Cats Sabotage Hydro Bill

I get home at 6 pm and Chewie is meowing thru the door. Nothing unusual, I think, until I open the door and hear the sound of running water, and notice the faint scent of chlorinated water. It could have been my swimsuit and towel from last night's swim, but I'm thinking that stupid cat number one or stupid cat number 2 turned on the tap by accident and the water had been running for quite some time. I hope my landlords don't notice it on the hydro bill! How much water would that be, say, if it was running for 1 hour? 5? Do the math.