This is hub dependent. The hub has no noticeable drag when its not doing anything. Drag kicks in when you ask it do work, like power a light or charge things.
This image shows it quite well, the green lower line is the drag you’d get from a non-dynohub, a good quality hub, you get friction as a linear function of speed so Watts lost is a function of speed.
The next line up shows a Dynohub which is not doing any work, in this case, lights off, you see it is only slightly worse than no dynohub.
Then look at that same line of a good dynohub and look up and see when lights on, yes it is adding drag, you see drag increases from about 0.5W to about 3W. At that speed you are outputting about 75W usually so you lose about 2.5W of 75W so about 3%.
What I like about the B&M Luxos U is it adapts to the situation. At speed on downhills, it INCREASES its power input, and directs it to a bright forward facing beam, to deliberately both cause more drag (natural braking) and to shine a brighter light to see further at speed, and then at slower uphills speeds it does the opposite uses lights pointed down at the ground to see the ground as you move slowly. Any unused power if you use the USB output, goes to USB after the light is furnished, so if you turned lights off, pretty much on downhills you are output say 6W, on the flat say 3W.
What I do is I keep the lights on the whole time, and plug in the phone the whole time and just ride, and let the electronics which sense speed and ambient light to do the thinking for me, it makes a bright forward beam when I need and at night I find on the flat, the USB doesn’t get power, but gets power on downhills at night. In daylight, more USB power is available. Overall, a good solution. You can pretty much get home most days fully charged (if you been paying attention, a day you get home not fully charged is if you rode in the dark and had a uphill or a slower on the level just before getting home).
Overall, it means you can basically tour the world with a cellphone and your phone is recharged during the day from riding to last through evening/night and fully off-grid. Might be useful to carry solar to add a bit more if you need to operate say a tablet also.
If you look at costs of all these ideas, remember that batteries age and die. A dynohub and a light has no batteries, they wear with use, not elapsed time. Since I moved to using dynohubs and hub lights, my average costs of lighting has gone down. I been riding about 40 years so far, do about 10,000miles/year.
Just add: beans, rice, beer. No Lithium required.
I can do about 40 miles off a a $3 Burrito. What is your car like for gas?