This kind of device (24 port Ethernet Switch) usually requires a small amount of power, between 20 and 50 VA. The 500 VA UPS you mention (APC BackUPS ES) is more than enough to support this device for more than 30 minutes (I should say about 1 hour). You even could use the model 350 (350 VA), which will give you at least 15 minutes of run time.
Of course, if you install another switch the power consuption will be as twice as before, and the run times one half. But even in this case, with 2 of these switches, you should be able to use the model 350 for at least the required 5 minutes.
With the NAS the story is different. Depending on the power requirements you could be short, even with the model 500 (at 500 VA, this model will give you less than 2 minutes of run time). If you plan to put a NAS in the near future, you should add the proper UPS to protect this solution in the NAS specification and budgeting. UPSs are cheap after all, and the protection offered is more than necessary.
Regards,
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Jose P. Mir
jpm@jpmir.net