A diesel generator size is selected based on the electric load it is intended to supply and the expected duty it is going to carry. Temperature, altitude, emissions should be taken into account as well. Many generator manufacturers will have the knowledge on which generators should be cater to your business/usage.
The power range differs from 8kW to 2000kW depends on the location and usage of the industries. It is pretty much depends on how big is your usage and you will go for the most suitable one. Apart from heavy usage industries, most of the small to medium sized businesses have bought in their own generators for backup power.
Every business will start losing money during blackout as they are forced to shut down. For example in retail line, you have 4 staff that you pay them $200/day. Your business daily revenue is $2000. Imagine the loss in revenue for having a blackout a few days. It is worth the investment if you have a generator ready.
Most industries, such as the oil and gas especially, will purchase their own custom made generator. Their production line cannot afford to have blackout even for 1hour as the loss will be more than millions.
They will work with their generator vendors to have the units designed especially for their production operation. It must be able to cater a smooth workflow for them during emergency. Most of the vendors will have the right knowledge to design and install the custom made units on the industry. They are also the people maintaining your unit for you. Probably at a quarterly schedule just to check on your generators. This maintenance will be able to prolong the lifespan on your machine.
It is also recommended that you have your technician equipment with the basic handling and maintenance skills of the diesel generators. They will be able to manage the generator during emergency. With this, you will not have to worry of the hassle of calling your vendor.
You have spent so much on your machine. It will be wise to invest in the maintenance rather than paying for the replacing parts as it will take more time for you to fix and replace the machine.