  The energy calibration and resolution of the electromagnetic calorimeter
(ECAL) of the CMS detector have been determined using proton-proton collision
data from LHC operation in 2010 and 2011 at a centre-of-mass energy of
sqrt(s)=7 TeV with integrated luminosities of about 5 inverse femtobarns.
Crucial aspects of detector operation, such as the environmental stability,
alignment, and synchronization, are presented. The in-situ calibration
procedures are discussed in detail and include the maintenance of the
calibration in the challenging radiation environment inside the CMS detector.
The energy resolution for electrons from Z-boson decays is better than 2% in
the central region of the ECAL barrel (for pseudorapidity abs(eta)<0.8) and is
2-5% elsewhere. The derived energy resolution for photons from 125 GeV Higgs
boson decays varies across the barrel from 1.1% to 2.6% and from 2.2% to 5% in
the endcaps. The calibration of the absolute energy is determined from Z to
e+e- decays to a precision of 0.4% in the barrel and 0.8% in the endcaps.
