Hi and welcome
The most important question is, what's the NAS of your choice? If it is some simple single/two disk station with 12V brick, the best choice would be to get DC UPS directly. The availabiltiy is still kinda limited, but there are at least some units on the market in most countries and you can get others directly from China. Cause inverting the power from 12/24 V DC (nominal - batetry voltage) to 230 (120) V AC and back to 12 V DC is very inefficient and robs you of runtime.
If it is some larger unit (or) with embedded power supply, still most NASes draw up to about 100 W thus the PSUs ion these are simple and do not have active power corection (usually boost type). These units gladly accept anything you throw at them as after retification is gets smoothed right away by the bulk capacitor. So actually I would really recommend sine-wave output UPS just in case you have some professional/enterprise NAS (or actual server being used as NAS) with many drives and some high-quality high-efficiency (thus digitally controlled) power supply.