Assuming a 100mm bore cylinder with a 1.5m stroke (~10L volume), operating at 150 bar, with a stroke time of 1 minute, the power required is:

Power in kW = Pressure in bar * Flow in liters per minute / 600

2.5 kW = 150 bar * 10 l/min / 600

or a little more than 3 horsepower. That gives you about 15 tons of force. Maybe the trucks use two cylinders, one on each side, so double it? Plus 10% for pump inefficiencies, and 5% for electric motor inefficiencies (200% for diesel-to-kinetic losses)?

Still negligible compared to accelerating a huge steel box plus dozens of tons of trash to roadway speeds 1000 times a day.

Looking up some existing PTOs and pumps on munciepower.com, it looks like typical refuse systems are provisioned with PTOs and pumps rated for on the order of 10-50 kW, and diesel "prime movers" of 200-400 kW.