generator question

Hello wild_amative,
    A backup generator is usually a smaller, 2 to 6 thousand watt generator that is stored in the shed or garage that you pull out and run when the power goes out or batteries need charged. A standby generator, usually 6000 watts or larger that is permanently mounted outside the home and connected to the house's main electric panel by a transfer switch that will automatically start and supply power when power is lost or batteries need charging.
   As far as the type you would need for an off grid setup, most smaller off grid solar installations would have a backup generator. Larger installations can go either way. It's a matter of necessity, money, convenience and/or personal preference.  As far as sizing the generator, it depends on the size of the battery bank and the loads to be used when the power is out and/or charging the batteries. Sizing solar power systems and generators are based on loads. The best way to save money on power production is conservation. The more exact the estimation of your off grid power consumption the better the system will meet your needs. That way you will not be inconvenienced by spending too little or waste money by spending too much.