Energy Resolution is the ability of the Detector to accurately determine the Energy of the Incoming Radiation.

Since no camera system is perfect, no system is capable of determining precisely what energy photon struck the crystal. Instead, the system can only determine within a range of values, what energy radiation it is detecting. The energy resolution is expressed as a percent of the energy of the incoming photons.

If Gamma Camera Energy Resolution = 10%

If the energy resolution of a camera is 10%, and only 140 KeV photons are striking the crystal, the system will "see" photons ranging from 133 KeV to 147 KeV. That is, it can only determine to within 14 KeV, what the actual incoming energy really is.
Energy resolution is a very important parameter in determining the overall performance of a gamma camera, because it is the parameter, which allows a camera to differentiate between primary photons and Compton scattered photons. This ultimately determines the spatial resolution of the system.
An important measurement to assess the efficiency of the scintillation counting equipment in a Nuclear Medicine department is the Full Width at Half Maximum (FWHM), which should typically be less than 10%.
The formula for determining the percent energy resolution for a particular radionuclide is:

%Energy resolution = FWHM x 100 /photo peak

Energy resolution is expressed as a percent of the FWHM of a specific energy.
Most cameras have an energy resolution in the range of 11% to 13% FWHM at 140 KeV.

The lower the number of FWHM (smaller the percent), the Better the Energy Resolution.