Pool swim season in Decatur, IL
Decatur, IL: how long the pool season really is
An unheated pool in Decatur never reaches 78°F in a normal year. It peaks around 78°F in Aug. The air allows up to 5 swimmable months with a heater (May to Sep).
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Heater options for a Decatur pool, side by side
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 3 | Jul to Sep | $0 | $50 |
| Solar panels + cover | 4 | Jun to Sep | $0 | $283 |
| Heat pump + cover | 5 | May to Sep | $188 | $738 |
| Gas heater + cover | 5 | May to Sep | $222 | $647 |
For a 450 sq ft pool held at 80°F, at IL prices of 19.2¢/kWh electricity and $11/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Decatur
Solar panels + cover: 4 of 5 possible months for about $283 a year all-in, which works out near $71 for each month it adds.
Before you pay for any heater, buy a solar cover: on its own it adds 3 months here for about $50 a year (why a cover comes first). If you get quotes, ask for gas as well as a heat pump: once the purchase price is counted, gas comes out cheaper all-in here (heat pump or gas).
Weather station: Decatur Wtp, 2.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Decatur month by month: air, sun and pool water
| Month | Air high | Air avg | Sun kWh/m²/day | Water, unheated | Water, covered | Heat pump $ | Gas $ |
|---|---|---|---|---|---|---|---|
| Jan | 34° | 27° | 1.9 | 28° | 34° | – | – |
| Feb | 39° | 31° | 2.7 | 29° | 35° | – | – |
| Mar | 51° | 42° | 3.7 | 37° | 43° | – | – |
| Apr | 63° | 53° | 4.9 | 50° | 56° | – | – |
| May | 74° | 64° | 5.5 | 61° | 67° | $146 | $169 |
| Jun | 82° | 73° | 6.4 | 72° | 78° | $25 | $32 |
| Jul | 85° | 76° | 6.3 | 77° | 83° | – | – |
| Aug | 84° | 74° | 5.6 | 78° | 84° | – | – |
| Sep | 78° | 67° | 4.7 | 72° | 78° | $17 | $21 |
| Oct | 65° | 56° | 3.3 | 62° | 68° | – | – |
| Nov | 50° | 42° | 2.2 | 48° | 54° | – | – |
| Dec | 39° | 32° | 1.6 | 36° | 42° | – | – |
Heating costs most in May (about $146 with a heat pump, $169 with gas). Sunshine peaks in Jun at 6.4 kWh/m²/day (NASA satellite data, 54.3 km grid cell). Highlighted months are swimmable with no heater.
How Decatur compares with nearby cities
Decatur ranks 68 of 225 Illinois cities for unheated swim months; Mattoon nearby gets a longer season without a heater.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Taylorville, IL | 28 mi | 0 | 5 | $161 |
| Lincoln, IL | 31 mi | 0 | 5 | $211 |
| Mahomet, IL | 37 mi | 0 | 5 | $262 |
| Springfield, IL | 38 mi | 0 | 5 | $202 |
| Champaign, IL | 39 mi | 0 | 5 | $206 |
| Mattoon, IL | 40 mi | 2 | 5 | $170 |