Pool swim season in Charleston, IL
When can you swim in an outdoor pool in Charleston, IL?
An unheated pool in Charleston reaches 78°F in Jul to Aug, about 2 months a year. The air allows up to 5 swimmable months with a heater (May to Sep).
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What each heater buys you in Charleston
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 2 | Jul to Aug | $0 | $0 |
| Solar cover only | 4 | Jun to Sep | $0 | $50 |
| Solar panels + cover | 5 | May to Sep | $0 | $283 |
| Heat pump + cover | 5 | May to Sep | $127 | $677 |
| Gas heater + cover | 5 | May to Sep | $151 | $576 |
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 Charleston
Solar cover only: 4 of 5 possible months for about $50 a year all-in, which works out near $25 for each month it adds.
Before you pay for any heater, buy a solar cover: on its own it adds 2 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: Charleston, 0.9 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Charleston 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 | 38° | 29° | 1.9 | 31° | 37° | – | – |
| Feb | 43° | 34° | 2.7 | 31° | 37° | – | – |
| Mar | 54° | 44° | 3.7 | 40° | 46° | – | – |
| Apr | 67° | 56° | 4.9 | 52° | 58° | – | – |
| May | 77° | 66° | 5.5 | 63° | 69° | $119 | $141 |
| Jun | 86° | 74° | 6.4 | 73° | 79° | $7 | $9 |
| Jul | 88° | 77° | 6.3 | 79° | 85° | – | – |
| Aug | 87° | 76° | 5.6 | 79° | 85° | – | – |
| Sep | 81° | 69° | 4.7 | 74° | 80° | $1 | $1 |
| Oct | 69° | 57° | 3.3 | 63° | 69° | – | – |
| Nov | 54° | 45° | 2.2 | 50° | 56° | – | – |
| Dec | 42° | 34° | 1.6 | 38° | 44° | – | – |
Heating costs most in May (about $119 with a heat pump, $141 with gas). Sunshine peaks in Jun at 6.4 kWh/m²/day (NASA satellite data, 27.7 km grid cell). Highlighted months are swimmable with no heater.
How Charleston compares with nearby cities
Charleston ranks 23 of 225 Illinois cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Mattoon, IL | 10 mi | 2 | 5 | $170 |
| Effingham, IL | 32 mi | 2 | 5 | $149 |
| Terre Haute, IN | 43 mi | 2 | 6 | $215 |
| Urbana, IL | 43 mi | 0 | 5 | $206 |
| Champaign, IL | 44 mi | 0 | 5 | $206 |
| Decatur, IL | 48 mi | 0 | 5 | $188 |