Capacity factors for electrical power generation from renewable and nonrenewable sources

Given the dire consequences of climate change and the war in Ukraine, decarbonization of electrical power systems around the world must be accomplished, while avoiding recurring blackouts. A good understanding of performance and reliability of different power sources underpins this endeavor. As an energy transition involves different societal sectors, we must adopt a simple and efficient way of communicating the transition’s key indicators. Capacity factor (CF) is a direct measure of the efficacy of a power generation system and of the costs of power produced. Since the year 2000, the explosive expansion of solar PV and wind power made their CFs more reliable. Knowing the long-time average CFs of different electricity sources allows one to calculate directly the nominal capacity required to replace the current fossil fuel mix for electricity generation or expansion to meet future demand. CFs are straightforwardly calculated, but they are rooted in real performance, not in modeling or wishful thinking. Based on the current average CFs, replacing 1 W of fossil electricity generation capacity requires installation of 4 W solar PV or 2 W of wind power. An expansion of the current energy mix requires installing 8.8 W of solar PV or 4.3 W of wind power.

Greenhouse gas emissions are the driving force behind climate change (1), which threatens biodiversity (2), food security (3), and cultural diversity (4). The main source of carbon emissions in electricity generation is the current mixture of inputs (5). The current state of affairs demands an energy transition, but numerous challenges emerge (6–8). Such a transition implicates different societies in different ways (9–11), but even the conservative United States wants a decarbonized future (12). Actions toward a sustainable future have been taken at different scales from city (13) to country level (14).The current energy system in place has a rigid structure with a modus operandi of “winner takes all” that hampers the establishment of alternatives (15). Policies can be implemented to overcome the status quo. However, unintended consequences can arise, e.g., carbon pricing policies tend to incentivize optimization of the current energy system instead of the required transformations to achieve a decarbonized one (16). This conclusion is not universally accepted (17–19). It seems, however, that renewables will lead the energy transition and solar photovoltaics will play a key role (20–22).Engaging different players in a society for the energy transition is essential. However, the diversity of stakeholders creates communication barriers, particularly when technical details are transmitted to a broad audience. The war in Ukraine and insufficient natural gas supply in Europe have added painful urgency to clear and truthful communication of the potential pitfalls of any energy transition that boil down to the clear understanding of what the different components of electricity generation systems can and cannot do. Regarding power-generation efficiencies of different sources, the use of CF is an excellent tool to connect with a broad set of audiences.CF is a measure of a power plant efficacy (23). In short, it is an indicator of how fully the power plant is used, relative to its thermodynamic and technological constraints and required spare capacity (24). For all technologies, CFs have typical values for a set time interval and input (a fuel, light, water, or wind).An electrical power plant’s CF gives this plant’s average output relative to its maximum capacity. This could be quite misleading for renewables. If a plant works at 50% of nominal capacity, its CF is 0.5. This does not mean that the plant worked 12 h at full capacity and was off over the remaining 12 h. This plant could be down for different reasons such as repairs, maintenance, refueling, or intermittency for renewables. Despite its limitations, CF is a straightforward indicator that can be easily calculated and predict the amount of electricity that will be obtained on average from a specific nominal capacity installed.Comparing CFs across different technologies can be tricky, mainly when some are well established and mature, while others are at pilot-scale or not fully deployed. In the last two decades, solar PV and wind have been growing exponentially. This explosive growth allows one to obtain reliably their CFs. When a technology is more established, the effects of pilot plants, learning curves (25–28), or optimal sites no longer dominate, giving reliable estimates about that technology’s performance. Field tests are the ultimate answer. Theoretical estimates can differ significantly from the measured ones. Such discrepancies have been presented for wind (29) and concentrated solar power plants (30) (SI Appendix, section 10). Knowing the real value of CFs is fundamental to estimating costs, power production, and the future roles of specific technologies.In this work, we analyze the average CFs of different electricity sources (i.e., biomass, fossil fuels, geothermal heat, water, uranium, solar light, and wind) over the period 2000–2017. Global and regional values are estimated to highlight the differences in the performance of different technologies. These average CF values are then used to calculate the required nominal capacity to be installed in the future for our unavoidable energy transition.