A NAS (network attached storage) is a small always-on computer whose job is to hold drives and serve them to everything else on your network. It replaces a scattering of external disks and cloud subscriptions with one place that stores your files, backs up your computers, streams your media library, and increasingly runs the services your smart home depends on.
The cheapest models are simple file boxes; the expensive ones are home servers that happen to have drive bays. Almost every price difference between them comes down to five things: bay count, processor, memory ceiling, network speed, and which software runs on it.
A NAS sits alongside the other always-on hardware in a home network, next to Mini PCs and Routers, and it is a common place to host the controller for Cameras and Hubs.
Start with drive bays, not capacity
Drive Bays is the first number to settle: it constrains everything downstream and is the one thing you cannot upgrade later.
Two bays only allow mirroring: two 8 TB drives give 8 TB of usable space and survive one failure. Four bays are the sweet spot for most homes, because RAID 5 spends only one drive on parity and leaves three quarters of the raw capacity usable. Six bays and up make sense when you want RAID 6 and its tolerance for two simultaneous failures.
Be careful reading Max Raw Capacity: it counts every bay filled with the largest supported drive, before redundancy. A four-bay NAS rated at 80 TB raw gives roughly 60 TB usable in RAID 5, and 40 TB in RAID 6. Hybrid RAID modes such as Synology Hybrid RAID or TerraMaster TRAID are worth looking for if you expect to buy drives gradually, since they reclaim most of the space classic RAID wastes on mismatched drive sizes.
Hot Swappable bays let you replace a failed drive without powering the NAS down; entry-level enclosures often require a shutdown. Expansion Unit Support adds bays later through a proprietary external chassis.
RAID is not a backup
Redundancy protects against a drive dying. It does nothing about deleting the wrong folder, a ransomware run, a fire or a theft, since all of those hit every drive at once. That is what Snapshots and Cloud Sync & Backup are for.
The processor decides what the NAS can become
CPU Architecture is the biggest single fork. x86-64 chips from Intel and AMD run essentially the whole software catalogue. ARM chips from Realtek and Rockchip are cheaper and much more power efficient, but they run a reduced set of packages and frequently cannot run containers at all.
That matters because Docker Containers is how nearly all self-hosted software ships now, including Home Assistant, ad blockers, download clients and databases. If you intend the NAS to be a home server rather than a file share, check this first. Virtual Machines go further by giving a guest its own kernel, at the cost of a lot of memory.
Hardware Transcoding is the other one people get wrong. It is an integrated GPU, such as Intel Quick Sync Video, that converts video formats in dedicated silicon. Without it a media server does the work on the main cores, which pegs the processor and usually fails outright on 4K. It does not follow from raw speed: several fast AMD Ryzen Embedded models have no integrated graphics at all, so a cheaper Intel chip will out-stream them. Pair it with Plex Media Server if you plan to watch away from home. Separately, a Hardware Encryption Engine means AES instructions in the CPU: encrypted shared folders on a machine without one can lose most of their transfer speed.
Memory, and how much of it you can add
Plain file sharing does not need much Memory Capacity. Everything else does: containers, a media server, photo indexing and virtual machines all compete for RAM, and Btrfs File System and ZFS File System use whatever is free as a read cache.
So the number to weigh is Maximum Memory Capacity, not the shipped figure. Entry models solder their memory, which makes the shipped capacity a permanent ceiling. Being able to move from 4 GB to 32 GB or 64 GB two years from now is what keeps a NAS useful as you pile services onto it. ECC Memory corrects single-bit errors before they reach the disk, and appears on business-oriented models.
Network speed is usually the bottleneck
A single modern hard drive already saturates Gigabit Ethernet, so 1 GbE Ports cap real transfers at around 110 MB/s no matter what is inside the box. Fine for backups and documents, painful for video work or bulk copies.
2.5 GbE Ports have become the sensible baseline: roughly double the throughput, over the Cat 5e cabling already in most homes, and now common on consumer switches and laptops. 10 GbE Ports and 10 GbE SFP+ Ports are worth the money only when the array can actually feed them, which means several drives striped together or NVMe volumes, and only if you also own a 10 GbE switch.
Link Aggregation bonds two ports together. It raises total bandwidth across several clients at once and adds a measure of redundancy, but it will not make a single transfer faster than one port.
M.2 NVMe Slots are usually a cache in front of the spinning disks, which mainly helps thumbnail generation, database work and browsing a large photo library. Check whether they also support M.2 as Storage Pool, the difference between a cache and an all-flash share you can genuinely store files on.
The software is what you actually live with
The Operating System decides the app catalogue, how snapshots and backups are managed, and how long the unit keeps getting security patches. Synology DSM is the most mature and conservative. QNAP QTS goes deeper on virtualization and advanced networking. UGREEN UGOS Pro is newer, pairs strong hardware with a catalogue that is still filling in, and defaults to Btrfs. TerraMaster TOS, ASUSTOR ADM and Ubiquiti UniFi Drive each trade breadth for price or for fitting into an existing ecosystem.
On top of that sit the services worth comparing: AI Photo Recognition as a local replacement for a cloud photo subscription, a Surveillance App that turns the NAS into a network video recorder for IP cameras, and PC & Mac Backup agents that image whole computers rather than just holding copies of files.
For reaching any of it from outside the house, prefer a VPN Server or the maker's Remote Access Service over opening router ports, which is the most common way a home NAS ends up exposed to the internet. DDNS keeps a stable hostname pointing at a changing home IP address, and SSH Access lets you script jobs and inspect the system directly.
Power, noise and where it will live
A NAS runs every hour of the year, so Power Consumption is a real line on the bill: the gap between a 15 W unit and a 40 W one compounds. Scheduled Power On / Off with Wake on LAN lets it sleep overnight and still come up on demand.
Noise Level decides which rooms are viable. Below roughly 20 dB(A) a NAS disappears into a living space. Rack-mount chassis with small high-speed fans can pass 40 dB(A) and belong in a cupboard, so check Form Factor before assuming a unit can sit on a shelf.
Finally, UPS Support is cheap insurance. Losing power mid-write is one of the few events that can damage an array rather than merely interrupt it, and a NAS that talks to a battery turns an outage into a clean shutdown.
