  We demonstrate a spin-based, all-dielectric electrometer based on an ensemble
of nitrogen-vacancy (NV$^-$) defects in diamond. An applied electric field
causes energy level shifts symmetrically away from the NV$^-$'s degenerate
triplet states via the Stark effect; this symmetry provides immunity to
temperature fluctuations allowing for shot-noise-limited detection. Using an
ensemble of NV$^-$s, we demonstrate shot-noise limited sensitivities
approaching 1 V/cm/$\sqrt{\text{Hz}}$ under ambient conditions, at low
frequencies ($<$10 Hz), and over a large dynamic range (20 dB). A theoretical
model for the ensemble of NV$^-$s fits well with measurements of the
ground-state electric susceptibility parameter, $\langle k_\perp\rangle$.
Implications of spin-based, dielectric sensors for micron-scale electric-field
sensing are discussed.
