  This paper examines how to minimize the energy consumption of a user
equipment (UE) when transmitting short data payloads. The receiving base
station (BS) controls a reconfigurable intelligent surface (RIS), which
requires additional pilot signals to be configured, to improve the channel
conditions. The challenge is that the pilot signals increase the energy
consumption and must be balanced against energy savings during data
transmission. We derive a formula for the energy consumption, including both
pilot and data transmission powers and the effects of imperfect channel state
information and discrete phase-shifts.
  To shorten the pilot length, we propose dividing the RIS into subarrays of
multiple elements using the same reflection coefficient. The pilot power and
subarray size are tuned to the payload length to minimize the energy
consumption. Analytical results show that there exists a unique
energy-minimizing solution. For small payloads and when the direct path loss
between the BS and UE is weak compared to the path loss via the RIS, the
solution is using subarrays with many elements and low pilot power and vice
versa. The optimal percentage of energy spent on pilot signaling is in the
order of 10-40%.
