    Factor for wind:

    Diffuse Radiation

    If you're down in a canyon or surrounded by lush forest you aren't getting as much reflected or "diffuse" solar radiation on the pool or the collectors. The diffuse component of solar radiation can be 30% of the total. If you have to squint on sunny days then this factor is just 1 but if its dark you should factor up by as much as 1.3 to make up for it.

    Factor for diffuse radiation: 

    Shade

    Shade on the pool is a huge factor. For every square foot of shade on the pool you need 75% of that area in solar collector area to make up for it. If the pool is half shaded for half the day then it is 25% shaded. The factor to input here is 75% of 25% which is 19% so use a factor of 1.19. Only consider shade in the middle 6 hours of the solar day.

    Shade factor:

    Calculate: Pressing "Calculate" will multiply all the above factors together and multiply by the top surface area of the pool. This is your desired total solar surface area.

    sq. ft.

    Now that you know how many sq ft of solar collector you want ideally, measure your roof space. You may have to scale down your expectations or consider a different space. Remember it takes more than 500 sq ft to accomodate 500 sq ft of solar collectors. Now go to our "Buyer's Guide" where you can specify the type of solar heater that makes sense for your situation and cost it all out.

    Buyer's Guide

    If you increase the sizing because you have the space and the budget, what happens? Is there any point? As the plot below demonstrates, absolutely! 8 collectors is the "right" sizing for this partriculoar example pool but extra collectors aren't a waste of money. Results are for a covered pool open 4 hours a day for pool use.

     

    The above graphs are created using data from computer simulations but not just any computer simulation. We've been involved in extensive development of this capability with the help of universities and government groups for almost 2 decades. The simulation has been verified with high level monitoring of real systems. Click here to learn about the software and the extensive effort behind creating these curves.

    The biggest draw to solar is that it completely changes the way you use your pool. With solar you start the pool very early in the year and even in early spring months when good weather strikes, your pool can be useable months before the neighbors with gas heaters or heat pumps have dared to fire up the beasts. The use of a cover in the early and late months will give the ultimate pool heating experience. If you only need the cover for the fringe season months when pool use is lowest, you really don't have to fuss much with that cover. Just leave it on all week and off all weekend if you use the pool. Solar with the occasional help of a cover on the pool negates the need to ever turn on a gas heater. The only time you'd want to run a gas heater would be a time when weather was unusually poor or you were so far off season the pool use wouldn't justify the gas use. At these times the cost to heat with gas will be highest. That's why solar and a cover used occasionally is the ultimate pool heating solution for your home if you have enough space for a full sized solar heater.

    Go thru the buying guide to give us the info we need on your pool so we will be to discuss it with you specifically.

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