100 AMP SERVICE IN GARAGE

Can I install 240 volt, 1500 watts electric baseboards - 4 in new 100 amp service in 500 sq feet garage?

Everything else will use 110 volts - power saw, TV, stereo. I just to keep it around
50 degrees F when not in use.

P=VI

240*100

24000 Watts at 100 percent

Baseboard will use 6000 max. So your ok.

...and still have service left over for an air compressor, welder, plasma cutter, kitchen stove, microwave and a little more!

I assume you will be installing a 100 amp sub feeder panel feed from your main house fuse panel?

what is the capacity of the main panel and how tapped out is it? You have to figure this out to determine if you can pull a separate sub feeder panel of of it
now if you are going with a completely new drop from the street you will have plenty of amps to run that garage
A side bar if you do not have a 200 amp main breaker into your original home, 100 to the garage is not a good situation

You have to look at the main breaker many times a 200 amp panel is used with only a 150 main amp in

Yes, I just upgraded the home to 200 amp, which has 100 amp breaker for the garage.
both panels have at least a dozen single breaker slots available.

Without getting into a "long discussion on 'load calcs'..." a panelboard (regardless of it's rating) 'design criteria' is 125% of the total connected load and can only be loaded to 85%...

Load a 100 amp panel to 98 amps...(or a 200 amp panel to 198 amps... seen it happen) and you "will have a few laughs".

But as to the "original question" I would say "yes it is acceptable"....

Is it OK to wire them in series or have them isolated one per breaker?