  In this paper, the cooling of 72 M- and X-class flares is examined using
GOES/XRS and SDO/EVE. The observed cooling rates are quantified and the
observed total cooling times are compared to the predictions of an analytical
0-D hydrodynamic model. It is found that the model does not fit the
observations well, but does provide a well defined lower limit on a flare's
total cooling time. The discrepancy between observations and the model is then
assumed to be primarily due to heating during the decay phase. The decay phase
heating necessary to account for the discrepancy is quantified and found be
~50% of the total thermally radiated energy as calculated with GOES. This decay
phase heating is found to scale with the observed peak thermal energy. It is
predicted that approximating the total thermal energy from the peak is
minimally affected by the decay phase heating in small flares. However, in the
most energetic flares the decay phase heating inferred from the model can be
several times greater than the peak thermal energy.
