does imac need clean power from a standby genertor

I don't believe it's necessary to clean up generator power. Apple computers are designed to auto switch and adapt to the power supplied all over the world on so many grids w voltage ranging from 100-240. I would not worry at all about it.

I think you're asking about popular UPS marketing claims of implementing a "pure sine wave". That's completely unnecessary and overkill for any device that uses switching power supplies like Macs, and just about every other consumer appliance these days because they are highly energy-efficient. They are required to be.

By design a switching power supply draws power discontinuously, hundreds of times per second. All desktop Macs as well as any other well-designed power supply will include robust filtering and regulation. The characteristics and advantages of a sine wave are thereby lost.

Don't waste your money on a "pure sine wave" UPS for an iMac. I do recommend avoiding certain manufacturers though. APC is reputable, and theirs are the only ones I use.

As far as standby (diesel / gas) generators go, they are unaffected by grid anomalies and should produce very "clean" power. Their output regulation may not be as good as public utility power, but again they don't have to be for consumer devices like iMacs.

Wall power is more than sufficient. Add-ons (except for Apple Care +) are only good for complicating things - best avoided.