This is old—the USAAF bike (middle) has yellow film over it now. There is no flash or extra light in this room, it was taken at night and entirely lit by the bikes with the cheapest ~2015 iphone and its tiny light sensor.
Keep in mind that I am not an electrician, so this is what I've picked up trying to learn and it is hopefully correct, but I can't guarantee it. If anyone out there knows I'm wrong, please correct me.
I don't know what you're using for lights, so you have to figure out capacity yourself. It's not as straight forward as the simple formula: W=A*V, but it might be the only set of numbers (hopefully not too exaggerated) you'll get to work with and it should get you in the ball park. You might also get a C rating, which is the rate of charge or discharge the pack can handle, but for small LEDs, I don't think that's too important. That said, feel free to look up battery C ratings. The lesser the power of the lights or the higher the capacity of the battery, the closer it should be to matching the AH rating (I believe) since there will be less heat loss to discharge. I think that's why Retro Rocket's headlight runs on a shorter time than the advertised capacity of its battery would suggest, but my less powerful lights are pretty close. So, say you have a 12V 3W light you want to run for 2 hours. 3(W)/12(V)=0.25(A). One hour of run time would be 0.25 AHs, so to run on 2 hours, you'd want a battery of at least 0.5 AH rating or the more likely published conversion of 5000 mAh. The battery I use for Interrobang is about 9600 mAh and it's about the size of my hand with outstretched fingers. I think I paid about $30. If you get cheap li-Ion, keep in mind that they
are cheap, so I wouldn't leave them charging unattended as I don't really trust their BMS and thermal runaway is a potential issue (Tesla's most important advantage is that their battery management is really good—possibly the best—and they can run the batteries down further than most because of it, increasing range as well as managing rapid high-power discharge, increasing power delivery). LiFePo (lithium iron phosphate) are a little less power dense and more expensive, but are much less prone to thermal runaway. For the motorized kayak I'm building and electric bike that will follow, that's what I'm using as power usage is much higher, but for bicycle lights I use the cheap stuff.