So how far is the trip down, 10 miles? The potential energy of moving 65 tons 10 miles down (think of pulling a rope with a 65 ton weight) is a freaking lot of energy.
The ratio will tell you how much "excess" energy you get, the efficiency of converting that from mechanical energy to stored charge and back to mechanical energy will tell you how much energy you will have 'left'.
And as long as you don't run out of battery capacity to hold energy you're capturing on the way down, the math just works.
The weight is 45 tons going up, and 45 + 65 = 110 going down, right?
In a 100% efficient no friction wonder world, the load needed would be 45 tons going up, and 45 + 45 = 90 tons going down. This would balance out to 0 battery usage.
Since our going down weight is 110 tons and not 90 tons, we have 110 - 90 = 20 "extra tons" that can go to things like friction and imperfect conversion of energy forms.
efficiency * 110 tons * gh = 45 tons * gh
efficiency = 45 tons / 100 tons = 40.9%