A generator is a machine used to convert mechanical energy into electricity. Powered by a fuel source such as oil, gasoline, wind or moving water, generators create electrical current through electromagnetic induction. Generators widely serve as backup power sources for factories and hospitals, where the facility can set them up to work immediately if the main power goes out. Commonly used by residential homes and small businesses, commercial generators are usually the size of a large barbecue grill and allow for easy storage.

Every machine contains an engine, which is generally the portion of the machine that converts the fuel source into usable energy and allows it to move or perform its mechanical function. For this reason, engines are sometimes called the machine’s prime mover. In a generator, the engine uses its fuel source (gasoline, diesel, natural gas, propane, bio-diesel, water, sewage gas or hydrogen) to create mechanical energy the generator will convert into electricity. Each generator engine's design aims to create a maximum supply of electrical current by running on a specific fuel or other power source. Some engines commonly used in the design of generators include reciprocating engines, steam engines, turbine engines and microturbines.

Generators running on fuel have a system that stores and pumps the appropriate fuel to the engine. The tank stores enough fuel to power a generator for an equivalent number of hours. The fuel pipe connects the tank to the engine, and the return pipe connects the engine to the fuel tank for return of fuel. The fuel pump moves the fuel from the tank through fuel pipe and to the engine. A fuel filter filters any debris from the fuel before delivery to the engine. The fuel injector atomizes the fuel and injects it directly into the combustion chamber of the engine.

The alternator converts mechanical energy produced by the engine into electrical current. The alternator comprises the stator and the rotor (or armature). The stator is a stationary part containing a set of coils that conduct electricity. The rotor moves to create a constantly rotating electromagnetic field around the stator. The alternator generates electrical voltage. The generator must regulate the voltage to produce a constant current suitable for practical use.

The temperature of generator components requires regulation to prevent overheating during use. Generators can use a fan, coolant or both to control the temperature of the generator at work. The generator will also produce exhaust as the combustion chamber converts fuel. Exhaust systems dispel the harmful gases emitted by the generator during use. Generators comprise many moving parts, each of which requires oiling to ensure smooth functioning. The lubricating system keeps the generator well oiled.