Pool swim season in Mercer Island, WA
Mercer Island, WA: how long the pool season really is
An unheated pool in Mercer Island never reaches 78°F in a normal year. It peaks around 70°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 Mercer Island
| 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 | 4 | Jun to Sep | $0 | $283 |
| Heat pump + cover | 4 | Jun to Sep | $240 | $790 |
| Gas heater + cover | 4 | Jun to Sep | $582 | $1,007 |
For a 450 sq ft pool held at 80°F, at WA prices of 14.4¢/kWh electricity and $18/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Mercer Island
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.
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: Seattle Boeing Field, 4.1 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Mercer Island 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 | 49° | 43° | 1.0 | 40° | 46° | – | – |
| Feb | 51° | 44° | 1.9 | 42° | 48° | – | – |
| Mar | 55° | 47° | 2.8 | 45° | 51° | – | – |
| Apr | 60° | 52° | 4.2 | 51° | 57° | – | – |
| May | 67° | 58° | 5.2 | 57° | 63° | – | – |
| Jun | 71° | 63° | 5.7 | 63° | 69° | $95 | $225 |
| Jul | 77° | 67° | 6.3 | 68° | 74° | $46 | $116 |
| Aug | 77° | 68° | 5.4 | 70° | 76° | $32 | $82 |
| Sep | 72° | 63° | 3.9 | 66° | 72° | $67 | $159 |
| Oct | 61° | 54° | 2.3 | 58° | 64° | – | – |
| Nov | 53° | 46° | 1.2 | 48° | 54° | – | – |
| Dec | 48° | 42° | 0.9 | 42° | 48° | – | – |
Heating costs most in Jun (about $95 with a heat pump, $225 with gas). Sunshine peaks in Jul at 6.3 kWh/m²/day (NASA satellite data, 21.4 km grid cell). Highlighted months are swimmable with no heater.
How Mercer Island compares with nearby cities
Mercer Island ranks 24 of 91 Washington cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Newcastle, WA | 4 mi | 0 | 4 | $249 |
| Bellevue, WA | 4 mi | 0 | 4 | $249 |
| Renton, WA | 6 mi | 0 | 4 | $249 |
| Tukwila, WA | 6 mi | 0 | 4 | $255 |
| Seattle, WA | 7 mi | 0 | 4 | $240 |
| Burien, WA | 8 mi | 0 | 4 | $255 |