  We propose an index to quantify and analyse the impact of climatological
variability on the energy system at different timescales. We define the
Climatological Renewable Energy Deviation Index (CREDI) as the cumulative
anomaly of a renewable resource with respect to its climate over a specific
time period of interest. For this we introduce the smooth, yet physical, hourly
rolling window climatology that captures the expected hourly to yearly
behaviour of renewable resources. We analyse the presented index at decadal,
annual and (sub-)seasonal timescales for a sample region and discuss scientific
and practical implications.
  CREDI is meant as an analytical tool for researchers and stakeholders to help
them quantify, understand, and explain, the impact of energy-meteorological
variability on future energy system. Improved understanding translates to
better assessments of how renewable resources, and the associated risks for
energy security, may fare in current and future climatological settings. The
practical use of the index is in resource planning. For example transmission
system operators may be able to adjust short-term planning to reduce adequacy
issues before they occur or combine the index with storyline event selection
for improved assessments of climate change related risks.
