Emergency or Longer Term Use
If you only plan on using your home generator during emergency power cuts you will probably not need as high a wattage as if you plan to use it for longer periods and want to run more appliances. If you just want to run basic services, such as the lights, refrigerator, freezer and small appliances, then a portable generator of 4 kilowatts will suffice.
If you want to keep running central heating, central air conditioning, water heating or other major appliances, such as a washer and dryer, then you may need a generator of up to 20 kilowatts.
Normal Running of the House
If you want to run your house normally, with all possible appliances able to function correctly, including major systems and appliances that require high wattage, such as the toaster and hairdryer, then you will need a generator that produces at least 35 kilowatts.