I drink a lot of tea. About 1 liter a day as a baseline, but if I do some ultrahike or something I can easily drink 3 liters a day. Sometimes for some reason it's really hard to drink my tea (e.g. I'm in a hotel that doesn't have drinkable tea), so I tried to substitute with other caffeine products. Unfortunately I couldn't find out any reliable data on the caffeine content of the tea that I drink. All the information I could find is the worthless X-5X mg caffeine/cup, usually without even defining what a cup is!

Does anyone know a better resource?

Anyway, subjectively speaking I find that all the online estimates are too low, at least for the type of tea that I drink and in the concentration I prefer.

Also, subjectively it seems that tea has a more complex psychoactive effect that either pure caffeine or coffee. If I drink enough caffeine, I can avoid the symptoms associated with caffeine withdrawal, but I don't get nowhere near as strong as an effect as I get from from tea, even if I drink a lot of caffeine. It appears tea has other stuff in it apart from caffeine.

Now when I travel I take my own tea, cup, and immersion heater with me.

As I said, I really like tea, but only good tea that properly prepared is good. What passes for tea these days, sometimes even in tea shops (!) is usually poison. An easy litmus test is this: do they use a thermometer when making green or white ea? If not, that means they use boiling water which will destroy the tea. I think most people never had good tea in their life. I strongly dislike coffee, but I somehow suspect I only drank bad coffee that was not prepared correctly, and that I would enjoy good coffee.