Power density: why land and space matter: How dense is energy use?

4.1 Buildings stack demand on small footprints

A single-family house in a cold climate uses roughly 10 to 50 W per square metre of floor area as an annual average, for heating, hot water, cooking, lighting and appliances. In a warm country with little heating, such as most of India, a home's average is lower, though air conditioning raises it sharply in summer. Per square metre of the ground it stands on, a one or two storey house uses about the same as its floor-area figure.

Stacking changes the picture. An apartment block of 15 floors draws 15 floors' worth of energy through one footprint. Smil notes that a high-rise office tower, with lighting, computers, lifts and air conditioning on every floor, can consume a few thousand watts per square metre of the ground it occupies. Supermarkets, with refrigeration and lighting running constantly, reach hundreds of W/m² on a single storey.

Industrial plants are the densest users. A steel mill, refinery, cement works or aluminium smelter concentrates enormous flows of fuel and electricity into compact sites. An integrated steelworks making 10 million tonnes a year uses on the order of 200 PJ of energy annually, an average of over 6 GW, and can do so on a site of perhaps 10 to 20 km², giving several hundred W/m². Individual furnaces and electrolysis halls run far higher. The 'Energy-intensive industry' tile in the Energy table lists the main ones.

The most extreme modern users are data centres. A conventional facility might draw a thousand or more watts per square metre of floor. The newest racks built for artificial intelligence draw on the order of 100 kW each on less than a square metre of floor, and individual processor chips reject heat at around 100 W per square centimetre, about a million watts per square metre. In 'Energies' Smil uses microchips as the extreme case of heat rejected per unit of area, and removing that heat has become one of the main engineering limits on computing.

• Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M8 'Power densities of energy use'
• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M8 'Power densities of energy use'

4.2 Cities use 10 to 100 W/m², far more than renewables supply on the same land

Smil puts the energy use of modern cities at roughly 10 to 100 W per square metre of built-up area, with the densest cores higher still. The figure combines two things: how much energy each person uses and how many people live on each square kilometre.

India shows how they interact. Primary energy use averages about 850 W per person (roughly 40 EJ a year shared among 1.45 billion people; calculated), a fraction of the 5,000 to 9,000 W of a typical person in a rich country. But Indian cities are very crowded. A dense core with 20,000 residents per square kilometre, each drawing about 850 W of primary energy, runs at about 17 W/m². A sprawling American suburb with 1,500 people per square kilometre each drawing 9,000 W runs at about 13 W/m². Crowding makes up for low consumption per head. As Indian incomes rise and air conditioning, cars and appliances spread, dense Indian cities will move towards the upper end of Smil's range.

Now compare supply. If a city uses 20 W/m² and solar parks deliver 10 W/m², supplying the city entirely from solar needs an area twice the city's size. At 50 W/m² of demand and wind at 1 W/m², it needs fifty times the city's area. In general, renewable supply at 1 to 10 W/m² serving demand at 10 to 100 W/m² needs land from about the same as to a hundred times larger than the city itself, and more for dense industrial districts.

Rooftops help but rarely suffice. In a dense city, roofs might cover a third of the land, of which perhaps half is usable for panels after allowing for water tanks, shading and access. If those panels average 30 W per m² of panel area in Indian sunshine, they add about 5 W per m² of city land, a quarter or a third of a dense Indian city's total energy demand. Tall, crowded cores and industrial zones will still need power brought in from outside, as they do today.

This mismatch is not new. It is the normal condition of cities. What changes in a renewable system is the kind of land that supplies them: large areas of collectors rather than small, dense mines and power plants.

• Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M8 'Power densities of energy use'
• Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M8 'Power densities of energy use'
• Energy Institute, Statistical Review of World Energy 2025 web ↗ · world primary energy 592 EJ in 2024; India roughly 40 EJ (approximate), https://www.energyinst.org/statistical-review

4.3 Every watt used ends as heat that a dense place must shed

The second law of thermodynamics guarantees that all energy used, whether in a motor, a computer, a lamp or a human body, ends as low-temperature heat. In a sparsely populated area this extra heat is trivial compared with the sunlight the ground absorbs. In a dense city it is not.

If a city core uses energy at 50 to 100 W/m², that heat is released on top of the roughly 150 to 250 W/m² of sunlight the surface absorbs on average. Add the effect of dark roofs and roads absorbing more sunlight than fields would, little vegetation to cool by evaporation and tall buildings trapping warm air, and city centres run several degrees warmer than their surroundings, especially at night. This is the urban heat island. In Indian cities it raises demand for air conditioning, which releases still more heat outside, a feedback the IEA notes is a growing part of electricity demand in hot countries.

At smaller scales heat rejection becomes a hard engineering limit. Data centres must remove all the electricity they draw as heat, and the densest AI facilities are moving from air cooling to liquid cooling piped directly to the chips. Power plants face the same issue in reverse: steam power plants reject one to two times their electrical output as heat into rivers or air, which is why they cluster on coasts and rivers.

The heat argument also cuts in favour of electrification. An electric motor or heat pump rejects far less waste heat per unit of useful service than an engine or a furnace, so an electrified city sheds less heat for the same comforts. Shading, reflective roofs, trees and water, the passive measures Smil favours, lower heat islands at almost no energy cost.

Power density thus links supply, use and disposal. The denser the use, the more energy must be brought in and the more heat must be pushed out.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M8 'Power densities of energy use'
• IEA, Electricity 2026 (February 2026) · cooling share of India's demand growth, https://www.iea.org/reports/electricity-2026 (executive summary)
• IPCC, AR6 Working Group I, Summary for Policymakers (2021) · total anthropogenic effective radiative forcing 2.72 W/m² in 2019, https://www.ipcc.ch/report/ar6/wg1/chapter/summary-for-policymakers/
• Energy Institute, Statistical Review of World Energy 2025 web ↗ · world primary energy 592 EJ in 2024; India roughly 40 EJ (approximate), https://www.energyinst.org/statistical-review

See it in the table

Check your understanding

A city uses 40 W/m² and solar parks nearby deliver 8 W/m². How much land would solar need relative to the city?

Why do dense Indian cities have energy-use densities comparable to rich-world suburbs despite much lower use per person?