When conventional electrical power is down, during a power outage, or not available at a job site or campsite, a portable generator provides power where and when you need it. Remember, the best time to buy a portable generator is before you need it. Portable generators provide electricity by running a gas-powered engine and must be manually started. That turns an on-board alternator to generate electrical power. Power outlets on the unit allow you to plug extension cords, electric-powered tools and appliances into it. In general, the more powerful the generator, the more outlet combinations are available.