Season length
Every extra week costs more than the last one
Pool heating cost is not distributed evenly across the calendar. Extending your season from May–September to April–October adds about two months and considerably more than two months' worth of money, because the months you are adding are the expensive ones. Knowing the shape of that curve is what lets you decide whether opening in April is worth it, instead of finding out from a bill.
Why the shoulders are expensive
Three things move against you at once as you step outward from midsummer. The air is cooler, so both convection and the vapour-pressure gap widen. The sun is lower and weaker, so the free solar gain that was quietly covering much of the load in July is not there. And the nights are longer, so the hours spent radiating to a cold sky go up.
None of these is dramatic on its own. Together they mean a day in April can cost several times what a day in July costs on exactly the same pool at exactly the same temperature.
The cost of your season, month by month
Change the start and end months and watch the per-day figure at the ends of the season against the middle.
Days in this season 214April through October
Heat to supply 196.18 million Btuafter the sun is credited
Season cost $1,662on a heat pump
Average cost per day $7.77the number the shoulders distort
Most expensive month Aprilby peak hourly loss
Per-day cost across the default season
| Month | Mean air | Month cost | Per day |
|---|---|---|---|
| April | 69.1 °F | $415 | $13.83 |
| May | 78.7 °F | $311 | $10.04 |
| June | 89.5 °F | $191 | $6.38 |
| July | 93.0 °F | $55 | $1.78 |
| August | 91.3 °F | $57 | $1.85 |
| September | 85.9 °F | $198 | $6.61 |
| October | 74.2 °F | $416 | $13.43 |
The per-day column is the one to read. It is the honest price of swimming on any given day in that month, and it is the number that tells you whether the last two weeks of the season are worth having.
What each extension actually costs
| Extension | Days added | Cost added | Cost per added day |
|---|---|---|---|
| May to September → April to October | 61 days | $912 | $14.95 |
| April to October → March to November | 61 days | $1,363 | $22.35 |
| March to November → February to December | 59 days | $1,869 | $31.55 |
| February to December → All year | 31 days | $1,033 | $33.33 |
Compare the last column against the season average of $7.77 a day. Each step outward is priced at a premium to the average, and the premium grows. Going to a full year in a climate that has a real winter is not an incremental decision at all — it is a different equipment conversation, usually involving a much larger heater and a cover that is genuinely always on.
Three ways to buy season length cheaper
- Lower the setpoint at the shoulders. Every degree you drop cuts the vapour-pressure gap that drives evaporation, and the effect is not linear — the top few degrees are the expensive ones. Dropping from 86 °F to 82 °F in April removes far more than a twentieth of the cost.
- Put the cover on in exactly those months. The cover page shows the saving scaling with hours on. Shoulder months have long nights, which is when a cover has the most hours available to work.
- Swim in a shorter window, warmer. Two heated weekends in April cost a fraction of a heated April, and a gas heater is the equipment that makes that pattern practical.
Why nobody publishes a season length for you
Search for what a pool costs to heat and you will be handed a season figure — a number of months, or an annual total — with no statement of how long the heater was assumed to run. That assumption is not a detail. It is the largest single term in the answer, and doubling it doubles the bill. When the Department of Energy built its own estimate of what American pool heaters consume, it set out plainly how unstable that term is:
DOE notes that the operating hours vary significantly based on several factors including geographic location (which accounts for ambient temperature conditions), consumer preference in terms of pool or spa usage (limited usage to year-round usage), installation location (indoor vs. outdoor pools), application (swimming pool only, spa only, swimming pool and spa using the pool heater), market segment (residential and commercial applications), and whether a pool cover is used, etc.
What it settles here The federal analysis needed an operating-hours figure and had to construct one from housing survey data because no such figure can be looked up for an individual pool — location, indoor against outdoor, pool against spa, and whether a cover is used all move it — which is exactly why this page asks you for a first and a last month instead of publishing a season total and calling it yours.
Two of the factors in that list are decisions rather than facts. Whether a cover is used, and how many months you choose to run, are yours to set, and between them they span most of the range of plausible answers. The largest term in your heating bill is therefore the one you control most directly.
What this does not model
The second thing it does not model is you. The arithmetic assumes the water is held at the setpoint every day of the season you set. Most people do not do that — they let it drift during a fortnight away, or drop the setpoint when nobody is swimming — and each of those habits removes cost that this page charges you for. Read the season total as an upper bound on a season run at a constant temperature, and the per-day column as the price of one more day at that temperature.