Very easily exploitable. Just leave your computer connected to the server overnight and rack up some hours while keeping the server slot occupied. I wouldn’t be surprised to see 40/40 servers with nobody playing.
A good way around that would be to use an “upkeep fuel” that you can obtain from a special crafting station that works like a dismantling bench, but instead gives you different amounts of “upkeep fuel” depending on what you put inside. That way you can get your upkeep covered with any play style, as long as you actually play the damn game 
At least, that’s how I tried to propose it in my suggestion for upkeep system.