Based on my initial testing, it's been powering two tanks across all the heating and cooling, pumps, powerheads, skimmer, and partially fuge lights, and a hydra26, and is running fine for about 5 hours so far.
In that time it averaged about 0.6% of the battery an hour. I wish I had a graph to tell if it's constantly doing so.
Tomorrow it'll get a full charge, and switch a couple plugs around. I'll either run just the return pumps or just the wave makers off the car, and leave everything else as is. In the event there's a power outage I can have my neighbor flip the entire power strips over to the extension cord when they get around to it. In the event somehow that went awry and the car stopped powering, I'd still have the other, powered from the house and grid, pump/powerhead moving water.
I'm honestly pretty chuffed at how cool this is. I literally can run two reef tanks off my car, for 6+ days with full everything. If that's not the future I don't know what is.