If your 100W light bulb is a USB bulb running at 5V (to make the math a bit easier), with 20 Ah you’ll get 100Wh, ergo it’ll run your bulb for a hour. You just need to specify the voltage. There are numerous online calculators to do this math for you.

https://www.inchcalculator.com/ah-to-wh-calculator/

Watt-hours won’t save you, because we don’t know what voltage your bulb needs. Don’t assume it’s 120/240V.

I mean, I know what voltage/wattage my devices need. I have no idea what voltage the battery bank is going to offer.

...by that logic if my 100W bulb is running on 10V, with my same 20Ah I'll get 200Wh, so I'll get a bulb of the same wattage, with the same battery, running twice as long? Magical double energy!

...this is why we should measure the total energy in Wh, not Ah.