How to get rid of 19kHz electrical fields
As others stated,the cause of the radio frequency interference (RFI) is likely separate inverters for a solar power system.

You can easily calculate values for an L-C filter for ~20 kHz region. About 10 nF and 6.3 mH, for example,should block most of that RFI coming over the wires. That said, you'd need an effective ground, e.g. a copper water pipe, to connect to the capacitors.

However, since the solar panels have comparatively high currents, you'll also have problems from the radiated magnetic field inducing 20 kHz in a length of wire, such as power cords and USB cables. You probably can't build a mu-metal shielded room, so get as far from the inverters and panels as possible. For sensitive measurements, run from battery (not UPS, which have their own inverters).