Sizing a back-up / standby / emergency diesel or natural gas generator set

The first step would be to find out your required load.  Contact your electricity supplier and ask them for a copy of your average load and peak load over the last 12 months.  You should also know the Amps of your incoming service.  Once you have the load information size the generator so the peak load is about 75% of your generator rated output.  New and Used Generator Inventory