Geothermal heating: 5,000 kWh in real use vs the brochure COP

Geothermal: 5,000 kWh a year versus the brochure COP

The sales pitch promises COP 5, COP 6, efficiency figures that multiply every kilowatt consumed by six. The real measurement, taken with a dedicated meter over twelve months, tells a different story. A home in the Madrid mountains, with underfloor heating, cooling and domestic hot water, closed a full year with 5,000 kWh of electricity as the total consumption of the system. Not a single month of gas. Not a single backup boiler.

The figure is not a catalogue estimate. It is the reading of a meter installed in February 2016 and read in February 2017, which adds up the heat pump, the circulation pump of the external loop, two internal pumps and a fifth one for the hot water return. The gap between what is sold and what is measured is the heart of this debate, which has been going on for years and still has no official answer.

How much does a geothermal heating system really use?

The COP calculation is where the story falls apart. If you divide the machine's nominal heating output by the electricity consumed, the result comes out low, because the pump does not always run at 100%. To get the real instantaneous COP you need a flow meter at the heat pump inlet, something almost no domestic installation includes. Without that flow rate, you only have inlet and outlet temperatures, and that is not enough to close the equation.

Some argue that the COP of a non-inverter pump, which always runs at 100% when it starts, can indeed be calculated against its nominal output. In that case, the measured performance falls well short of the brochure. On the opposite side, others argue that a modulating machine with return water at 30 °C and 17 °C outside works effortlessly, with barely any noticeable consumption.

The underlying problem is that geothermal cannot be judged on COP alone. You have to add the circulation pump of the external loop, which moves the fluid through the boreholes, and the internal distribution pumps. All of that consumes electricity and never shows up in the commercial comparisons.

The cost of drilling versus the annual savings

Here is the calculation nobody wants to say out loud. Two 150-metre boreholes for a single-family home were quoted at almost 9,000 euros. The difference between a geothermal installation and a gas condensing boiler was estimated at 13,000 euros. With annual savings counted in the hundreds of euros, payback stretches over decades.

The rule of thumb among installers is clear: one 150-metre borehole for every 125 m² of floor area to be heated. That means a 240 m² house needs two boreholes, and the upfront cost shoots up. In areas without natural gas, the comparison changes, but the gap is still a five-figure sum.

The argument of the defenders is that geothermal does not only heat: it also cools in summer and produces hot water all year round, with ground collectors that outlast any boiler. The counter-argument is that the theoretical savings amount to nothing if the system is poorly sized, and that a miscalculation in the collectors can leave the ground thermally damaged for life.

Air-source heat pump on a geothermal schedule: the trick that cuts the bill

The alternative gaining ground is to use an air-source heat pump as if it were geothermal. That is, taking advantage of the midday hours, when the outdoor temperature approaches that of the ground, to heat the water. A documented case with 240 m² spread over three floors recorded 2.90 euros per day of consumption, with supply at 38 °C and return at 31 °C, switching on at 13:00 and off at 23:00.

The trick is not magic: it is thermodynamics. At 13:00, even in winter, the outdoor temperature can be 13 °C, and the COP soars. The modulating machine only makes noise on start-up or in extreme cold; once the return temperature is reached, consumption drops. Those who use this strategy say the condenser noise is a minor issue compared to the savings.

The contrast with pure geothermal is interesting. Air-source avoids the boreholes, but depends on the outdoor temperature. Geothermal is stable, but requires an upfront investment many never recover. The open question is whether the COP difference between the two justifies the cost of the wells.

Real monthly consumption of a geothermal installation

The monthly data from a real installation show the consumption profile. In October, with fifteen days of operation, the cost was 34 euros. In November, twenty-four days with six days off, 57 euros. In December, thirty-one days of operation, 112.5 euros, although the owner himself admits the Christmas dinners inflated the figure and that 90 euros would have been expected.

These are figures including taxes and heating only. Annualised, the total comes to around 600 euros for heating, cooling and hot water in a 150 m² home. Compared with the 90 euros a month of natural gas in a 150 m² flat in a village near Madrid, geothermal does not win on operating cost. It wins on independence and summer cooling.

The comparison with heating oil or bottled gas does favour geothermal, mainly because it removes the cost of storage tanks and dependence on external deliveries. But natural gas, where available, remains unbeatable in cost per kilowatt hour.

The sizing mistake that sends the bill soaring

There is a recurring pattern: oversized installations that force customers to contract absurd electrical capacities. One documented case describes how the customer was expected to contract a 10 kW fixed supply so as not to void the machine's warranty. The owner refused, contracted 5.5 kW and has gone years without the limiter tripping.

The same pattern appears in another 150 m² home where two 14 kW machines were installed. That is not heating, it is nonsense. The sizing tables circulating in the industry, according to this experience, are designed to dazzle and do not hold up in practice.

The conclusion drawn from real cases is that the performance of a geothermal system depends less on the machine and more on the sizing, the insulation of the home and the scheduling. A well-calculated, well-managed system can get close to 600 euros a year. A badly sized one can ruin both the bill and the owner's patience.

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With these numbers on the table, geothermal stops being a technological promise and becomes an investment equation. Is it worth paying 13,000 euros more to save a few hundred a year? It depends on whether there is natural gas on your street, on whether you value summer cooling, and on whether the installer gets the boreholes right. The answer, as almost always, is not in the brochure.