Pool swim season in St. Charles, MO
Swim season in St. Charles without a heater, and with one
An unheated pool in St. Charles 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 St. Charles
| 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 | $94 | $644 |
| Gas heater + cover | 5 | May to Sep | $202 | $627 |
For a 450 sq ft pool held at 80°F, at MO prices of 14.3¢/kWh electricity and $15/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in St. Charles
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: St Charles Elm Pt, 1.3 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
St. Charles 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 | 40° | 31° | 2.1 | 32° | 38° | – | – |
| Feb | 45° | 35° | 2.8 | 33° | 39° | – | – |
| Mar | 55° | 44° | 3.8 | 41° | 47° | – | – |
| Apr | 67° | 55° | 5.0 | 52° | 58° | – | – |
| May | 76° | 66° | 5.6 | 63° | 69° | $89 | $189 |
| Jun | 84° | 74° | 6.4 | 73° | 79° | $5 | $13 |
| Jul | 88° | 78° | 6.3 | 80° | 86° | – | – |
| Aug | 87° | 76° | 5.6 | 80° | 86° | – | – |
| Sep | 80° | 69° | 4.8 | 74° | 80° | – | – |
| Oct | 69° | 57° | 3.4 | 64° | 70° | – | – |
| Nov | 55° | 45° | 2.4 | 51° | 57° | – | – |
| Dec | 44° | 35° | 1.8 | 39° | 45° | – | – |
Heating costs most in May (about $89 with a heat pump, $189 with gas). Sunshine peaks in Jun at 6.4 kWh/m²/day (NASA satellite data, 32.9 km grid cell). Highlighted months are swimmable with no heater.
How St. Charles compares with nearby cities
St. Charles ranks 52 of 80 Missouri cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| St. Peters, MO | 5 mi | 2 | 5 | $102 |
| Bridgeton, MO | 6 mi | 2 | 5 | $71 |
| Maryland Heights, MO | 6 mi | 2 | 5 | $80 |
| Hazelwood, MO | 6 mi | 2 | 5 | $71 |
| St. Ann, MO | 8 mi | 2 | 5 | $71 |
| Creve Coeur, MO | 10 mi | 2 | 5 | $80 |