  One of the promising platforms for creating Majorana bound states is a hybrid
nanostructure consisting of a semiconducting nanowire covered by a
superconductor. We analyze the previously disregarded role of electrostatic
interaction in these devices. Our main result is that Coulomb interaction
causes the chemical potential to respond to an applied magnetic field, while
spin-orbit interaction and screening by the superconducting lead suppress this
response. Consequently, the electrostatic environment influences two properties
of Majorana devices: the shape of the topological phase boundary and the
oscillations of the Majorana splitting energy. We demonstrate that both
properties show a non-universal behavior, and depend on the details of the
electrostatic environment. We show that when the wire only contains a single
electron mode, the experimentally accessible inverse self-capacitance of this
mode fully captures the interplay between electrostatics and Zeeman field. This
offers a way to compare theoretical predictions with experiments.
