The xl250 k series are 6v,  the generator is made by  Denso (headquartered in the city of Kariya, Aichi Prefecture, Japan. Established December 16, 1949 as Nippondenso Co. Ltd now known as Denso Corporation)  with output of a maximum of 80 watts at 5000  revs, and the AC (alternating current) generated is used as AC for the ignition but also transformed into DC (direct current) by a  silicon bridge rectifier mounted near the battery. This is item 6 in the schematic below.  
The generators have 3 generator coils  but look at the generator and you will only see  two, one is a bit bigger than the other, that's because it is  two, one wound on the other. 
Of the three real coils there are  also three separate functions;  the first is to start and run the bike  by providing ignition system power, the second runs the headlight, so it is said to be important to replace even the main beam indicator light on the headlight shell to present a full load to the coil and help stabilise the voltage, as the K3 doesn't have the 3.6 Ohm resistor that earlier models have. The third one charges the battery.


At 6 Volts, every 6 Watts is one amp so a 24 Watt bulb is going to need 4 Amps to run it.
At 12 Volts every 12 Watts  is one amp so a 24 Watt bulb is going to need only 2 Amps to run it. 


As the  generator/alternator  produces alternating current (AC), which light bulbs are happy to run on, like in your home, there is no need for this to be rectified into direct current (DC).
However the battery can only hold DC so the rectifier changes the AC to DC by a diode  network that is a rectifier. The battery  then powers the side lights, the horn and the indicators.  This is so that if your engine dies  say in the pitch dark of an unlit  crossroads, you are still visible to other road users and can still express your annoyance by sounding your horn. Its a safety thing.


Having a blown bulb is therefore a bad idea, The usual problem of blown bulbs is due to off road riding where the filaments get shaken till they break, or old age. It is noted that some apparent headlight bulb failure is mainly attributable to poor quality Chinese bulbs, rather than an electrical problem.

As tungsten filaments age they deposit a layer on the inside of the glass, which can look black or silver, but this not only makes the bulb dimmer but also  traps infra red (heat) inside the bulb which them runs too hot so shortens the life further. Any bulb with this effect apparent should be thrown away and replaced with a nice new shiny one. It is possible to regulate the voltage with a silicon voltage regulator, with a little ingenuity,  but it isn't fitted by Honda.

This means that the bike does not need a battery to start. However, it also means that there is an easy way to steal one as the ignition switch and the kill switch both turn off the engine by grounding (sending to earth = short circuiting) the ignition. Therefore if the ground wire from the ignition switch is cut, even with no key the bike will start,
so best to  lock it up.  The act of kicking the kickstarter turns the engine and the generator to produce enough voltage to energise the coil, so a good kick is preferred. 
My favoured method of starting is to turn on the fuel (down is main, up is reserve), check kill switch is in 'off' position, check ignition is 'on' and choke on full. Then a hard kick (or 2) with NO THROTTLE TWIST and the engine starts.
I push the choke in full most days  as soon as I am moving as it otherwise runs too rich and the plug can soot up. Note the engine doesn't have to be in neutral, as long as you pull in the clutch the kickstarter will work;  handy if you stall it.

The output of the generator means that it is  unwise to fit substantially bigger wattage headlight bulbs, they won't be much  brighter. 

The standard are 35W/35W (Bosch type base) you can buy 25/25  and even 45/40. Indicator bulbs are 18W and the stop/tail are 5/21.The neutral lamp is 3W and the dial  illumination and turn signal lights on the tacho/speedo as well as the high beam indicator  are all 1.7W
         

So, if everything is on, that's:-
35W main beam
5W   tail
21W brake
6.8W panel lights
36W  indicators

that's nearly 104 Watts plus ignition.  I appreciate we don't ride with indicators and brake light on constantly. Remember the alternator is 80W, and that doesn't leave  much left over.


Because the engine needs to be running to generate power for the headlight you only have side lights or parking lights if it isn't,   these being powered via  the battery. 
You can however  improve the headlight with a halogen version which is whiter/brighter, even though it is 'only'  a 25/25W. 

I use the standard lamp fitted by Honda San, but I keep a spare halogen one inside the headlight just in case  (they are smaller head than a regular tungsten one and take up less room in that crowded headlight).


I recently received an email from Michael C. which is interesting, which I should share with you, and reads like this:

"Hello Rich.

So it might not last as long.  The point really is bigger wattage is not the solution.


When I bought my bike  I found the front indicators had 12v instead of the correct 6v bulbs in, so that explained why they were dim. It is worth checking. 
There is a 10 amp (60 watts @ 6 volts) fuse in a little white tubular fuse case
(item 11 below) near the battery,  and a holder for 3 spare fuses inside the battery cover.

Since the generator output increases with engine speed, the headlight will be dimmer at tick over (1200 rpm) and get brighter with an increase in engine speed. This does however give a quick check to show the generator is working. 

If you look on the left side of the bike where the generator multipin block (white) joins to the loom you will see  an odd black unconnected short wire with a female bullet connector on it. 

This is the one.