Can a data centre heat a city?

A server turns almost all the electricity it consumes into heat. The European Commission recalls this in its September 2026 report on data centres: "practically all the electricity consumed by a data centre ends up as heat". Data centres installed in France on dedicated sites consumed 4 to 5 TWh of electricity in 2024, according to RTE. The country's 1,041 heating networks delivered 28.3 TWh of heat in the same year.

Compared with this delivered heat, the 4 to 5 TWh of the dedicated sites would represent 14 to 18%. ADEME estimates the recoverable heat of French data centres at about 1.77 TWh in 2024, equivalent to 150,000 homes and about 6% of what the networks deliver, without counting the difficulties of carrying out the projects. Very little is used. The European Commission cites the readiness of the sites, the investment required, heat offered out of step with demand and the lack of takers; the report by Loup Cellard (CNRS), published in October 2026, adds the negotiation of cost sharing between the operator, the local authority and the heating network.

The air or water that cools the servers comes out lukewarm. In most French data centres, this heat comes out between 20 and 35 °C, according to the CNRS report; liquid cooling in contact with the chips brings it to about 47 °C at OVHcloud. Existing heating networks most often operate at 65 to 75 °C, and up to 80 to 110 °C for the oldest.

To go from one to the other, a heat pump is needed, which itself consumes electricity to raise the temperature. The same report notes that recovery often has a neutral or negative effect on the energy efficiency of the site, because heat pumps increase its total consumption. The climate balance then depends on the heat that the data centre's heat replaces in the network.

The case of Odense, in Denmark, is documented by the network operator. Meta's data centre there supplies its heat to the city network, and heat pumps raise it to 70 to 75 °C. According to the operator, Fjernvarme Fyn, this heat supplied about 11,000 households in January 2025. In Stockholm, the 20 suppliers to the network's open heat scheme, including data centres, delivered more than 100 GWh in 2022, equivalent to the heating of 30,000 recent flats; the scheme covered 1.5% of the network customers' needs at the time of the study, in 2023.

In Courbevoie, the sub-network that heats 14,000 homes needs 45 MW in winter and about 9 MW in summer; a 100 MW data centre located on this network could supply 20 MW all year round, according to the managing director of Generia, the heating network authority of La Défense. In summer, another outlet must be found for the heat, or it must be rejected. The European Commission makes the same observation at the scale of the Union: even in cold regions, the seasonality of demand holds back reuse.

This figure comes from the first mandatory reporting by data centres with an IT power of at least 500 kW, from May to September 2024. The 770 reporting centres represent about 36% of the centres concerned, and 6 Member States reported none. The Commission warns that the value may not be representative: some reported values seemed too high, and known projects were not reported. For France, a study carried out for ADEME by the firm Hubblo estimates that fewer than 1% of data centres make use of their heat.

Since 1 January 2026, a decree transposing the European energy efficiency directive sets the threshold for the heat recovery obligation provided for by the energy code: a data centre subject to this obligation is deemed to make use of its heat when its energy reuse factor reaches 0.20, equivalent to 20% of the energy it consumes; the threshold may be raised to 0.40 by order. A new data centre may commit to achieving it no later than 5 years after commissioning. Data centres whose permit was applied for before 2026 and which do not make use of their heat must submit a cost-benefit analysis no later than 1 October 2027. A derogation remains possible where the threshold cannot be reached under acceptable technical and economic conditions.

For us, at Projet Celsius, recovered heat does not reduce the electricity of a data centre; it replaces another source of heat, and its value for the climate depends on that source: it is greater against gas, which supplies 35% of the energy of French heating networks, than against renewables, which supply 46%. The cases where the heat is used, Odense and Stockholm, have in common a nearby network, planned with the arrival of the data centre. The location of a data centre also counts for the electricity it consumes.