The plate sells kilowatts, but the calendar counts the days

A pool heater is bought looking at one number — the kilowatts on the plate — and judged waiting for another: the days that pass between switch-on and the first dive. In between sits the most honest physics there is: a pool does not warm up in degrees, it warms up in kilowatt-hours — it takes 1.16 kWh to raise one cubic metre of water by one degree, and no brand, no catalogue, no promise discounts them. This calculator does the count in the right order: how many kilowatt-hours the basin asks for, how many effective machine hours it takes to pour them, and how many calendar days they become with the running hours you actually have.

The formula: the kilowatt-hour per cubic metre per degree

The model is a chain of three divisions. The energy to pour is E = 1.16·V·ΔT: volume times temperature rise times the physical constant of water. The effective hours are E/P: the energy divided by the heater's delivered output — the thermal one, not the one absorbed at the socket. The calendar days are the effective hours divided by the daily running hours: the same machine that «makes it in 39 hours» takes three and a half days if it works half the day. The count is currency-free and brand-free: it holds for the heat pump, the boiler exchanger and the electric resistance — what changes is what you pay for each kWh poured, not how many kWh are needed.

The scale of the volume: the climb grows with the basin, cubic metre by cubic metre

The table grows the basin with everything else held equal (8 °C rise, 12 kW delivered, 12 hours a day):

The reading is linear and merciless: the double basin asks for double the days, machine being equal. It is why the same heat pump that makes the 20 m³ above-ground pool happy becomes a pursuit on the 120 m³ in-ground one: kilowatt-hours are not negotiated, they are poured.

A complete numerical example

Typical pool: 50 m³ — an eight-by-four in-ground — to raise by 8 °C, with a heat pump delivering 12 thermal kW and working 12 hours a day. The energy is 1.16 × 50 × 8 = 464 kWh; the effective hours are 464 / 12 = 38.7; the calendar days are 38.7 / 12 = 3.2. Stopping at the effective hours tells half the story: the machine does not work at night if the running hours are daytime ones, and the calendar — the only number the weekend guest understands — says a good three days. To close in two days it would take 19.3 kW: nearly the size above.

How to read the verdict of the days

The four bands tell four pools. Under 1 day the warm-up is lightning: event-cover size, the basin switches on like a room — and deserves the honest question whether a smaller size would do. Between 1 and 3 days the weekend warms it: switched on Thursday evening, you swim on Saturday — the physiology of the well-sized seasonal pool. Between 3 and 6 days the climb is planned: it works, but the season opening moves up the diary. From 6 days up the pump chases the pool: every night gives back to the air part of the degrees bought by day, and without a cover the pursuit may never close.

The power to make it in two days: sizing on the calendar

The calculator's fourth number flips the count: it fixes the days — two, the long weekend of the warm-up — and asks how much power is needed: P₂ = E/(2·h). It is the number to bring to the dealer, because power is sized on the calendar, not on the catalogue: the right size is not the one that consumes least or the one on offer, it is the one that brings the water to the promised temperature on the promised day. And it has a property the power table shows well: P₂ does not depend on the machine you have — only on basin, rise and running hours. It is the requirement, not the equipment.

The cover: half the heating sits above the water

The model computes the net climb: the kilowatt-hours that end up in the water. But the free surface is a loss machine — evaporation above all, then convection and night radiation — and every uncovered night gives back to the air part of the degrees bought by day. The thermal cover during the pause hours is the silent half of the heating: with the cover, the real count closely resembles the computed one; without it, prudence asks a good third of margin on the hours — and on big rises it is the cover, not the machine, that makes the difference between climbing and chasing.

International perspective: gallons, Fahrenheit and BTU

The physics of water is universal, the units are not. Whoever thinks in gallons divides by 264 and gets cubic metres; whoever measures in Fahrenheit converts the rise — not the temperature — multiplying by 5/9: a 9 °F rise is a 5 °C rise. North American plates speak BTU/h: 1 kW delivers 3,412 BTU/h, so a 100,000 BTU/h machine is a good 29 kW. The calculator's count does not change: kilowatt-hours per cubic metre per degree, everywhere — it is the clothing of the units that changes, not the body of the physics.

Limits of the model and correct use

The model photographs the warm-up, not the maintenance: keeping the basin warm is another count — the one of daily losses — and asks much less power than the climb. The power P is the delivered one: for the heat pump the thermal plate is already the delivery (the COP only says how much electricity it absorbs to produce it: 12 kW delivered at COP 5 are 2.4 kW at the socket); for the electric resistance, delivered and absorbed coincide. And the climb is half of the management: the pool turnover calculator says whether the filtration pump closes the loop, and the pool chlorine calculator keeps the water you are heating in balance — because a warm basin with still or unbalanced water is still a pool nobody swims in.

The scale of the temperature rise: where the pursuit is born

The second table turns a single knob: the degrees to gain, basin and machine being equal (50 m³, 12 kW delivered, 12 hours a day):

Here too the physics is linear — double the degrees, double the days — but the verdict is not: from 12 °C up it changes register and the cover clause enters, because on big rises the uncovered nights give back to the air a large part of what the day poured. The «might as well» two-degree raise is the dearest way to stretch the climb: it is paid in days, not in degrees.

The rise is chosen first: target temperature and season

The temperature rise is not a plate datum: it is a choice made twice. The first is the target temperature — swimming comfort sits almost always between 26 and 28 °C: every degree beyond is pleasure paid in kilowatt-hours, in evaporation and in chlorine working worse. The second is the season: the same target is 4 degrees away from June water and 12 from early-May water. It is why the same machine is «right» in July and «slow» at the opening: the pump has not changed, the rise has — and the calculator should be used with the worst rise you truly intend to ask.

The scale of the power: the column that does not move

The third table changes the machine with the request held equal (50 m³, 8 °C rise, 12 hours a day):

Two readings. The first: the days fall in proportion — the double size halves the wait, no discounts and no miracles. The second is the column that does not move: the power to make it in two days stays 19.3 kW whatever machine sits in the plant room, because it is the basin's requirement, not a property of the equipment. When the dealer asks «what power do you need?», the right answer is this column — not the plate of what is already there.

The running hours: the silent multiplier

Among the four knobs, the running hours are the only nearly free one: going from 12 to 24 hours halves the days exactly like doubling the power — without buying anything. The real limits are three: the electricity tariff's time bands, which can make the wrong hours expensive; the heat pump's night noise near the bedrooms; and the yield, because the machine that also works at night works with colder air and delivers less. For whoever has photovoltaics the reading flips: the sun hours are nearly free kilowatt-hours, and the climb is scheduled where production abounds.

Pouring free kilowatt-hours: solar and hybrid heating

The count E = 1.16·V·ΔT does not ask where the kilowatt-hours come from — and this neutrality is an opportunity. Pool solar thermal panels, or the simple black mat on the filtration circuit, pour energy at zero marginal cost: every solar kWh is a kWh the machine does not have to produce. Hybrid systems use solar as the first stage and the heat pump as the closer: the calculator stays valid — you enter as power the sum of the average deliveries during the running hours — and the climb shortens without touching the bill. On maintenance, then, solar alone is often enough: it is the climb, not the cruise, that asks for the big machine.

Typical mistakes and how to avoid them

Four recurring errors. Confusing absorbed and delivered: the heat pump reading «2.5 kW» on the energy label delivers 12 thermal ones — the delivery enters the count, and in doubt the thermal plate is the one with the BTU beside it. Ignoring the cover: the model is the net climb, and the basin uncovered at night can give back in eight hours what the machine poured in twelve. Not converting the rise in Fahrenheit: 10 °F are not 10 °C — they are 5.6. Sizing on the catalogue: the right size is born from the calendar — the two-days column — not from the current offer; the «almost right» machine costs a little less and waits much longer.

Frequently asked questions

How long does a pool take to warm up?

With the chain E = 1.16·V·ΔT, hours = E/P, days = hours/h: for 50 m³ to raise by 8 °C with 12 kW delivered and 12 running hours a day it takes 464 kWh, 38.7 effective hours and 3.2 calendar days.

How much energy does heating a pool take?

1.16 kWh per cubic metre per degree: it is the physical constant of water, equal for every brand and technology. A 50 m³ basin raised by 8 degrees asks 464 net kWh — how you pour them (heat pump, boiler, resistance) changes the cost, not the count.

What power must a pool heater have?

The one that respects your calendar: the calculator flips it into P₂ = E/(2·h), the power to close the climb in two days. The number depends only on basin, rise and running hours — it is the requirement, not the catalogue — and is compared with the delivered thermal plate.

Why calendar days and not just hours?

Because the machine does not work 24 hours: with 12 running hours, 38.7 effective hours become more than three days. The calendar is the number that matters for the season opening and the weekend guests — effective hours tell the machine, days tell the pool.

Is the cover really needed during heating?

Yes, and it is the silent half of the count: the free surface loses by evaporation and radiation, and every uncovered night gives back part of the bought degrees. The model computes the net climb: with a cover during pause hours the real count resembles it, without one a good third of margin is needed.

Heat pump, boiler or electric resistance?

For the climb time only the delivered thermal power counts: 12 kW are 12 kW. What changes is the cost of each poured kWh: the heat pump absorbs one to deliver four or five (COP), the resistance one for one, the boiler depends on the fuel. The calculator is technology-neutral on purpose.

What is the COP of a heat pump?

The ratio between delivered heat and absorbed electricity: COP 5 means 12 thermal kW with 2.4 kW at the socket. Only the delivery enters the days count; the COP enters the bill — and it drops when the air is cold, exactly when the rise to cover is biggest.

How early should I switch on for the weekend?

The verdict of the days answers directly: under 1 day the eve is enough; between 1 and 3 you switch on Thursday evening; between 3 and 6 the climb goes in the diary; above 6 it pays to rethink size, running hours or rise — or accept that the season opening starts a week earlier.

Does the calculator work with gallons and Fahrenheit?

Yes, converting: gallons divide by 264 to get cubic metres, and the rise in °F multiplies by 5/9 (a 9 °F rise = 5 °C). Plates in BTU/h divide by 3,412 to get kW. Physics does not change with units: 1.16 kWh per cubic metre per degree, everywhere.

Why does my pool never reach temperature?

Almost always because the losses match the power: basin uncovered at night, wind on the surface, a rise too big for the size. If the verdict marks 6 days or more, the machine chases: every night gives part back. A cover and more running hours pay more than one size up.

Is it cheaper to keep the pool always warm?

Two different counts: the climb is a one-off investment of E kWh, the maintenance pays the daily losses. For frequent use, maintaining (with a cover) often beats redoing the climb every time; for rare use, heating per event wins. The model computes the climb — maintenance depends on climate and cover.

What does the computed time NOT consider?

The losses during the climb (uncovered basin, wind, cold nights), the heat pump output drop with cold air, and the warming of shell and bottom at the first switch-ons. It is the net, physical climb: in the real world it is the honest lower bound — with a cover and mild weather it comes close.

Updated July 2026