  In this paper, we examine the energy consumption of a user equipment (UE)
when it transmits a finite-sized data packet. The receiving base station (BS)
controls a reconfigurable intelligent surface (RIS) that can be utilized to
improve the channel conditions, if additional pilot signals are transmitted to
configure the RIS. We derive a formula for the energy consumption taking both
the pilot and data transmission powers into account. By dividing the RIS into
subarrays consisting of multiple RIS elements using the same reflection
coefficient, the pilot overhead can be tuned to minimize the energy consumption
while maintaining parts of the aperture gain. Our analytical results show that
there exists an energy-minimizing subarray size. For small data blocks and when
the channel conditions between the BS and UE are favorable compared to the path
to the RIS, the energy consumption is minimized using large subarrays. When the
channel conditions to the RIS are better and the data blocks are large, it is
preferable to use fewer elements per subarray and potentially configure the
elements individually.
