When I noticed that the in-game codex doesn't outline any specifics for upkeep, I had to dig in and figure things out. In fact, it turned into all I really wanted to do in the beta, and without wanting to ruin an actual game with other people, I had 2 opportunities of free time with a friend to jump into a 1v1 and experiment while they tested out the game mechanics. This is what I found:
Your upkeep is 0 until you go above 30 supply. From there, for every 2 supply you gain, you lose 4 requisition per minute and 1 power per minute. These were the numbers I jotted down as I built workers, then some other units, then spot-checked it later on in the game:
I think the biggest takeaway here is that your power generators are of extreme importance. If you are at +100 power per minute, and you are sitting at 50 supply, you are losing 10% of your power per minute because of your army. If you get into intense early fights and bloat your supply up to 80 with cheap units, you are losing 25% of your power per minute.
Do you ever suddenly notice you have 1500 req and 100 power? What are you compelled to do? If it is "purchase more units to spend the req", you are potentially obliterating your power economy in the process. If you spend 1200 req on 3 Tac squads as SM, and you are above 30 supply, you are taking a -15 power per minute hit to your resource generation due to the 30 supply you are also accruing.
Even if a unit only costs req to build, it is costing you power above 30 supply.