I found a battery that might be good to use it's a: Rechargeable 12volt 7.0AH Sealed Lead Acid Battery. Is that good to use with a lighting setup?
Just try it and see how it goes, then tweak as required from there.
At least you'll have 95% of the work done with wiring, clips etc.
If THAT battery don't suit the application, just try another battery to the work/wiring that's already been established.
12 v 7 ah is what I run!
I expect an hour of power with 55 watts, with some in reserve. Have heard of a H3, 35 watt bulb which I may persue. 35 watts= low beam headlight.
It's bad to repeatedly drain a battery to less than half its capacity, with a 50 minute ride I am just making it to this point.
One will have to charge said battery on shore power until they figure out the on-bike charging, but my electric rates are still reasonable compared to the cost of gasoline, (even more reasonable if I charge at work) and even a coal fired plant is way greener than my 2-stroke.
Watts are just volts times amps so a 20 watt would draw 1.666 amps, a 7 ah battery would last 4.2 hours at 100% capacity so your ride time should stay under ~2.5 at the most.