I'd think the 20W would be the smallest you could get away with for a driving light. I've used mine for a couple years now and the cops haven't looked twice. But my light does look a lot dimmer than a comparable headlight side by side.
Check out the 'snowbum' - he's got all the numbers for all the different brands of those halogens. I got mine at NAPA for around ten bucks. Pretty expensive for such a little light. There must be a cheaper way to get these things - If I could find them for three or four bucks I'd get a bunch for the rest of the herd, and spares.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*