Pool swim season in Foster City, CA
Swim season in Foster City without a heater, and with one
An unheated pool in Foster City never reaches 78°F in a normal year. It peaks around 72°F in Aug. The air allows up to 6 swimmable months with a heater (May to Oct).
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Heater options for a Foster City pool, side by side
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 0 | no month | $0 | $50 |
| Solar panels + cover | 5 | May to Sep | $0 | $283 |
| Heat pump + cover | 6 | May to Oct | $660 | $1,210 |
| Gas heater + cover | 6 | May to Oct | $884 | $1,309 |
For a 450 sq ft pool held at 80°F, at CA prices of 32.6¢/kWh electricity and $22/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Foster City
Solar panels + cover: 5 of 6 possible months for about $283 a year all-in, which works out near $57 for each month it adds.
A solar cover alone adds no swim months here, so if you want a longer season you need a heater (why); still use one at night to hold the heat you pay for. If you get quotes, ask for a heat pump sized to your pool's surface area; here it beats gas all-in (heat pump or gas).
Weather station: Redwood City, 6.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Foster City 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 | 59° | 50° | 2.5 | 49° | 55° | – | – |
| Feb | 62° | 52° | 3.4 | 51° | 57° | – | – |
| Mar | 65° | 55° | 4.6 | 55° | 61° | – | – |
| Apr | 69° | 58° | 6.0 | 59° | 65° | – | – |
| May | 73° | 62° | 6.7 | 63° | 69° | $212 | $274 |
| Jun | 79° | 66° | 7.3 | 68° | 74° | $108 | $149 |
| Jul | 81° | 69° | 6.5 | 71° | 77° | $53 | $76 |
| Aug | 81° | 69° | 5.8 | 72° | 78° | $44 | $62 |
| Sep | 79° | 67° | 5.1 | 70° | 76° | $76 | $105 |
| Oct | 75° | 62° | 4.0 | 66° | 72° | $167 | $218 |
| Nov | 65° | 54° | 2.8 | 58° | 64° | – | – |
| Dec | 59° | 49° | 2.1 | 51° | 57° | – | – |
Heating costs most in May (about $212 with a heat pump, $274 with gas). Sunshine peaks in Jun at 7.3 kWh/m²/day (NASA satellite data, 23.1 km grid cell). Highlighted months are swimmable with no heater.
How Foster City compares with nearby cities
Foster City ranks 307 of 376 California cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| San Mateo, CA | 3 mi | 0 | 5 | $731 |
| Redwood City, CA | 4 mi | 0 | 6 | $660 |
| Belmont, CA | 4 mi | 0 | 6 | $660 |
| San Carlos, CA | 5 mi | 0 | 6 | $660 |
| Hillsborough, CA | 6 mi | 0 | 5 | $731 |
| Burlingame, CA | 6 mi | 0 | 5 | $731 |