I am diagnosed ADHD rather than autistic (I do sometimes wonder though) and have been trying to work out the same thing.
I've found the concept of "Energy accounting" which I think is very helpful. That involves trying to make a note over a few days/weeks of how activities, tasks, people, environments, sensory inputs, experiences, etc (anything really) feel like they affect your energy or are affected by energy.
For example, I have found that walking is NOT inherently energy draining for me, but carrying something heavy is, being aware of the location of my children is, giving instructions to other people (e.g. children) is, experiencing uncomfortable temperature is (and my threshold is quite low for this - there is a very narrow band of what is comfortable temperature). This was surprising because I thought that walking physically exhausted me - it's not, it's all the other things around it.
And when I feel totally drained, it is difficult for me to watch certain TV programmes or read books, but I can read and respond to posts on forums. I can describe something in detail to another person by typing, but I find it physically difficult to open my mouth and speak out loud.
If I have to get up before 8am, it really depletes my energy for the entire day. Getting up after 10am is the best. But between 8-10 as long as I'm on my ADHD medication and have been to sleep before midnight is fine. (I suspect that I have a delayed sleep phase disorder but haven't looked into testing for this).
As for what gives me energy - this seems more tricky. Sometimes fresh air does, but only if it's not the wrong temperature. Singing can, but I feel a bit self conscious about singing where people are listening. Eating and drinking can help. Sometimes momentum helps so just getting up and doing something IRL rather than being sucked into the computer which is probably more draining again.
Obviously all the normal stuff as well like ask GP for blood tests. It might be worth experimenting with non-standard sleeping patterns and different elimination diets.