  The 2011 PhD thesis of Farris demonstrated that the ECH of a prequantization
bundle over a Riemann surface is isomorphic as a Z/2Z-graded group to the
exterior algebra of the homology of its base. We extend this result by
computing the Z-grading on the chain complex, permitting a finer understanding
of this isomorphism and a stability result for ECH. We fill in a number of
technical details, including the Morse-Bott direct limit argument and the
classification of certain J-holomorphic buildings. The former requires the
isomorphism between filtered Seiberg-Witten Floer cohomology and filtered ECH
as established by Hutchings-Taubes. The latter requires the work on higher
asymptotics of pseudoholomorphic curves by
Cristofaro-Gardiner--Hutchings--Zhang to obtain the writhe bounds necessary to
appeal to an intersection theory argument of Hutchings-Nelson.
