That is not correct. The alternator output is regulated by the regulator (hence it's name) and when the battery is fully re-charged after engine start the regulator reduces the Field voltage which in-turn reduces the output of the alternator (and subsequently the alternator does not require as much energy from the engine.)
An electric fan (or any other accessory) will draw from the battery and the regulator will then demand more from the alternator to keep the battery at full charge. The additional output from the alternator will require more HP from the engine to produce it, ... Therefore, no free ride (but it's a miniscule drag on the engine relative to the need to motivate the tractor so engine/fuel economy is virtually unmeasurable for the average guy)...HOWEVER the alternators on most Kubotas these days are 40 Amp units. If you are running lights or other accessories an added electric fan may exceed the alternator capability to keep the battery fully charged. This may result in a discharged battery at the end of the operating period...OR it may wear our your alternator prematurely due to increased demands.
On another note regarding aftermarket parts for a Kubota: The genuine Kubota 40A alternator is priced in the several hundreds ($299). An identical unit sells on Amazon for $62
https://www.amazon.com/Electrical-A...sr=8-6&keywords=kubota+alternator+34070-75601 which comes with the computer test analysis from the mfr'r (which may be the same who makes it for Kubota because I cannot tell the difference. When I took the genuine Kubota alternator off and opened it up to replace a defective regulator (cost $18 online) the internals were identical in every detail between the two alternators. Now I have a spare (the original) sitting on the shelf .
Addendum: The local auto-electric shop who specializes inrebuilding starters/alternators wanted $185 to "rebuild" my original Kubota alternator even when they agreed that all it needed was a new regulator. In my profession (retired from aircraft) anytime something is "rebuilt or overhauled" then ALL wear-parts are replaced, I.E., brushes, bearings, etc. are completely replaced and electronics (I.E. regulator, etc) are fully tested/recertified. It's my belief the automotive world merely replaces defective items and paints the unit for appearance. In other words, $18 did everything a "rebuild" would have accomplished for 1/10th the cost. Don't be afraid to dig into these things.... they're not airplanes.