Portable generator
Often the lower-cost choice for selected circuits or individual loads. It requires safe outdoor placement, proper connection equipment and manual fuel management.
Step 1: Decide what must stay on
Make a list of essential loads rather than assuming the whole house needs power. Common priorities include the refrigerator, freezer, furnace or boiler controls, well pump, sump pump, selected lighting, internet equipment and a few receptacles. Air conditioning, electric water heating, electric ranges and clothes dryers can dramatically increase required generator size.
Step 2: Add running watts
Use the nameplate or manufacturer data for each appliance. Add the running wattage of the loads you expect to operate at the same time.
Step 3: Allow for starting watts
Motors and compressors can require a brief surge when they start. Refrigerators, pumps, furnaces and air conditioners are common examples. Your generator must handle both the normal running load and the largest expected starting surge.
Standby generator
A permanently installed unit can automatically start and transfer selected or whole-house loads. Correct sizing also depends on available natural-gas or propane supply.
Think in load tiers
Essential-load system: refrigerator, heating controls, lights, communications and a pump or two.
Comfort system: adds more kitchen circuits, more receptacles and possibly smaller HVAC loads.
Whole-house approach: may include central air conditioning and other large loads, but can require substantial generator capacity, fuel supply and load-management controls.
Don't forget the fuel system
A natural-gas generator can only deliver its rated output if the gas service, meter and piping can supply the required input while other gas appliances are operating. Propane systems also need adequate storage and vaporization capacity. Portable gasoline units require safe fuel storage and rotation.
Transfer equipment matters
A generator must not backfeed utility wiring. A listed transfer switch, interlock system or automatic transfer equipment appropriate to the installation is a core part of the design. Permits and licensed electrical work may be required.
A useful rule
Size from a measured or well-documented load list, not the square footage of the house alone. A smaller home with electric heat can need more generator capacity than a larger home with gas appliances.
General educational information only. Generator systems involve lethal electrical, carbon-monoxide and fuel hazards. Never operate a combustion generator indoors or in an attached garage, and never connect a generator in a way that can backfeed utility lines. Use qualified professionals and follow manufacturer and local code requirements.