Most standby generators are about the size of a central air conditioning unit. But standby generators will need a minimum of 5 feet of clearance out the back of the unit for the hot exhaust. If landscaping, sidewalks, animal pens, or other delicate items are placed closer than 5 feet then they risk being damaged or destroyed. Otherwise generators will need 3 feet of clearance on the other three sides to be able to access the unit for its routine maintenance and any other service needs throughout the lifetime of the unit.