Power density: why land and space matter: How dense are renewable flows?

2.1 Direct sunlight gives the densest renewable conversions

Sunlight sets the upper bound for all solar conversions. At the top of the atmosphere about 1,360 W/m² arrives on a surface facing the sun. After the atmosphere has absorbed and scattered part of it, and after night, seasons and the angle of the sun are averaged out, the ground receives roughly 100 to 250 W/m² over a year. Northern Europe sits at the low end, while most of India receives about 200 to 230 W/m², and desert regions slightly more.

The simplest conversion is also the most effective. A rooftop solar water heater can turn half or more of the light it receives into hot water. At noon this can exceed 700 W per m² of collector, and as an annual average about 40 to 100 W/m². That is why Smil regards solar heat as the densest renewable conversion and why the 'Solar heat collectors' tile describes such high efficiency.

Photovoltaics turn a smaller fraction of light into a more valuable product, electricity. A modern module converts about 21 to 23% of the light at its surface. Over a year, after accounting for the gaps between rows, inverter and wiring losses and high panel temperatures, Smil finds that large ground-mounted parks deliver about 3 to 7 W/m² in less sunny places and 7 to 11 W/m² in sunny ones. Rooftop panels avoid spacing losses on the roof area they cover: Smil cites about 12 W/m² of panel-covered roof in cloudy Germany, and sunny Indian rooftops do considerably better.

Concentrated solar power, which uses mirrors to heat a fluid, needs direct rather than diffuse sunlight and large spacing between mirrors. Its land power density is broadly similar to PV, typically under 10 W/m², with the advantage of built-in heat storage.

Solar parks are thus the densest practical way to make renewable electricity at scale, roughly ten times better than wind per unit of land fully occupied, though that land is more completely given over to the panels.

• Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M8 'Power densities of renewable flows'
• Smil, Energy Transitions: History, Requirements, Prospects (2010; 2nd ed. 2017) · via Smil Mastery Course, M8 'Power densities of renewable flows'

2.2 Wind and hydro are diffuse except at exceptional sites

A single wind turbine looks powerful, but turbines must be spaced widely. Each machine slows the air behind it, and the wake takes many rotor diameters to recover. Developers typically space turbines five to ten rotor diameters apart, which for modern machines means several hundred metres to over a kilometre. Smil finds that large US wind farms have installed capacity densities of about 2.5 to 4 W/m² over their whole area. Multiply by capacity factors of 25 to 40% and actual average output falls to around 1 W/m², and under 1 W/m² in many European farms. There is also a regional ceiling: very large arrays slow the wind itself, so the extractable power across a whole region tends towards about 1 W/m² however many turbines are added.

The saving grace is that the land between turbines is barely touched. Pads, roads and substations occupy only a few per cent of a wind farm, so crops and grazing continue. Counted on that direct footprint alone, wind's power density is high. Both views are legitimate; they answer different questions about how much land is fenced off and how much landscape the project spreads across.

Hydro varies more than any other source because it depends on the height of the fall. Alpine stations, where small reservoirs feed water down hundreds of metres, exceed 500 W/m². Giant dams on large rivers manage about 10 to 20 W/m² of capacity over their reservoirs. But many reservoirs on flatter ground flood huge areas for modest heads and operate near 1 W/m² or even less, comparable to or worse than wind, and the flooded land is lost entirely rather than shared.

Run-of-river schemes, common in the Indian Himalaya, need little reservoir area, so their land power density is high, but their output falls sharply in the dry winter months when river flow drops.

• Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M8 'Power densities of renewable flows'
• Smil, Energy Transitions: History, Requirements, Prospects (2010; 2nd ed. 2017) · via Smil Mastery Course, M8 'Power densities of renewable flows'

2.3 Biomass is the least dense of all, and geothermal the odd one out

Plants are solar collectors with a very low efficiency. Even the most productive crops convert only about 1 to 2% of the sunlight falling on a field over a year into stored chemical energy, and most natural ecosystems manage well under 1%. Water, nutrients, cold seasons and the energy plants use for their own respiration all cut into the harvest. Smil's measure of realistic tree plantations is about 0.3 to 1.5 W/m² of harvested wood energy.

Turning crops into liquid fuels loses more. Only part of the plant is usable, fermentation and distillation consume energy, and farming needs fuel and fertiliser. Smil puts US corn ethanol at no better than about 0.25 W/m², Brazilian sugarcane ethanol, one of the best cases, at about 0.4 W/m², and European biodiesel from rapeseed at only about 0.12 W/m². These are one to two orders of magnitude below solar parks on the same land. A field of PV panels produces roughly 20 to 50 times as much usable energy as the same field of crops grown for fuel.

This is why Smil judges crop-based biofuels a poor route to replacing oil at scale. India's case illustrates the tension. It reached an average 20% ethanol blend in petrol in mid-2025, five years ahead of its earlier 2030 goal. NITI Aayog's E20 roadmap put the ethanol needed at about 10 billion litres a year, with distillery capacity planned roughly half on cane juice and molasses and half on grain. Sugarcane is a thirsty crop grown in water-stressed states, and grain diversion raises questions about food. Fuel from wastes and residues, such as molasses, crop stubble or used cooking oil, is far better because it uses no extra land, but its supply is limited.

Geothermal energy breaks the renewable pattern. In a good volcanic field, heat concentrated over geological time is tapped by a few wells and a compact plant, giving about 50 to 80 W/m² of the land actually affected. But such fields are rare. The ordinary flow of heat from the Earth's interior averages less than a tenth of a watt per square metre, far too weak to use directly for power.

• Smil, Harvesting the Biosphere: What We Have Taken from Nature (2013) · via Smil Mastery Course, M8 'Power densities of renewable flows'
• Smil, Biomass Energies: Resources, Links, Constraints (1983) · via Smil Mastery Course, M8 'Power densities of renewable flows'
• Smil, Energy Myths and Realities: Bringing Science to the Energy Policy Debate (2010) · via Smil Mastery Course, M8 'Power density and the energy transition'
• NITI Aayog and MoPNG, Roadmap for Ethanol Blending in India 2020-25 (2021) · 1,016 crore litres needed for E20, summary at https://prsindia.org/policy/report-summaries/report-of-niti-aayog-on-roadmap-for-ethanol-blending-in-india-2020-25
• All India Radio News, 'India hits 20% ethanol blending in petrol 5 years ahead of target' (24 July 2025) · web

See it in the table

Check your understanding

Why is the actual power density of a typical onshore wind farm only about 1 W/m²?

Roughly how many times more usable energy does a solar park produce than the same land growing sugarcane for ethanol?

Which renewable source can exceed 500 W/m² at the best sites?