Zero energy consumption. The goal sounds simple and is presented excessively in variations all over the world. Energy and environmental politics demand zero consumption as a long-term goal, marketing has discovered the concept and first buildings and settlements aiming at balanced energy or emission budgets have been constructed. As an example, the German Federal Government specifies in its fifth energy research programme (2005): For new buildings, the goal is to reduce the primary energy demand, i.e. the energy demand for heating, domestic hot water, ventilation, air-conditioning, lighting and auxiliary energy, again by half compared to the current state of the art. The long-term goal is zero-emission buildings. England and the USA aim for zero carbon developments and net-zero energy buildings (DOE, 2009) in political programmes. The Vatican accepted the offer of climatic “indulgence”—and thus became the first country in the world to completely compensate its carbon emission (Spiegel online, 2007). Megaprojects in the growth regions of the Arabian Gulf and China advertise with a CO₂-neutral balance. A Zero Carbon Community is to be created in Masdar, Abu Dhabi (Foster, 2007), and the first Chinese carbon-neutral ecocity was planned for Dongtan, Shanghai (Pearce, 2009). Not only to aid international communication, but also to further the processes required to solve energy-related problems, it is essential that key words, central concepts, their usage and their relationships be clarified. This article intends to contribute to this clarification based on the monitored example of a solar estate. Net zero energy building, equilibrium building, carbon neutral city—the accounting method varies, depending on motivation and point of view. If the focus is on finite and scarce resources, energy is the currency; CO₂-equivalent emissions are considered if global warming and public health is the issue; the cost of energy is what concerns a tenant paying for heating and electricity. A balance in one set of units can be converted to another, but the conversion factors often also shift the balance point. Energy will be used as the reference quantity in the following article, which prevents confusion with non-energy measures (e.g. carbon credits for forestry) and avoids the nuclear power debate, in which nuclear power is partly calculated as being CO₂ neutral. The diversity of concepts is an indicator that a scientifically based methodology is still lacking, though initial publications focus hereon (Pless et al. 2009). Since October 2008, a group of experts in the International Energy Agency has been addressing this issue under the heading, Towards Net Zero Energy Solar Buildings (Riley et al. 2008). The goal is to document and analyse outstanding examples that are close to being net zero-energy buildings, and while doing so, to develop the methodology and tools for working with such buildings. The Chair of Technical Building Services, University of Wuppertal, is co-ordinating the methodological work. The zero-energy approach—still under construction—will here be presented using a solar estate as an illustration.

Electricity generation from renewable energy reduces greenhouse gas emissions and, in the long term, the cost of electricity in power grids. However, there is currently no strong positive correlation between greenhouse gas intensity and electricity spot prices in Germany, despite increasing renewable energy penetration. Therefore, energy flexibility programs that rely on demand response may not be fully effective in reducing carbon emissions unless the energy market aligns consistently with carbon emission factors. To address this issue, we propose a model for joint signals consisting of power grid climate gas intensity and price signals that can achieve both environmental and economic benefits for building energy flexibility applications. Next, to assess the maximum possible flexibility hours from the grid side, we explore penalty signal threshold limits with daily and biweekly aggregation. Using a case study, we analyze energy flexibility with joint signals to explore their effect on greenhouse gas emissions and building operation cost. Our results suggest that joint signals can be more effective than a single type of signal in promoting energy flexibility.