Heating a pool: solar, heat pump or gas

Almost every heating quotation starts with the equipment. Starting there is starting at the end, and it is why so many people pay for a heat pump they then never switch on, because the first bill frightened them.

One sentence puts the whole problem in order: a pool is not heated, it is stopped from cooling. Warm water loses heat three ways — evaporation, convection to the air, and radiation to the night sky — and in an uncovered pool the first takes most of it. Whatever you install is competing against that loss, every day, forever.

First: how much energy this actually is

One constant settles half of these arguments, and it depends on no rule and no supplier: raising one cubic metre of water by one degree Celsius takes 1.163 kWh. That is the specific heat of water, and nobody negotiates it.

With an efficient heat pump, those 419 kWh of heat cost in the order of $80,000 in electricity at August 2026 tariffs. That sounds perfectly bearable, and it is. Raising the temperature is not the problem: holding it is.

Evaporation takes nearly all of it

When a kilogram of water evaporates, it carries away the heat it took to evaporate it: around 2,430 kJ. Translated into the unit on your bill, evaporating one millimetre of depth over one square metre costs 0.675 kWh. Now multiply.

Eighty-six kWh a day is 2,592 kWh a month. Set that beside the 419 kWh of raising the temperature once and you can see where the cost really sits: not in heating, in replacing.

A thermal cover is the cheapest heating system there is

A floating cover on the water surface heats nothing. What it does is cut evaporation, which is where those 2,592 kWh a month come from. Every percentage point of evaporation it prevents is a percentage point your equipment does not have to replace, on any fuel and at any tariff.

The three systems, compared per useful kWh

Comparing gas equipment with electric equipment needs a common unit, and the only honest one is the kilowatt hour that actually enters the water. Every tariff below is from EPM, all from Medellín and all from August 2026: same utility, same city, same month. Mixing markets would make the table fuller and completely useless.

Taken to a full month on the reference pool, uncovered: an electric element would cost in the order of $2.49 million, gas between $0.90 and $1.09 million, and a heat pump at COP 5 about $498,000. With the surface covered, all of it divides down. That is the whole article in two sentences.

Gas: why a cubic metre is not always worth the same

You are billed volume, but what you buy is energy, and how much energy a cubic metre carries depends on the composition of the gas. The rule governing transport puts bounds on it: the gross calorific value of gas delivered to the transporter must sit between 35.4 MJ/m³ and 42.8 MJ/m³ — between 950 and 1,150 BTU per cubic foot.

That is between 9.8 and 11.9 kWh per cubic metre, and it is where the range in the table comes from. The figures above use the low end, which is the conservative assumption against gas. And the tariff changes from market to market: the same cubic metre billed at $3,517.06 in Medellín costs $3,125.89 in Valle and northern Cauca in the same month. Redo the sum with the number on your own bill, not with the one in this article.

The heat pump: the COP nobody verifies

The coefficient of performance — the COP — says how many kilowatt hours of heat the unit delivers for each kilowatt hour of electricity it draws. A COP of 5 means it moves five times more heat than it spends. It is real, and it is why the heat pump wins the table comfortably.

That matters because COP is not a fixed number: it falls as air temperature falls, and air is the source the unit draws its heat from. A COP measured with air at 27 °C is not the one you will have at five in the morning on the Bogotá savannah. Always ask for the test condition alongside the COP: without it the number means nothing. And if you are sold a COP without being told at what air and water temperature it was measured, you know what it is worth.

Solar: the sum that gives you the collector area

Colombia receives, on average, in the order of 4.5 kWh per square metre per day of global solar radiation, according to the IDEAM and UPME atlas; on the Bogotá savannah the average is somewhat below that and on the Caribbean coast considerably above.

A pool collector — the unglazed polypropylene kind, working at a temperature very close to ambient — captures a high fraction of it. Assuming 60% as a daily average, replacing the 86 kWh evaporation carries off calls for 86.4 ÷ (4.5 × 0.6) ≈ 32 m² of collector.

Altitude decides before the equipment does

This is the part that saves the most money and is almost never asked. In Cali, in Jamundí or on the coast, an uncovered, well-oriented pool settles on its own at a temperature many people find perfectly comfortable, and what is needed is not a heater but a cover for the nights. On the Bogotá savannah the night air is another thing and the balance changes completely: there really is a heating problem there, and there the cover stops being optional. It is the same logic as the bioclimatic house by altitude band, applied to 45 m³ of water.

The right order of decisions

1. 1Decide the temperature you want, and for how many months of the year. 26 °C at weekends is not 30 °C year-round, and the difference on the bill is several times over.
2. 2Settle the cover. It is the cheapest heating system there is and it sets the size of everything else.
3. 3Look at the site before the catalogue. Orientation, shade and wind decide a good part of the balance at no cost at all.
4. 4If there is clear, well-oriented roof, fit solar. It is the cheapest kWh there is and it supplements any other system.
5. 5To hold the temperature, a heat pump, asking for the COP with its test condition in writing.
6. 6Keep gas for fast warm-up, for back-up and for cold climates, where a heat pump loses performance just when most is asked of it.

The person who leads this line of work at the practice brings fifteen years of designing hydraulic systems and bodies of water, and the heating lesson that recurs most is this: almost no temperature problem is solved with a bigger machine. It is solved by covering the pool, correcting the siting, and sizing against the real loss rather than the volume. A big machine on an uncovered pool is a big bill, and nothing else.