Generally speaking, a standby generator is not designed for long operation periods or often use. They are meant to just sit for most of their life, and run for a few hours to a day tops, then be shut down and go back to sitting.
A key example is, on a standby generator things such as the charger for the engine battery (that runs the ECM/ignition circuit) are connected to a charger/maintainer that feeds from the grid. When grid power goes down, this charger stops topping up the battery. This is a classic case of an upset Generac customer – their power goes out and then the generator starts, and then 20 hours in, the battery goes dead, the generator shuts down, and they are in trouble because they have no power to recharge the battery.
Furthermore, standby generators are built much less robust because the use case only demands a few hundred hours of use in their lifetime. Often times they are purely air-cooled versus water-cooled, and are much noisier. They have service intervals more often and just don’t last in continual, heavy use.
A true off-grid generator is commonly equipped with an alternator, so as to keep the battery charged at all times. Off grid generators usually run at much lower engine RPM’s, have superior water cooling systems and acoustic enclosures to keep noise down because they are meant to run, and then Run, and then RUN.
They can last tens of thousands of hours before requiring major maintenance, and the regular maintenance is going to be less often than a standby generator.
A fine example is a 14kw Wacker-Neuson with a 3cyl Kubota motor installed at a customer’s 40-acre off-grid property. As of this post, it has 14,000 hours and has only had filters/oil/fuel, at ~9000 hours the AVR board was replaced, and there has not been a single issue since.