How many auxiliary lights can your motorcycle power?

Where do you find your bike's number?

In the specifications at the back of the owner's manual or the shop manual, under a name like generator output, alternator output or charging capacity. Derryn Wong, writing for webBikeWorld in 2009, gave the method in one line: look in the specifications section for an entry called "Generator Charging Capacity" or "Generator Output". His Honda Magna's manual gave 0.374 kW at 5,000 rpm, which is 374 W.

Read the engine speed beside the number, because it matters as much as the number. The Clymer shop manual for the DR650 gives the maximum output as 200 watts at 5,000 rpm, and the regulated voltage as 13.0 to 16.0 volts at the same speed. KTM's 2013 repair manual for the 690 Enduro R gives "Alternator 12 V, 224 W" and no speed at all. Both are the most the system makes, not what it makes at a walking pace up a rocky trail.

How do you add up what the bike already uses?

List everything that is on at the same time while you ride, in watts, and add it. Watts are volts times amps, so a figure given in amps on a 12-volt bike converts at about 14 V, the voltage a charging system runs at. The worst case is the one to plan for, and on most harnesses that is the high beam, because auxiliary driving lights are usually wired to come on with it.

Rick Korchak did this for his DR650 in 2011 and the result is a fair picture of any small dual-sport. The headlight "uses 55 Watts on low beam and 60 Watts on high." The brake and tail light draws 21 W, the plate light 5 and the instruments 6. He added "35W or so to run the ignition", which brought the bike to about 195 W with his two auxiliary lights on, against a 200 W rating. Our note on his build goes through what he did next.

Why is the rated output not what you get on a trail?

Because it is a peak at one engine speed, and nobody publishes the curve below it. Korchak's own advice for his bike: "figure on less than 200 Watts most of the time." A stator's output rises with rpm, so a bike crawling through rocks in first gear, or idling at a trail junction with the lights on, makes much less than its rating.

The one real-world figure we found for a stock DR650 comes from Pro Cycle's how-to, written by a rider who tested his own bike with a voltmeter. He "could power an additional 70 watts of lights/accessories and still keep the battery voltage high enough to stay charged." That is the useful kind of number: not what the stator makes at 5,000 rpm, but what the bike can carry in use without slowly draining the battery.

Running at the limit is not harmful in itself. The same author explains how the common shunt regulator works: "The stator is always putting out as much power as it can at any given rpm." What the battery and lights do not use is shunted to ground as heat. "So using the extra power actually takes some of the load off of the regulator and stator." The risk of adding too much is a battery that runs down over a night ride, not a stator that burns up.

How much do auxiliary lights draw?

Less than they used to. Wong's 2009 rule was built on halogen: a typical driving light used a 55 W bulb, so a pair needed 110 W, and a bike needed room for 110 to 150 W to run them. A DR650 does not. LED lights changed the arithmetic, and dimmable ones changed it again.

Korchak's Clearwater Kristas show both. By the maker's figures, each draws 36 W on the high beam, so a pair is 72 W, more than the headlight. On the low beam the dimmer runs each from about 30 W down to about 5 W, so the pair can be turned down to 10 W. A light whose draw is set by a dial lets the rider match it to what the bike can carry at the moment, which is a better answer to a small alternator than any fixed wattage.

What can you do before buying a bigger stator?

Replace before you add. The Rolling Hobo wired a single LED spot on his KTM 690's navigation crossbar to the high-beam switch in place of the stock high beam, so he gained a better high beam without adding a load on top of the old one. Our note on his build covers it. An LED tail and brake bulb, and an LED headlight bulb that is a genuine low-draw unit, free watts the same way.

Then fit a voltmeter or a voltage monitor where you can see it, and ride with your lights on at the speeds you actually ride. If the voltage holds, you have your answer for free. Korchak's monitor "always showed green with the original stator, even with the Krista lights on full blast", which is how he learned that the upgrade he went on to fit was margin rather than a fix.

If it does not hold, a higher-output stator is the next step, and it is a real one. For the DR650, Rick's Motorsport Electrics sells a 240 W stator through Pro Cycle, 20 percent over stock, for $231.95. Korchak measured his 2011 equivalent at about 30 percent more charging current than the original, at an engine speed he did not record. The stator sits in the engine's oil, so the job means draining it and a new cover gasket.

What we do not know

No maker we found publishes output against engine speed for these bikes, so the most useful number, what the stator makes at 2,500 rpm in second gear, has to be found with a voltmeter on your own bike. The DR650's 200 W comes from a third-party shop manual rather than Suzuki's own, though webBikeWorld cites the shop manual for the same figure. Every auxiliary-light draw on this page is the light maker's own number.