  This paper presents the energy resolution study in the JUNO experiment,
incorporating the latest knowledge acquired during the detector construction
phase. The determination of neutrino mass ordering in JUNO requires an
exceptional energy resolution better than 3\% at 1 MeV. To achieve this
ambitious goal, significant efforts have been undertaken in the design and
production of the key components of the JUNO detector. Various factors
affecting the detection of inverse beta decay signals have an impact on the
energy resolution, extending beyond the statistical fluctuations of the
detected number of photons, such as the properties of liquid scintillator,
performance of photomultiplier tubes, and the energy reconstruction algorithm.
To account for these effects, a full JUNO simulation and reconstruction
approach is employed. This enables the modeling of all relevant effects and the
evaluation of associated inputs to accurately estimate the energy resolution.
The study reveals an energy resolution of 2.95\% at 1 MeV. Furthermore, the
study assesses the contribution of major effects to the overall energy
resolution budget. This analysis serves as a reference for interpreting future
measurements of energy resolution during JUNO data taking. Moreover, it
provides a guideline in comprehending the energy resolution characteristics of
liquid scintillator-based detectors.
