You can determine what capacity generator you need by comparing the wattage rating of the generator to the wattage of lights and appliances you intend to operate during a power outage. You need to decide what you must keep running, such as a refrigerator, electric furnace, propane furnace blower motor, water heater, heat pump, or convenience appliances (TV, computer, etc.).
Motors typically require four times the power to start as they do to run. Estimate power requirements from equipment nameplates when possible. As a guide, electric motors require approximately 4,000 watts of power to start and 1,000 watts of power to run for every horsepower of output.
Backup generator safety
You must follow some basic safety rules when using a backup generator.
Do not plug a generator into an electrical outlet to feed power into your house wiring. It is illegal in Oregon. The generator can feel electricity back through the utility power lines and possible electrocute those working to repair the power.
Do not operate the generator in an enclosed or partially enclosed space. Gasoline or diesel engines may produce deadly levels of carbon dioxide/carbon monoxide. Propane generators can produce carbon monoxide. Fumes can accumulate in a building even with a large door, such as an open garage door. Wind blowing into an attached garage can push the carbon dioxide into the house. If a generator is operated in an enclosed building, the engine exhaust must be vented outdoors away from the building using engine exhaust ducting. We recommend a carbon monoxide gauge when using a generator.
Choose a generator that provides the power at the same voltage and frequency as the power lines supply. Most power lines supply 120/240-volt, single-phase, 60-cycle alternating current to homes and farms.
Connecting Portable Generators
Direct hookup using extension cords to connect to lights and appliances. Use only heavy-duty orange or yellow extension cords with adequate capacity to handle the wattage. Do not connect more wattage than the continuous duty rating of the generator.
Permanent connection using a transfer switch. This provides a safe way to connect a generator and usually is installed by an electrician. It assures that no power from the generator can reach the grid. (The main breaker of the household panel is NOT an approved disconnect device; never connect a generator to a household wiring by extension cords.) Because most people do not want to power their entire house, the transfer switch usually controls a sub panel, which feeds a few circuits for lights, outlets, and critical appliances throughout the house.
Locate the generator outside so exhaust fumes do not enter the house or attached garage. Follow all safety instructions included with the generator.
If you hear the motor slow down while using the generator, it means the generator is overloaded. If a generator is overloaded for prolonged periods, it will overheat and shut down.
A storage battery can be recharged with jumper cables using an automobile alternator.