I THINK I understand the basic difference between a stator and a regular alternator...an alternator can put out more charge based on load, a stator sort of puts out a "set" charge, correct? By that I mean it will put out a full charge at road speed, say 4000+ rpm, but at idle, there's less watts being produced by the stator and the battery will start to draw down.
If the stator is putting out a full charge at road speed, any excess watts go to the voltage regulator or rectifier (or whatever the heck it's called), ending up as excess heat. More load equals less heat to the rectifier; less load equals more heat to it, right?
I am responding mainly to correct the common mistaken impression, expressed above, that power not used by the loads ends up in the regulator. When I first saw that notion stated on this board, I knew it could not be right because the regulator simply did not get hot enough. (My work has resulted in assessing lots of heat dissipation concerns in electronic systems.) So I undertook the study and analysis necessary to develop the understanding outlined next.
First, a few fundamentals necessary to understand this, put in lay terms:
1. Voltage is the force exerted upon electrons (or other charged particles.) It is analogous to water pressure or the force of gravity; it tends to produce charge flow (aka "current") if such flow is allowed. It is not power or current. Just as you can have plumbing pressurized with all the faucets off, or a heavy object sitting on a shelf, there need be no current (or water moving or things falling) just because there is a force tending to move the charge (or water or things.)
2. Current is the flow of charge. It is not, by itself, power. For example, current can flow indefinitely in a superconductor loop without power being used to keep it flowing. Or, a satellite can keep falling around the earth, in a stable orbit, without power used to sustain its motion, (if it is well out of the atmosphere.)
3. Electrical power is the product of voltage and current. It is usually stated in units of Watts, which is the product of voltage (in units of Volts) and current (in units of Amperes, often called "Amps".)
4. Current or voltage can be AC, which means regularly alternating in direction, or DC, which means steadily in one direction. The motorcycle loads need DC voltage and current. The battery provides DC voltage, and is charged by DC current or discharged by DC current.
The motorcycle magneto (which is the combination of the stator and the rotating permanent magnets enclosing it) generates a set of 3 AC voltages proportional to engine speed. The magneto output current, collectively among its 3 windings, is limited to about 28 Amps at all engine speeds. (The limiting mechanism is the stator inductance, whose impedance increases directly with frequency which is proportional to engine speed.)
If the magneto output is left open, so that no current can flow, there is negligible power delivered through the stator. The product of voltage and current is zero because the current is very close to zero.
If the magneto output is shorted, so that no voltage can appear across the stator terminals, the stator ouput power is zero. The product of output voltage and current is zero because the output voltage is very close to zero. If the stator windings were made with superconductors, the magneto with shorted output would convert no mechanical power to electrical power. Average torque applied to its rotor would be zero.
Unfortunately, the stator is not made with superconductors; the windings are made with copper and have some resistance. It is about 0.1 Ohms per winding, meaning that about 0.1 Volts must be applied to each winding just to get an Ampere of current to flow. Because of this resistance, the 28 Amperes of current flow from the stator dissipates over 60 Watts in the copper windings regardless of whether the output is shorted or delivering useful power to the loads. It is that 60+ Watt power dissipation which stresses the stator on stock bikes.
The stock, shunt regulator (sort of) shorts (or "shunts") the stator output as necessary to keep excessive power from reaching the 12V DC bus and destroying the battery and (most of) the loads. While that "shorting" occurs, about 50 Watts is dissipated in the regulator because it does not truly short the stator output at zero Volts. It uses diodes to convert the AC stator output to DC, and those drop a couple of Volts when conducting current. This 50 Watts is also dissipated when the regulator is not shunting but passing stator current to the DC bus. So the stator resistance and regulator are always consuming over 100 Watts when the stock bike runs.
The series regulator, instead of shunting the stator output to avoid excess DC bus power, blocks current flow between the stator and the DC bus. During this blocking, negligible power is dissipated in the stator or in the regulator's AC-to-DC conversion devices. So, for a typical bike load of a couple hundred Watts, the stator dissipation is reduced by about half. Likewise, the regulator dissipation is reduced. This reduced wasted power will translate to a small gas mileage improvement which you will be able to see if you measure it carefully before and after replacing the shunt regulator with a series one.
The bottom line here is: While there is 400W available
from the magneto, that much power is not consumed when the loads do not take 400W. A significant fraction of the 400 available Watts, nearly a third, is wasted and converted to heat in the shunt-regulated system at all times when riding. (It drops a bit when idling.)
To answer the thread title's question: There is very little difference in regulator or stator dissipation, with the shunt-regulated system, between the stock load, a load closer to 400W (heated gear, etc.), or a load reduced by use of LED headlights. However, with the series-regulated system, regulator and stator dissipation are substantially reduced when the loads consume substantially less than 400W. (And conversely, when the loads take all of the available power, the series regulator yields no significant reduction in stator or regulator dissipation.)
Everything I can find says my '13 Wee's stator puts out ~290 watts. Using the load values here: Calculating Excess Electrical Capacity - Learning Center - Powerlet Products, it looks like these are the "common operating loads":
High Beam 55 watts
Low Beam 55 watts
Number Plate 5 watts
Brake/Tail 21 watts
Instrument Panel 2 watts
Computer 25 watts
Fuel Pump 60 watts
Cooling Fan 60 watts
Electronic Ignition 50 watts
The incandescent headlights consume about 55 Watts for each filament. (Many V-Strom's have dual headlights.) I doubt the computer takes anything like 25 Watts. I suspect the pump and fan figures are a little high, and may represent peaks rather than averages. The usual stock loads take only about half of the available 400W from the magneto. (I have measured the loads by temporarily replacing fuses with a current meter. I do not have the results at hand.)