Pool swim season in Derby, CT
When can you swim in an outdoor pool in Derby, CT?
An unheated pool in Derby never reaches 78°F in a normal year. It peaks around 75°F in Aug. The air allows up to 4 swimmable months with a heater (Jun to Sep).
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What each heater buys you in Derby
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 2 | Jul to Aug | $0 | $50 |
| Solar panels + cover | 4 | Jun to Sep | $0 | $283 |
| Heat pump + cover | 4 | Jun to Sep | $210 | $760 |
| Gas heater + cover | 4 | Jun to Sep | $266 | $691 |
For a 450 sq ft pool held at 80°F, at CT prices of 27.5¢/kWh electricity and $17/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Derby
Solar panels + cover: 4 of 4 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 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: Ansonia 1 NE, 1.7 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Derby 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 | 36° | 27° | 1.8 | 29° | 35° | – | – |
| Feb | 38° | 29° | 2.7 | 28° | 34° | – | – |
| Mar | 47° | 37° | 3.7 | 34° | 40° | – | – |
| Apr | 58° | 48° | 4.8 | 44° | 50° | – | – |
| May | 69° | 58° | 5.4 | 55° | 61° | – | – |
| Jun | 77° | 68° | 5.9 | 66° | 72° | $126 | $161 |
| Jul | 82° | 73° | 6.0 | 73° | 79° | $12 | $17 |
| Aug | 82° | 72° | 5.3 | 75° | 81° | – | – |
| Sep | 75° | 64° | 4.3 | 70° | 76° | $72 | $88 |
| Oct | 63° | 52° | 2.9 | 58° | 64° | – | – |
| Nov | 52° | 42° | 2.0 | 46° | 52° | – | – |
| Dec | 41° | 32° | 1.5 | 36° | 42° | – | – |
Heating costs most in Jun (about $126 with a heat pump, $161 with gas). Sunshine peaks in Jul at 6.0 kWh/m²/day (NASA satellite data, 39.8 km grid cell). Highlighted months are swimmable with no heater.
How Derby compares with nearby cities
Derby ranks 16 of 21 Connecticut cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Ansonia, CT | 1 mi | 0 | 4 | $210 |
| Shelton, CT | 3 mi | 0 | 4 | $210 |
| West Haven, CT | 7 mi | 0 | 4 | $172 |
| Milford, CT | 7 mi | 0 | 4 | $119 |
| New Haven, CT | 8 mi | 0 | 4 | $172 |
| Bridgeport, CT | 11 mi | 0 | 4 | $119 |