How the wattage calculator works
Every appliance with a motor or compressor needs two numbers. Running watts are what it draws once it is going. Starting watts are the short surge it pulls for a second or two as the motor spins up. A fridge that runs on about 700 watts can briefly ask for more than 2,000.
The calculator adds up the running watts of everything you tick, then adds the single biggest start-up surge on top. That reflects how a house behaves in practice: two motors rarely start at exactly the same moment, so you size for everything running plus the worst surge. Most manufacturer sizing charts use the same method.
Turning the total into a generator size
Look for a generator whose running (rated) watts cover your running total with about 10 to 20 percent to spare, and whose starting (peak) watts cover the peak figure. Headroom matters. A generator working flat out all night burns more fuel, runs hotter and has no margin when the well pump kicks in.
If the result is more than you expected, decide what really has to run at the same time. Many people run the fridge and freezer in turns with a window AC or a microwave rather than all together. That one habit can drop the size you need by a whole class.
Where the numbers come from
The figures are typical values for common household appliances. They are not measurements of your appliances. Check the data plate or manual for the real running and starting draw, especially for well pumps, central air and anything with a large motor. Central air conditioners and heat pumps vary so much that I left them out; use the label figures or ask an HVAC tech.
For a whole-house or standby setup, an electrician can do a proper load calculation for your panel. For more help, see the sizing and wattage guides. Once you know the size, the fuel run-time calculator shows how long your fuel will last.