The arithmetic of sustainable energy necessitates tackling consumption first. In Britain, total consumption stands at approximately 125 kWh per day per person. We have already established that electrification can radically reduce the energy consumed by surface transport, transforming the 40 kWh/d fossil fuel burden into a much smaller electrical demand. The next formidable challenge—the second largest of the “three biggest fish” in our consumption picture—is heating.

The national expenditure on heating buildings, including space heating, water heating, and workplace heating, is estimated at roughly 40 kWh/d per person, with a more precise estimate putting it at 45 kWh/d per person. In the United Kingdom, this immense demand is overwhelmingly met by burning natural gas. To achieve the necessary deep cuts in greenhouse gas emissions—which must be greater than 85% for a high-GDP country like Britain—we must simultaneously execute two major strategies for heating: radical conservation (reducing the heat demand) and complete decarbonization (replacing fossil gas with efficient electric sources).

Phase I: The Conservation Imperative—Stopping the Leakiness

Energy conservation, particularly in buildings, is crucial because the energy required to heat a space is continuously lost to the outside world. The power required to keep a building warm is primarily lost through two routes: conduction (heat passing directly through solid materials) and ventilation (warm air escaping and being replaced by cold air).

Understanding the Physics of Heat Loss

To effectively reduce consumption, policies must target the physics of these losses.

1. Conduction loss: This is the rate at which heat transfers through surfaces like walls, floors, roofs, and windows. It is mathematically defined by the product of the surface area, the thermal transmittance (U-value), and the temperature difference between inside and outside. The U-value is measured in watts per square metre per kelvin (W/m²/K), and a smaller U-value indicates better insulation.

Counter-intuitively, conventional double-glazing is only about as effective at insulating as a solid brick wall. Reducing heat loss via conduction requires introducing materials with low U-values, such as fiberglass or foam, or dramatically thickening existing structures. For instance, putting 1.8-cm-thick insulated wallboards over an existing wall can halve its U-value, from 2.2 W/m²/K down to about 1 W/m²/K.

2. Ventilation loss: This occurs when warm indoor air escapes, replaced by cold outdoor air. It is often measured by the rate of air changes per hour (ACH). The losses are proportional to the number of air changes, the volume of the space, the heat capacity of air, and the temperature difference. Ventilation is necessary for health, to safely combust fuels, and to prevent moisture damage to the building fabric.

The Power of Insulation

Insulating existing building stock represents a massive opportunity to achieve big energy savings. In older, leaky houses, improving insulation can drastically cut the heat required.

Studies comparing typical UK houses demonstrate the impact of successive improvements:

Insulation savings: 43% reduction in heating demand with full insulation (53 to 30 kWh/d)

  • A detached house with no insulation requires about 53 kWh/d for space heating.
  • Adding loft insulation reduces this to 43 kWh/d.
  • Adding cavity insulation and double glazing reduces the demand further to 30 kWh/d.
  • A semi-detached house starts at 37 kWh/d, falling to 27 kWh/d with full insulation and double glazing.

In well-insulated, modern buildings, the problem shifts: ventilation loss often becomes the principal loss of heat, rather than conduction through the walls. This challenge can be countered effectively using heat recovery ventilators, often analogized to a “nose.” A nose warms incoming air by cooling down outgoing air through a temperature gradient or dedicated counter-current air passages, maximizing heat recovery.

The Role of Behavioural Change (Woolly Jumpers)

Simple lifestyle changes involving personal comfort levels can rival major technical interventions in their energy saving potential. The heat demand of a building depends heavily on the temperature set by the thermostat. This demand can be visualized using degree-days, which is the total area on a graph between the external temperature and the desired internal temperature over time.

Quantifiable reductions based on lowering the thermostat from a baseline of 20°C include:

Thermostat reduction: 45% heating demand reduction by lowering to 15°C

  • Turning the winter thermostat down to 17°C reduces the temperature demand for heating by 30%.
  • Turning it down further to 15°C results in a 45% reduction in demand.

This behavioral shift—often summarized by “put on a woolly jumper and turn down your heating’s thermostat”—is listed as an individual action that can save approximately 20 kWh/d.

Phase II: The Decarbonization Solution—The Heat Pump Revolution

Even after aggressive conservation efforts, a substantial heat demand remains, which must be met without using fossil fuels. Natural gas heating, predominant in the UK, must be replaced.

A common misstep in policy discussions is the promotion of combined heat and power (CHP), or co-generation, which attempts to increase efficiency by simultaneously generating both electricity and useful heat from a single fuel source. While potentially useful for large buildings, gas-powered CHP would be a mistake because it uses fossil fuels, locking society into their continued use.

The superior technology for sustainable heating is the heat pump. Heat pumps are “back-to-front refrigerators” that transfer existing heat from a colder external reservoir (the air or the ground) into a warmer internal space. Because they only move heat rather than creating it by combustion or simple electrical resistance, they deliver much more heat energy than the electrical energy they consume.

The Coefficient of Performance (COP)

The efficiency of a heat pump is measured by its coefficient of performance (COP): the ratio of useful heat delivered to the electrical energy consumed.

Heat pump efficiency: 4× more efficient than electric heaters (COP = 4)

Heat pumps are inherently far more efficient than ordinary electric heaters, which have a COP of 1 (1 kWh of electricity yields 1 kWh of heat). Efficient heat pumps are approximately four times more efficient than these simple heaters. Some air-source heat pumps (like the Eco Cute water heater in Japan) have achieved a COP of 4.9. Aggressive government regulation has been shown to improve average COP from 3 to 6 within a decade in some markets.

The universal switch to electric heat pumps is a foundational element of the future consumption plan for Britain, dramatically increasing the total demand for delivered electricity. The plan assumes that electric heat pumps will supply the residual heating demand, transforming the original 40 kWh/d gas input into a delivered electrical requirement of just 12 kWh/d per person.

Heating electrification: 70% reduction from 40 kWh/d gas to 12 kWh/d electricity

Ground-Source Heat Pumps and the Thermal Limit

Ground-source heat pumps (GSHPs) draw heat from the ground, which typically maintains a more stable temperature than the outside air. However, the ground is not an infinite source of heat; it is a poor thermal conductor. If heat is sucked out too quickly, the ground temperature drops, diminishing the pump’s efficiency.

For highly populated areas, such as a typical British suburb with a population density of about 6200 people per km² (or 160 m² per person), there is a physical limit to sustainable heat extraction. Detailed calculations show that if everyone in such a neighbourhood attempted to pull their typical winter heat demand (48 kWh/d per person) from the ground, they would end up freezing the ground.

Based on physical constraints and average soil conductivity, the maximum power deliverable by GSHPs without causing unreasonable long-term ground cooling is about 12 kWh/d per person. This figure is precisely the amount required for heating in the sustainable future plan.

To enable this limit to be met sustainably, particularly in dense urban settings, system designs must incorporate substantial summer heat-dumping. This involves running the system in reverse during warmer months, depositing heat back into the ground to replenish the reservoir for the subsequent winter.

Phase III: Complementary and Marginal Solutions

Beyond radical conservation and heat pumps, other technologies play a role in reducing the overall energy burden.

Solar Water Heating (SWH)

Solar thermal panels, used to heat water, are a promising and effective solution. They absorb sunlight and transfer the heat to water flowing through pipes, providing hot water for domestic use. Installing panels on a south-facing roof area of 10 m² per person can deliver approximately 13 kWh/d per person of thermal energy.

If installed on all available rooftops, solar hot water could significantly reduce the energy needed for water heating. The future energy plan assumes that solar HW will contribute 1 kWh/d per person.

Cooling Demand

The need for cooling in the UK is minor compared to heating, estimated at only 1 kWh/d per person. This demand can often be met efficiently using the same heat pump infrastructure operating in reverse, where the transfer of heat from the inside to the outside also conveniently serves the purpose of ground replenishment during summer.

Summary: The Tripling Electrical Burden

The integrated solution for heating—aggressively reducing demand through insulation and behavioural shifts, and then meeting the remaining demand via high-efficiency electric heat pumps—successfully eliminates the 40 kWh/d reliance on fossil gas. This is a big and necessary reduction.

However, this success fundamentally transforms the nation’s energy challenge: it moves the problem from gas consumption to electrical generation. The combined requirements of electrifying surface transport (18 kWh/d of electrical input) and electrifying heating (12 kWh/d of electrical input), added to the existing delivered electricity demand for gadgets and lights (18 kWh/d), result in a total future demand for delivered electricity of 48 kWh/d per person (or 120 GW nationally).

This means the UK must prepare for nearly a tripling of its current delivered electricity consumption. The focus must now shift entirely to quantifying the maximum plausible output of the “green stack” to meet this monumental demand.