Half as many watts.
Will that affect my lights in any way?
Yes it will; they won't be as bright.
Low powered dynamos started about 5 years ago, after the German authorities noticed that modern LED lights were easily producing more than double the brightness required by the StVZO tests. The intention was to encourage the use of smaller, lighter and cheaper low power dynohubs in place of the less reliable side wall generators that were still found as OEM on most German utility/town bikes.
As
@JhnBssll says, it will depend on the lamps. An incandescent bulb would be half as bright with the 1.5 than with the 3, but an LED may not switch on at all.
WRONG
An under powered incandescent bulb may give no more than a dull red glow, but an under powered LED will merely be less bright..
Hub dynamos are best thought of as being a current source. They are rated using a simple 12 Ω resistor, with a standard (3N, 3W nominal) dynamo giving 0.5 A, and a low power (1N, 1.5 W nominal) dynamo giving about 0.35 A.
A standard front bulb has a resistance of 12 Ω, so 0.5 A from a regular 3N dynamo will give 6 V across the bulb, and 3 W power.
Using the same bulb in the same light on a 0.35 A 1N dynamo will give only 4.2 V across the bulb, and 1.5 W power.
However, the bulb filament doesn't have to get so hot to radiate the reduced power, so an even higher proportion than normal will be radiated as invisible infra red, leaving very little to see with. You could shift the emitted light back towards the visible by fitting a different bulb, but finding something in a suitable flanged fitting wouldn't be simple.
In contrast, using an LED light, the driver electronics ensures that the voltage across the LED itself is the 3.5V (approx) required, and brightness is proportional to current, with no spectrum shift to worry about. In fact, you'll get a little more than half the brightness with half the current, as the LEDs are more efficient when they are cooler.
With an LED, what energy doesn't come out as visible light doesn't get emitted as IR - it goes into making the LED itself hot. Most powerful LED lights will automatically turn down the power to protect the LED(s).
