  Electron spins in solids constitute remarkable quantum sensors. Individual
defect centers in diamond were used to detect individual nuclear spins with
nanometer scale resolution, and ensemble magnetometers rival SQUID and vapor
cell magnetometers when taking into account room temperature operation and
size. NV center spins can also detect electric field vectors, despite their
weak coupling to electric fields. %even that of an isolated fundamental charge,
despite their weak coupling to electric fields. Here, we employ ensembles of NV
center spins to measure macroscopic AC electric vector fields with high
precision. We utilize low strain, $^{12}$C enriched diamond to achieve maximum
sensitivity and tailor the spin Hamiltonian via proper magnetic field
adjustment to map out the AC electric field strength and polarization and
arrive at refined electric field coupling constants. For high precision
measurements we combine classical lock-in detection with aspects from quantum
phase estimation for effective suppression of technical noise. Eventually, this
enables $t^{-1/2}$ uncertainty scaling of the electric field strength over
extended averaging periods, enabling us to reach a sensitivity down to
$10^{-7}$ V/$\mu$m.
