The Arithmetic of Sustainability - Part 2: The Red Stack

The vast appetite of affluent societies, quantified as the “red stack” of consumption—approximately 125 kWh per day per person in Britain—is overwhelmingly fueled by transport and heating. For the typical car driver, road transport alone consumes about 40 kWh per day. Successfully migrating away from fossil fuels and achieving the drastic emissions reductions required (potentially greater than 85% for Britain) demands a fundamental, numerically sound strategy for tackling this consumption pillar.

This strategy must pursue two critical goals simultaneously: first, delivering the biggest possible reduction in transport’s energy use, and second, achieving the elimination of fossil fuel use in the sector entirely. Arithmetic, not emotion, is essential for identifying solutions that deliver big results.

The eventual path to sustainable surface transport hinges on electrification, a technological shift that provides a radical increase in efficiency, dramatically reducing the energy consumed per kilometer travelled.

The Arithmetic of the Road: Why Petrol Cars Are Inherently Wasteful

To understand how to reduce consumption, one must first grasp the physics of where the energy goes in a standard fossil-fuel car.

75% of fuel energy is lost as waste heat in petrol cars

The overwhelming truth about a standard fossil-fuel car is its inherent inefficiency: approximately 75% of the fuel energy is lost as waste heat generated by the engine and radiator. Only about 25% of the fuel’s energy is converted into useful motion. Therefore, the first and most critical strategy for consuming less energy is to make the entire energy-conversion chain far more efficient.

Beyond the engine losses, the remaining useful 25% of energy is spent overcoming two primary forms of resistance: kinetic energy loss (due to stopping and starting) and air resistance (drag). The dominance of one factor over the other depends entirely on the driving scenario.

1. City Driving: Braking Dominated

In short-distance travel characterized by frequent starts and stops, most energy is directed toward accelerating the vehicle, which is then wasted as heat in the brakes when the vehicle slows down. This scenario is “braking-dominated” when the distance between stops is less than about 750 meters.

In this environment, energy consumption is highly dependent on the vehicle’s mass. Key strategies for saving energy in city driving include:

• Reducing the mass of the car
• Utilizing regenerative braking, which can capture and reuse the kinetic energy otherwise lost
• Driving more slowly and going further between stops

2. Motorway Driving: Drag Dominated

When traveling long distances at a high, steady speed, the energy expenditure is dominated by the need to overcome air resistance. The car pushes air aside, leaving a swirling tube of kinetic energy behind it.

The physics of drag reveals a highly sensitive relationship to speed: the power dissipated by the vehicle scales roughly as the cube of the speed ($P \propto v^3$).

• Reducing the speed by half reduces the power consumption by a factor of eight, meaning the total energy consumed to cover the same distance is four times smaller.
• In this drag-dominated scenario, the weight of the car matters much less.
• Key strategies here are reducing the car’s cross-sectional area and, most effectively, driving more slowly.

Shared Journeys: The Efficiency of Trains and Buses

The foundational principles of efficient surface transport—reducing frontal area per person, reducing mass per person, and reducing speed—can be dramatically satisfied through public transport. Public transport systems exploit the principle of shared volume, resulting in an inherently smaller frontal area per passenger compared to a car.

Rail

Electric trains are highly energy efficient and offer independence from fossil fuels. A full commuter train has a frontal area per passenger of just 0.02 m².

• An average electric high-speed train, if full, uses just 3 kWh per 100 passenger-km. This makes it 27 times smaller in energy cost than the baseline single-occupancy car (80 kWh per 100 km).
• London Underground trains, at peak times, achieve 4.4 kWh per 100 passenger-km.
• The total energy cost of London’s entire Underground system in 2006–07 was 15 kWh per 100 passenger-km—five times better than the baseline car.

Buses and Coaches

A diesel-powered coach carrying 49 passengers can be 13 times better than a single-occupancy car, using just 6 kWh per 100 passenger-km. London buses, however, had an average energy cost of 32 kWh per 100 passenger-km in 2006–07, highlighting that occupancy rates significantly affect real-world efficiency.

Cycling

A bicycle is one of the most efficient forms of transport, achieving roughly 1.6 to 2.4 kWh per 100 km. This is at least 25 times more efficient than a standard petrol car. Walking uses about 22 times less energy per distance than a car.

Despite the efficiency advantages, public transport adoption faces the hurdle of preference: many people still demand the flexibility of a private vehicle.

The Technological Leap: Electric Vehicles

If individuals refuse to abandon private transport, the only way to achieve massive energy savings and decarbonization is through a radical technology shift: electrification.

The electric vehicle (EV) overcomes the major efficiency flaw of the fossil car by eliminating the 75% heat loss associated with the internal combustion engine. Electric motors are inherently far more efficient.

EVs achieve a 5x efficiency improvement over fossil cars (15 kWh vs 80 kWh per 100 km)

EVs can deliver transport at an energy cost of roughly 15 kWh per 100 km. This is a five-fold improvement over the baseline fossil car (80 kWh per 100 km) and superior to current hybrid technology.

Electric Vehicle Performance Metrics

EV performance varies by design and use case, but even high-performance models demonstrate efficiency:

The energy consumption savings are so significant that they make solo electric travel economically viable as a cornerstone of a sustainable energy plan.

Regenerative Braking: Capturing Kinetic Energy

Electrification also enables regenerative braking, crucial for maximizing efficiency in city driving where kinetic energy loss dominates. Regenerative braking, using an electric generator coupled to the wheels, can charge a battery or supercapacitor.

• It typically salvages roughly 50% of the kinetic energy in a braking event.
• This feature alone can lead to an estimated 20% reduction in the energy cost of city driving.
• Systems using flywheels and hydraulics can salvage at least 70% of the braking energy.

The False Hope of Hydrogen and Limits of Hybrids

When discussing methods to replace fossil fuels, several technologies are often hyped. For surface transport, hydrogen fuel cells and existing hybrids offer a comparison point against pure electrification.

Hydrogen: An Inefficient Carrier

Hydrogen is not a source of energy; it is merely an energy carrier, like a rechargeable battery. The critical flaw in promoting a “hydrogen economy” is the massive inefficiency of converting energy into, and back out of, hydrogen.

The available data on hydrogen vehicles confirm this high energy cost:

• The BMW Hydrogen 7 consumed 254 kWh per 100 km. This staggering figure is 220% more energy than the average European car uses.
• This makes the Hydrogen 7 over ten times less energy-efficient than the electric Tesla Roadster.

If limitless green electricity were available for free, inefficient solutions like hydrogen might be contenders, but acquiring green electricity on the required scale is already extremely challenging.

Hybrids and Diesel Efficiency

Hybrid cars (combining a fossil engine with an electric motor and regenerative braking), such as the Toyota Prius, are often perceived as highly efficient. While they do offer improved fuel efficiency, they don’t represent a radical leap forward compared to the best modern diesel cars.

• The hybrid Prius emits about 100 g of CO₂ per km, but several non-hybrid vehicles, like the VW Polo BlueMotion, also achieve similar low emissions (99 g/km).
• Some luxury hybrids, like the Lexus RX 400h, have emissions (192 g/km) that are worse than the average new UK car (168 g/km).

Hybrids are an incremental step, but they do not solve the fundamental problem of eliminating fossil fuel use, nor do they achieve the dramatic efficiency gains of pure EVs.

The Necessary Shift: The New Energy Burden

The conclusion is clear: electrification of surface transport is the only available technology that delivers the necessary big reduction in energy consumption while fully eliminating fossil fuel use. Assuming we switch to electric vehicles, the consumption required for transport drops from 40 kWh/d per person (fossil fuel input) to roughly 18 kWh/d per person (electric input).

Transport energy reduction: 55% drop from 40 kWh/d to 18 kWh/d with EVs

However, this seismic shift does not eliminate the energy challenge; it merely changes its nature, relocating the problem from the fuel pump to the power socket. When combined with the necessary electrification of heating (using heat pumps), the total demand for delivered electricity in the future sustainable plan for Britain is projected to rise to 48 kWh/d per person (120 GW nationally). This is nearly a tripling of the UK’s current electricity consumption.

This monumental need for clean electricity—the production side, or the “green stack”—is the critical hurdle, requiring the exploration and exploitation of every substantial sustainable resource available, starting with wind and solar power.