Pool swim season in Portsmouth, NH
Portsmouth, NH: how long the pool season really is
An unheated pool in Portsmouth never reaches 78°F in a normal year. It peaks around 72°F in Aug. The air allows up to 4 swimmable months with a heater (Jun to Sep).
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Heater options for a Portsmouth pool, side by side
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 1 | Aug | $0 | $50 |
| Solar panels + cover | 3 | Jul to Sep | $0 | $283 |
| Heat pump + cover | 4 | Jun to Sep | $381 | $931 |
| Gas heater + cover | 4 | Jun to Sep | $527 | $952 |
For a 450 sq ft pool held at 80°F, at NH prices of 26.9¢/kWh electricity and $19/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Portsmouth
Solar panels + cover: 3 of 4 possible months for about $283 a year all-in, which works out near $94 for each month it adds.
Before you pay for any heater, buy a solar cover: on its own it adds 1 month here for about $50 a year (why a cover comes first). 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: Greenland, 3.6 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Portsmouth 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 | 33° | 24° | 1.7 | 26° | 32° | – | – |
| Feb | 36° | 27° | 2.5 | 25° | 31° | – | – |
| Mar | 43° | 34° | 3.7 | 31° | 37° | – | – |
| Apr | 56° | 45° | 4.7 | 42° | 48° | – | – |
| May | 66° | 55° | 5.3 | 53° | 59° | – | – |
| Jun | 75° | 65° | 5.6 | 63° | 69° | $179 | $246 |
| Jul | 81° | 70° | 5.9 | 70° | 76° | $54 | $80 |
| Aug | 79° | 69° | 5.3 | 72° | 78° | $29 | $43 |
| Sep | 72° | 62° | 4.3 | 67° | 73° | $119 | $158 |
| Oct | 60° | 50° | 2.8 | 56° | 62° | – | – |
| Nov | 48° | 40° | 1.8 | 44° | 50° | – | – |
| Dec | 38° | 30° | 1.4 | 33° | 39° | – | – |
Heating costs most in Jun (about $179 with a heat pump, $246 with gas). Sunshine peaks in Jul at 5.9 kWh/m²/day (NASA satellite data, 53.9 km grid cell). Highlighted months are swimmable with no heater.
How Portsmouth compares with nearby cities
Portsmouth ranks 7 of 11 New Hampshire cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Dover, NH | 10 mi | 0 | 4 | $418 |
| Somersworth, NH | 14 mi | 0 | 4 | $347 |
| Amesbury, MA | 17 mi | 0 | 4 | $350 |
| Newburyport, MA | 18 mi | 0 | 4 | $292 |
| Rochester, NH | 19 mi | 0 | 4 | $347 |
| Haverhill, MA | 25 mi | 0 | 4 | $356 |