There are many kinds of generators which are used when the power goes off either because of vagaries of nature or if something else happened that can put the lights off and they actually come in very handy. They are installed outside the house so as to keep the noise low and also prevent the fumes of the fuel they use to get inside the home and some are usually automatic and they switch on immediately the power goes off.
There are others which use diesel and petrol and these devices use electricity generation which makes conversion of motive power to power which is electrical and there are many sources which may include hand cranks, combustion engines, water turbines, gas turbines, and steam turbines. Many of the devices are driven mechanically so as to give generation of electricity and some electric generators which use direct current have magnetic field which is static and there are qualified companies which provide all kinds of generators and they also do the installing of these generators.
They turn on automatically and one does not need to go outside to crank it up. They know what particular home needs and they go to the site so as to find out what range of power output the home may need and also what kind of generator is suitable for the home.
The stand by generators providers may go and see if the devices are working well and in some cases repairs are done such as when fluid gets accidentally in the generator and makes it not to start or when the battery gets drained and goes cold because of vast coldness or after a period of time. Some home generators have multiple of them which are connected in series so as to produce more electricity depending on the requirement of the home which can be as much as a million of more amperes because they have internal resistance which is low.
The are many manufacturers of different types of home generators but they all work the same way by producing electricity when there is a power outage and this concept has helped very many homes to constantly have electricity and many people may not actually even know when the stand by devices are in use or when the normal electricity from the groin is being used.