[en] Uncertainties in fission product decay power calculated by the summation method have been studied. Error contributions from individual isotopes were computed by assignment of specific errors from recent evaluations and projections. In this procedure, variance algorithms were constructed to define the error contributions due to uncertainties in (1) the averge decay energy of each isotope, and (2) the disintegration rate which includes fission yield errors for each isotope and its β-decay precursors in a decay chain. Total uncertainties in the decay power in the thermal fission of ²³⁵U and contributions from decay energy and yield errors are presented for burst and 10⁷ sec irradiations in the cooling time range 1 to 10⁵ sec. Nuclides that control the decay energy and yield variances are identified for burst irradiations and short cooling times