The biggest mistake contractors make when buying a generator is underestimating the actual power requirements of their tools and equipment, generator manufacturers agree. Insufficient current will cause a tool's electric motor to overheat and fail. To select the best generator for a job, you need to acquire a basic understanding of electricity and assess your power needs. Portable generators, which can be transported easily to and around the jobsite, can satisfy the power requirements of masonry contractors until a site is hooked up to electric utility lines. There are two types of electrical loads--resistance and induction. Most power tools and equipment used on a jobsite impose induction loads. Two types of motors impose induction loads: universal motors (with brushes), which power drills, hand-held saws, and grout vibrators; and induction motors (without brushes) which power table saws, bench grinders, air compressors, and pumps. According to one manufacturer, the main way people wreck their hand tools is by attaching excessively long extension cords to the generator. This builds up a lot of resistance and raises the current in the system. Because the windings are usually sized for a certain maximum amperage, they become overheated if this current strength is exceeded, causing the tool to burn out after a period of time.