So Sturmey Archer sells the X-FDD 6v dynamo in two varieties: 2.4w and 3w

Does anyone know what the difference is? The 2009-2010 catalog shows some graphs:
https://www.sturmey-archer.com/pdfs/S..._Catalogue.pdf
but it's hard to say what the difference is, if there's any difference at all.

I think you'll find the 2.4W is for front light only and the 3W is for front and rear light 2.4W front + 0.6W rear.

I know what it's marketed for, but what is the actual difference?

I mean, electrically speaking, aren't we're talking about a simple voltage source? If they both put out 6V and the loads were the same, then wouldn't they both be delivering the same wattage?

The voltage is dependent on the load.

The wattage may well be dictated by the size of the windings.

A 2.4W hub may have less drag than a 3W hub.

A 6V 3W load is 12 ohms resistance. A 6V 2.4W load is 15 ohms resistance. Judging from the graphs, the 3W unit is designed to produce 6V at 15km/hr with a 12 ohm load while the 2.4W unit puts out 6V at 15km/hr with a 15 ohm load.

An ideal voltage source would put out its rated voltage no matter what the load resistance. The bicycle "dynamo" is not like this. It has a high internal resistance (actually more of a reactance to be precise). This means the voltage will change if you change the load resistance. To prevent premature bulb burn out, you should not use the 3W dynamo with a 2.4W load. If you use a 2.4W dynamo with a 3W load the bulbs will last a long time but will be dim. Best is to match dynamo to load; that is why they offer you the choice.

Any thoughts on drag resistance for the sturmey dynamo with drum brake? Am looking for reviews. Price is very reasonable.

That's not really true with LED's though, the lifetime of the bulb is so long it has become completely irrelevant.

If you're getting an LED dynamo light, there's no reason to go with the 2.4 version, get the 3w version.