Any body know what LEDs &/or resistors to buy to give me the equivalent light output to a 21w Bulb? They are to be run on a 12v circuit.

 

If you do some research on the bulb, you should be able to find it's brightness in Lumens.

then look for an LED with similar output and buy a resistor to use it on 12V.

But take into account that the light from MOST LEDs is in one direction, whereas the bulb is coming from a reflector designed to reflect light from ALL directions, so choose the LED carefully, SMD might do the job.

 

Yeah I'm aware of the directional properties of led's.

Having scoured the internet it seems a standard incandescent 21 bulb is around 180-200 lumens, the problem I'm having is led's are measured in mcd (millicandela) and as far as I can tell there is no direct conversion........