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The motorisations of a generator may be:

• Alternator engine:

- petrol;

- diesel;

- air-cooled;

- water-cooled;

- electric start-up;

- manual start-up;

- 3,000 revolutions;

- 1,500 revolutions;

- synchronous;

- asynchronous.

Petrol generators are normally less powerful, though they have the advantage of costing and weighing less than those with diesel engines. In general they are quicker to start and are more resistant to the cold.

Diesel engines are more stable, have less running costs and are more reliable for hard work, over 8 - 10 kVA almost all generators on the market have diesel engines.

Electric start-up requires the presence of a battery, start-up motor and alternator (to recharge the battery) making it heavier, bulkier and more expensive. It is indispensable for automatic start-ups (but not sufficient in itself).

Manual start-up is achieved with an energetic tug on a cable wrapped around a pulley linked to the drive shaft. Basically it acts as a start-up engine. If the engine is in good conditions a couple of tugs are enough. Manual start-up can also be found in diesel-engine generators, but is rare.

In synchronous-alternator generators the alternator is set to supply energy at a determined frequency (standard 50 Hz) turning synchronously with the engine. Once the running temperature is reached the engine is set to a standard number of revolutions: 1,500 or 3,000. Obviously, to have the same power at half the number of revolutions means that the engine must be basically double its maximum power (therefore heavier, bulkier, more expensive...). The advantage is that an engine set to work at 1,500 revolutions is used less than one at 3,000 revolutions, and is therefore more suitable for heavy and continuous work.