One of the most demanding challenges with ultra-high field MRI is the SAR10g assessment during a multi-channel transmit MRI examination. A worst-case estimate is often used. In this work a fast method is presented to assess the upper bound of the maximum SAR10g for body coil arrays with knowledge on only the power distribution among the channels. The performance is assessed for prostate imaging at 7T using our database with 23 realistic subject-specific models. The mean overestimation factor is 1.58 and the mean reduction in overestimation is 0.75 compared to a recently presented alternative estimation method.