First off, I want to say, I think it's cool to consider this scientifically. If a vampire is driven by hunger for human blood, it makes sense to ask the question, "how much is enough."
Using a vampire bat as a guide isn't good I don't think because due to the volume of the bat relative to the skin's surface area, the bat loses a lot of energy and therefore must eat more relative to its body weight than other creatures. The shrew for instance is so small, it loses so much energy it eats almost constantly. Vampires are much bigger and being dead, don't lose energy at all when still.
I think you can justify any amount you chose, but if I were to approach this scientifically, I think I would vary the amount used. The cool thing is, when at rest, no energy used. Humans use, I think, about 400 calories while they sleep just to stay alive. Vampires on the other hand classically have the ability to climb walls, change form, move with ethereal quickness and charm people.
I haven't done the calculation, but since it's imaginary (except my next door neighbor) you could just do a quick and dirty calorie calculation for everything the vampire does assigning them values and then use the values given from these answers about calories in blood and reconcile the two. Example: the CDC says a 150 pound person uses 110 calories to do moderate exercise and twice that do vigorous exercise. So our vampire sits in a chair monologuing to his would-be hunter: 0 calories burned.
Hunter makes a move toward his knife. Vampire moves at lightning speed to come up behind the hunter to prove his superiority. That's superhuman speed. I'm thinking 1000 calories, right there. Then the hunter plasters a cross on his forehead, burning him, weakening him. Maybe he has to make that up in blood. That cost him a couple hundred calories worth of energy. Now he has to fight. If he fought like a human, 220 calories for 30 minutes. If he uses superhuman strength, 900 calories. After he dispatches the human, he sucks down 5 liters of blood for 1785 calories. Sounds like he's still hungry and might want to go trolling for more victims.
But on a light night, he might just stroll down the dark streets at pedestrian speeds using very little energy and snack on a hooker for a nice positive energy total. Also, maybe vampires can go for a little while at a deficit, like humans can. I'd say in a normal night for a vampire he'd need to drain about 1 human. If he's been battling a group of five vampire hunters, depending on the ferocity of the battle, I'd say maybe 1 human for every 5 minutes of melee. On a light night, maybe he would just down a pint or two from someone he has chained in the dungeon. No need to kill him, just keep him weak, which incidentally is exactly what Dracula did. He didn't kill them all.