Work out zener diode resistor instantly with clear inputs, formula shown and shareable results.
The series resistor must pass the load current plus a minimum zener current: R = (Vin - Vz)/(IL + IZmin). The worst case for the zener is no load, when it carries the entire current, so its power rating must cover Vz times the full current.
Zener shunt regulator
R = (Vin - Vz) / (Iload + Izmin); Pzener(worst case) = Vz x (Vin - Vz) / R
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.
Because the resistor current is fixed by the input voltage, so with the load disconnected all of it flows through the zener.
Whenever load current varies much or efficiency matters. A shunt zener wastes the full current continuously and regulates poorly.