Engine sizes vary from small electric or gasoline powered, units with horsepower of 1 HP or less to larger 4 to 10 HP, usually gasoline powered units. Some of theses are available with an electric starter which can be a useful feature for larger engines. Some shredders are available that have a chute for receiving leaves near the ground so that the leaves can be easily raked into the unit. Some have the capability to be used as a vacuum to pull leaves into the unit and some have wheels to allow operation over the lawn and landscape much like a lawn mower. There are even self propelled models.
Shredders operate by a using fixed 'hammers' or hinged flail blades that spin at a high speed.
Some models have discharge screens which allow the shredded material to be discharged only after it has been ground to a certain fineness. Chippers usually operate with knives fixed to a rotary disk that chop wood fed through a chute into small pieces. Some chippers have knives that can be turned over when one side becomes dull. Chipping of green wood is usually easier and less damaging to the blades than drier wood which is harder. The size of the branches that can be chopped is limited by the capacity of the machine.