Porsche 930 Back Up Light Bulb Size

The Porsche 930 back up light requires a bulb size of P21W for the rear lighting assembly. Data for the specified component covers the range from 1978 to 1979.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1978-1979	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The P21W is the specified back up light bulb for the Porsche 930 across the 1978-1979 model years.
The P21W is an ECE-standard single-filament bulb rated at 12 volts and 21 watts, mounted on a BA15s bayonet base.
The BA15s designation refers to a single-contact bayonet base with a 15mm collar, which is not interchangeable with the BA15d double-contact variant despite sharing the same physical collar size.
As an ECE-regulated bulb, the P21W is designed for signalling applications rather than primary illumination, a classification that governs its light output and intensity characteristics.
The single-filament construction of the P21W distinguishes it from dual-filament bayonet bulbs such as the P21/5W, meaning substitution with a dual-filament variant would introduce an additional circuit contact that the 930's socket does not accommodate.