Note the swimming pool size
Measure the size of your pool's surface area. It is a general rule of thumb that your solar collectors (either panels or tubes) cover roughly the same amount of space as your pool's surface area. So if you have a larger pool, you will need more collectors to generate the desired heat.
Measure and assess your available roof space
As has been noted, you want an equal amount of roof space for your solar collectors compared to the surface area of your pool. You want your collectors on a part of your roof with minimal shade. It also must be noted that you want your solar collectors either facing South (in the Northern Hemisphere) or North (in the Southern Hemisphere) to maximize the amount of sunlight your collector receives.
Choose between panels or tubes
The shape of your roof is important here as panels are typically more effective for flatter, rectangular roofs as the total amount of sunlight during the day is higher. Tubes (sometimes called strip matting) work better on sloping, triangular roofs as this helps better with the efficiency of the water flowing through the tubes. As a general rule, black-strip tubing is cheaper and far easier to install yourself but this comes at the cost of a smaller heating capability and the fact that you will use more electricity to pump the water from the pool to the roof and back.
Assess the financial benefits
Solar heating systems typically cost between $2000 and $5000+ AUD ($1500-$3750 USD). There are cheaper systems, but these are not recommended for any but the smallest pools. Solar heating systems are usually slightly more expensive than alternative heating systems to install, but due to their low operating costs, the cost is generally made back up in savings within two years.
Check whether a solar heating system is financially viable
If your property is highly shaded or doesn't have much roof space, or if the collectors would be facing in the wrong direction, it may not be financially preferable to get a solar heating system. See 2018 Solar Pool Heater Guide
Solar heating systems typically increase heat to your pool between 5-8 degrees Celsius, but sometimes as much as 10 degrees Celsius. The aforementioned issues may cause your setup to hover around the lower mark or even struggle to reach a 5 degree gain. In these cases, alternative heating methods may be more preferable or you may decide that the environmental benefit outweighs the lower efficiency and higher initial cost.
Enlist the help of a licensed installer
This is the most important point as licensed solar panel installers will work with you and your budget to design the most affordable and efficient solar pool heating system for your property. You can buy DIY solar heating kits but these are not recommended unless you are very tech-savvy. In some states, it is illegal not to use a licensed installer so make sure to check your area's guidelines on installation. When choosing an installer, look for 10+ year warranties and brands which have been around for a long time. Considering you're investing in equipment which you'll have for at least a decade, you really don't want to be looking at the cheapest brands here. The European companies are particularly good, as they are backed by years of innovation and design. LG (from Korea) is also one of the best brands, especially when it comes to solar panels.
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Most solar pool systems are capable of heating a pool by around 5-8ᵒ C and often by as much as 10ᵒ C.
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