It’s best to have a pool expert ultimately determine the size heater you need because there are so many factors that go into the calculation. These include the surface area of the pool and the difference between the average air temperature in your region and your desired water temperature. In addition, wind and humidity levels and nighttime temperatures can have considerable impact. For instance, if you live in an area with high wind speeds, low humidity, and cool nights, you will need a larger heater.

To minimize energy consumption, use a pool cover and choose a high-efficiency heater whether you’re purchasing a gas heater (see Swimming Pool Heaters: Gas) or a solar heater (see Swimming Pool Heaters: Solar).

Calculating for a gas heater
To roughly calculate the size heater you need, you can plug a few numbers into a formula. The first number is the pool area in square feet. Then you need a couple of temperatures—the water temperature you prefer and the average air temperature for the coldest month you will be using the pool. Subtract the air temperature from the water temperature to get the “temperature rise.”

Now, plug these numbers into the formula to find the BTUs per hour you need your heater to generate. The formula is: Pool area x temperature rise x 12 = BTUs needed per hour. How much energy you need is based on an increase of about 1 degree (or a little more) in water temperature per hour with the wind by the pool at 3.5 miles per hour.

Calculating for a solar heater
How would you make this calculation for a solar heater? It’s a little more complicated because in addition to the factors mentioned above, your site’s solar potential and the solar collector’s efficiency also come into play.

Ultimately, you need to consult a solar-system contractor. But, as a ballpark figure, the surface area of your solar collector needs to be 50 percent of the surface area of your pool if you’re in a sunny climate and 100 percent if you’re in a region with less sun. The solar collector needs to increase in size if you want your water to be warmer and if you want to lengthen your swimming season. For instance, in a sunny climate like Florida, a solar collector might need to be 100 percent of the pool’s square footage to accommodate year-round swimming.