In article ,
says...

I typically ride 3 - 4k miles per year with just under half that at night. When
riding in the city, no headlight is needed really to see the road, so yeah, I
suppose 3 watts will keep the police at bay.

Realistically, no light at all is needed to keep the police at bay, at
least around here. Even many police bikes don't have headlights on at
night, and I've never seen anyone ticketed for riding without a light.

On dark roads at low speed or on
dark trails, a 10 watt narrow beam or 20 watt flood is okay, but going downhill
20+ mph, it's very easy to out ride your light, a 35 watt spot high beam will
allow you to ride 25mph down a hill with comfort. Even 20 watts is scary if
riding a paved bike trail after a summer storm ... there isn't enough light far
enough up the trail to detect fallen branches. (yes, even at only 12mph). A
flick of the high beam will easily show if the path is clear. It depends on
where you ride.

I find my SON generator at 3W adequate for 30 mph in the rain at night,
unless there's a lot of glare from oncoming traffic. I find it puts more
useful light on the road than the NiteRider dual-beam system it replaced.
The beam pattern is much narrower than the narrowest NiteRider bulb, and
it's shaped to fit a lane of traffic, rather than a simple round cone of
light.

But it isn't as good for off-road, it doesn't throw light up into the
trees so I don't see some head-height dangling branches as soon. For
off-road I add a helmet-mounted battery light.

I have a second lamp head for the SON that allows 6W operation at higher
speeds, but it's been sitting on the shelf for a few years, I've never
felt the need to install it.

Another BIG advantage of the 35 watt high beam is the ability to attract
attention. Drivers will often completely miss a single light comming at them
(many drivers will miss seeing a car comming at them!), but being able to flash
the high beam grabs their attention. This also works on bike paths with joggers
and dog walkers on summer nights.

A lower-wattage lamp with good optics allows the same thing, except
instead of turning the light on and off, you just wiggle your existing
headlight at them and away from them -- the narrow beam makes this just
as effective as turning the light on and off. (Lighthouses don't turn
their lights on and off, it just looks like it -- they just rotate a
narrow beam.)

One of the big advantages of "hardware store lamps" is the _relative_ ease of
finding replacement bulbs (yet try to find a 10watt, MR16 spot bulb on the
internet to order ... good luck !! ). Good optics are nice, but must you
replace the entire assembly, or just the lamp?

Just the bulb, a dollar or two, and small enough that I keep three spares
in a 35mm film can, packed in cotton balls. Replacing the bulb takes a
minute or less, and no tools are required. Bulbs usually last me a few
years -- I carry multiple spares in case one spare is bad, or in case
someone else needs a bulb.

--
is Joshua Putnam
http://www.phred.org/~josh/
Books for Bicycle Mechanics and Tinkerers:
http://www.phred.org/~josh/bike/bikebooks.html