A comprehensive energy plan is crucial to the development of a sustainable Net-Zero/Low-Carbon city that wants to move to a cleaner, cheaper, and near-zero energy future. Such a plan helps city officials identify opportunities and challenges for addressing the city’s energy needs in a sustainable way. The Comprehensive Energy Plan takes into consideration the city and surrounding communities (e.g. the metro area); assesses land-use patterns, waste-management requirements, and energy and water needs in different sectors such as buildings, industries, transportation, recreation, etc., and identifies efficient, cheap and clean ways of fulfilling them by mapping several local and other available resources.

The traditional approach is to study and plan each urban subsystem individually (and combine them, eventually, into an overall urban plan). In general, this will lead to a suboptimal system, because it neglects numerous interdependencies which may exist between the system components. In the process of carrying out local energy planning projects in some cities in the United States like New York City, San Francisco, Chicago, and others mentioned in this chapter during the last two decades, it was recognized that partial solutions for individual projects and a long-term strategy for the whole city have to be integrated simultaneously and that a variety of different decision-makers and interest groups have to be involved in the decision making and implementation process. However, only in recent years has the importance of a “consistent methodology” emerged in response to the requirements of such an “integrated” or “holistic” approach. Consistent systems integration is a prerequisite for achieving the goal of sustainability at the urban level since it allows for adequate consideration of the many interactions between the different components of the local energy system.8 For example, an analysis of integrated energy and water systems for long-term sustainability in New York City.9 Framework for this analysis is highlighted in Figure 4.

To understand the advantage of a systems approach and the need for long-term strategic planning, it is important to recall some facts about energy systems:

• Planning and operation of the energy system is generally carried out by different actors with sometimes conflicting goals. Local interest groups may have different opinions concerning the "optimal" energy supply system.

• A local energy system consists of long-lived infrastructures (planning horizon of 10 to 30, and eventually up to 50 years), which does not lend itself to quick modification or response. Changes in the energy system generally establish long-lasting facts. Thus, long-term developments of framework conditions (energy prices, economic growth, socio-economic changes, etc.) must be adequately considered in the planning process.

• There are many different options for technologies and energy carriers available to supply the energy demands and services. The energy system contains many interdependent subsystems (changes to one subsystem may have effects on other subsystems).

• Measures on the supply side compete with conservation measures on the demand side. Capital and human resources are scarce and must be directed towards the most effective measures.

• The economic success of investments must be evaluated in the context of uncertain socioeconomic factors, like general economic development, energy prices, taxes, and legislation.

• Energy system planning interacts with strategic planning in other fields. Planning tasks like environmental planning, urban planning, or transportation system planning may affect the energy system.

• Changes in the energy system affect residents, local industries, and the environment, and thus have a large impact on the urban environment.

• The exploitation of local renewable resources (biomass, wind, solar energy, hydro energy, waste heat, etc.) is often expensive and needs stable, long-term demand and commitment to justify the investment.

As indicated in Figure 4, energy sub-systems and technologies have an intricate interdependence and must be developed and deployed in close consideration with other sub-systems. A low-carbon energy system will feature more diverse energy sources. This will provide a better balance than today’s system, but it also means that the new system must be more integrated and complex and will rely more heavily on renewable and distributed generation. This would entail increased efficiency, decreased system costs, and a broader range of technologies and fuels. Success, however, will critically depend on the overall functioning of the energy system, not just on individual technologies, thus demanding more systems integration. The most important challenge for policymakers over the next decade will likely be the shift away from a supply-driven perspective, to one that recognizes the need for more demand-side generation and pulled resources, which needs integrated solutions. The term “integrated solutions” means that a combination of differently scaled measures shall be developed to realize a strategy that will achieve the given goals in the best possible way. Besides the traditional planning of individual measures, it is necessary to consider the municipal energy system as a whole, including possible interactions and interdependencies of its components: a comprehensive view of the overall “complex” system and its long-term behavior under different assumptions and influences. With this requirement, we enter the field of systems analysis, which so far has very little been applied in the context of local energy planning, despite its already long scientific tradition in other disciplines.