Harald Heinrichs · Pim Martens Gerd Michelsen · Arnim Wiek Editors

4 The “Energy Equation”
Impacts from societal energy use can be framed with the following simplified equa-
tions which are formulated here with regard to greenhouse gas emissions but can be 
used also for other environmental impacts
1
:
Population X Welfare
Capita
X
Activities
Welfare
2
X
Energy
Activity
X
GHG
Energy
= GHG
emission
Energy 
service 
intensity
Energy 
intensity
Emission
intensity 
(decarbonisation)
Impact
Sufficiency
Efficiency
Consistency
By pointing out the overall drivers of GHG emissions on a macroeconomic level,
this equation also provides for a rough structure of the three main elements to reduce 
GHG emissions and thus to increase sustainability of the energy system: while pop-
ulation and per capita welfare are not typically subject to energy policy, the energy 
intensity of a nation and its economy and the GHG emission intensity of the respec-
tive energy systems are the most important (overall) drivers to reduce energy use 
and its impacts on the environment, economy and society. Finally, the amount of 
activities (such as distances travelled or building space heated) per unit of wealth is 
another element that can be influenced in order to reduce emissions per capita.
In terms of strategies, these elements are often termed as follows:
Energy service 
intensity 
(sufficiency)
In the context of developed societies, this mainly 
means to use less energy services, e.g. travel less 
by car or have smaller flats and still enjoy a high 
standard of living and happiness. The reduction of 
energy service demand successively reduces 
energy demand and GHG emissions. For
developing countries, however, sufficiency often 
means access to an increased level of energy 
services.
Activities (energy 
services)/capita
1
Such functions, which are often defined slightly different, are known as IPAT, or ImPACT, equa-
tions (see Waggoner and Ausubel 
2002
) or as Kaya identity (Kaya 
1990
; Kaya and Yokobori 
1997
).
2
Both, activities and welfare, are often expressed in terms of GDP, which of course is to many 
respects a simplification. At least above certain levels, welfare is not a direct function of income 
levels nor is the income a very appropriate measure for the various activities which are supplied by 
the use of energy.
(continued)
19 Sustainable Energy Systems

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240
(Energy) efficiency
Increasing the efficiency of cars, machines and 
household appliances as well as reducing the 
energy consumption of buildings leads to a 
reduced energy use without changes in the 
demand of energy services (see box on rebound 
effects). This is done by technical improvements 
of technology which can be instrumented by 
various policies and measures.
Energy/activity 
(GDP)
Emission intensity 
(consistency)
This means reducing the GHG emissions per unit
of energy consumed, e.g. by increasing the share 
of renewable energy generation and by increasing 
the efficiency of the energy supply system (e.g. 
modern power plants and the use of waste heat 
from power plants). Consistency refers to the idea
that the emission intensity should be consistent 
with the goal of reduced GHG emissions.
GHG/energy
There are basically four technical options for reducing carbon dioxide emissions 
from energy systems: (i) energy efficiency, and in the field of consistency, there are 
(ii) renewable energy, (iii) nuclear energy and (iv) fossil fuels with carbon capture 
and storage. From the point of view of sustainability or their ability to meet energy 
policy goals, they score differently.
Energy efficiency is generally considered a robust strategy to improve on all the 
energy policy goals and contribute to sustainable development. The same is true for 
renewable energy although some forms and conversion technologies are still expen-
sive compared to other supply options. But it should also be warned that the use of 
renewable energy (notably bioenergy and hydropower), can be highly unsustainable 
with large environmental and social impacts.
Nuclear energy has essentially zero carbon dioxide emissions but is associated
with another set of major concerns: nuclear weapon proliferation, accidents and 
safety and waste handling issues. Fossil fuels are generally considered as unsustain-
able, but their climate impact can be considerably reduced through deploying tech-
nologies for capturing the CO
2
and storing it in underground geological formations. 
However, fossil fuels remain unsustainable since CO
2
storage capacity as well as 
fuel resources are limited (see Everett et al. 
2012
). The sustainability of these four 
technical options depends strongly on how they are implemented and how the con-
cept of sustainability is interpreted or defined.
• What are the key elements and strategies to create more sustainable energy 
systems?
S. Lechtenböhmer and L.J. Nilsson

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