This is a brief overview of currently-known alternator information for the Ninja 250. This information is only likely to be helpful to advanced mechanics.
Tiny alternator glossary:
Stator: The part of the alternator which is affixed to the bike, inside the left hand engine cover. This part may be replaced with some work.
Rotor: The part of the alternator which spins, affixed to the crankshaft. No aftermarket replacements available.
A stock alternator stator was sent to Electrosport for testing. They sent back the following results:
Note that all stators make nearly their full power by 1400 RPM, which is approximately idle speed. Kawasaki lists the stock alternator as being 17A at 10k RPM (or 238W at 14V), although this was not borne out by Electrosport's testing -- they say that the output curve is essentially flat above 6k RPM.
No information has been collected on how much power the bike requires while running. This information is necessary to accurately determine the actual power overhead available. Common wisdom is that there's about 60W overhead available for accessories (lights, heated clothing, etc.) with a stock stator, which would imply that the bike needs about 120W to run. Forum posts have led some riders to surmise that there may be a somewhat higher electrical overhead, in the range of 70-90W, but the only way you can find out for sure is to install whatever accessories you're thinking about, put in a voltmeter, and keep an eye on it.
Electrosport reports that the ESG110 stator can be installed without modifications to the engine case, although it does not come with the correct connector, and will require cutting and soldering to get the stock connector swapped on. The ESG636 can be installed in the Ninja 250's case, but it will "require some machining of the case." More-specific information isn't available at this time.
Electrosport suggested that the stock Ninja 250 regulator is capable of handling the increased power without modification or replacement. This seems likely, as Kawasaki is unlikely to make a variety of regulators, more likely using one regulator on many different bike models.
For the time being, a stator swap should be considered an expert-level modification, not suitable for most mechanics. If you're comfortable with soldering and have access to heat-shrink tubing (electrical tape is not an acceptable insulator for this job), and are capable of jobs such as a valve adjustment, you have the skill to swap in the ESG110. If you're willing to commit your engine cover to having at least 4mm machined off the stator mount-points, the ESG636 should be achievable. If considering either modification, contact Electrosport directly for more-specific fitment information.
This FAQ article should not be considered authoritative on the matter of stator installation.
Note: For reliability reasons, you may be better off sticking with your stock stator. Members of the Thumper section of ADVRider did a group buy on Electrosport stators, and they were less than impressed with their longevity. If you're really interested in Electrosport products you should do your own research.
Note 2:If you are looking at this information because you are thinking of getting some heated gear for winter, see Leon's comments on this page.