Wind and solar generation may reduce the inter-annual variability of peak residual load in certain electricity systems

Electricity systems worldwide are transforming from relying almost exclusively on firm, predictable generation (e.g., fossil, nuclear, and large hydropower) towards incorporating more variable generation (e.g., wind and solar PV). In these systems, the electric load minus generation from variable resources is known as the "residual load." The peak residual load provides an estimate of the dispatchable power capacity required to supply electric load during all hours. We analyze a decade of concurrent historical electric load and weather data from four electricity systems. For each system, we construct hypothetical, plausible residual load profiles to study the peak residual load values and their spread from year to year, the "inter-annual variability," as a function of wind and solar generation. The inter-annual variability in the peak residual load can be equated with the spread in dispatchable power capacity required to supply all load from year to year in electricity systems. In each system, adding variable generation changed the inter-annual variability in the peak residual load values. The introduction of variable renewable power is often thought to increase the variability of most electricity systems characteristics. In contrast, using our simple approach, we show the inter-annual variability in peak residual load may decrease with added solar generation in systems where peak load occurs in the summer months. We attribute these reductions to correlations between the availability of solar generation and the hours of peak electric load, which occurred during the hottest days each year, when electric cooling (air conditioning) was likely used. Also, we show the inter-annual variability in peak residual load decreased in certain circumstances when adding wind generation to the system with a winter peaking load. An understanding of how and why this spread in peak dispatchable power capacity changes with increasing wind and solar deployment could inform long-term planning and resource adequacy targets for electricity systems.