UPS Sizing Guide for Home Servers

Quick Recommendation

For a single mini PC or NAS (50–100W): a 600 VA / 360W UPS gives you 15–30 minutes of runtime — enough for a clean shutdown. For a full homelab rack with server + switch + NAS (200–400W): a 1500 VA / 900W UPS provides 10–20 minutes. Buy the smallest UPS that gives you at least 10 minutes of runtime at your actual load.

Why You Need a UPS

A sudden power outage with no UPS means:

ZFS: Writes in progress are lost. If you’re mid-scrub, the pool may need repair.
Databases: PostgreSQL, MariaDB, and Redis can corrupt data on unclean shutdown. WAL files may be incomplete.
Docker: Containers with pending writes lose data. Writable overlay layers can become inconsistent.
Filesystems: ext4 journals recover most corruption, but metadata-heavy operations in progress can still cause issues.
Hardware: Repeated hard power-offs stress SSDs (wear leveling cache loss) and HDDs (head crash on older drives).

A UPS does two things: absorbs brief power blips (the majority of outages) and provides enough runtime for a clean shutdown during extended outages.

UPS Basics

VA vs Watts

UPS capacity is rated in two units:

VA (Volt-Amperes): Apparent power — what the UPS can deliver
Watts: Real power — what your equipment actually consumes

The relationship: Watts = VA × Power Factor. For typical computer loads, the power factor is 0.6–0.7, so a 1000 VA UPS delivers 600–700W of real power.

Always size by watts, not VA. If your gear draws 300W, you need a UPS that delivers at least 300W (typically a 500+ VA unit). Running at 60–80% of rated capacity is ideal — it maximizes runtime and battery life.

UPS Types

Type	Protection	Price Range	Best For
Standby (Offline)	Basic: switches to battery on outage (5–12ms transfer)	$50–150	Low-power servers, routers, switches
Line-Interactive	Better: AVR corrects voltage fluctuations without battery drain	$80–300	Home servers, NAS, most homelabs
Online (Double Conversion)	Best: always runs on battery, zero transfer time	$300–1000+	Critical infrastructure, sensitive equipment

For home servers, a line-interactive UPS is the right choice. It handles both outages and voltage fluctuations (brownouts, surges) without burning through the battery. Standby UPS units work fine for low-power setups. Online UPS is overkill and noisy (the inverter fan runs constantly).

Sizing Calculator

Step 1: Measure your actual power draw

Plug a Kill A Watt meter ($20–30) between your equipment and the wall outlet. Note the idle wattage — this is what the UPS must sustain during an outage.

If you don’t have a meter, use these estimates:

Equipment	Idle Power (W)	Load Power (W)
Intel N100 mini PC	6–10	20–25
Dell OptiPlex Micro	15–25	40–65
Dell OptiPlex SFF	25–40	60–100
Synology DS920+ (4 HDDs)	25–35	40–55
Synology DS1621+ (6 HDDs)	35–45	55–70
Custom NAS (4 HDDs)	40–60	70–100
Tower server (Xeon)	60–120	150–300
Unmanaged 8-port switch	5–8	5–8
Managed 24-port switch	15–30	20–40
Router (consumer)	10–15	10–15
Raspberry Pi 4/5	3–5	7–10
Cable modem	8–12	8–12

Step 2: Add up your total idle wattage

Example homelab:

Device	Idle (W)
Dell OptiPlex Micro (server)	20W
Synology DS920+ (NAS)	30W
UniFi Switch 8 PoE	15W
Cable modem + router	20W
Total	85W

Step 3: Choose a UPS with adequate runtime

Target at least 10 minutes of runtime at your actual load. This is enough for NUT (Network UPS Tools) to detect the outage and initiate a clean shutdown.

UPS Model	VA/W Rating	Runtime at 85W	Runtime at 200W	Price (Feb 2026)
APC BE600M1	600 VA / 330W	~25 min	~8 min	$60–80
APC BR700G	700 VA / 420W	~30 min	~12 min	$80–110
CyberPower CP1000PFCLCD	1000 VA / 600W	~45 min	~18 min	$120–150
APC BR1500MS2	1500 VA / 900W	~60+ min	~25 min	$180–230
CyberPower CP1500PFCLCD	1500 VA / 1000W	~70+ min	~30 min	$180–220
APC SMT1500	1500 VA / 1000W	~65 min	~28 min	$350–450

For the example homelab at 85W: the APC BE600M1 provides ~25 minutes — plenty for a clean shutdown. No need for the 1500 VA unit.

Step 4: Verify the outlet count

UPS units have two types of outlets:

Battery + Surge: Connected to the battery — these stay powered during an outage
Surge Only: Surge protection but no battery backup

Put your critical equipment (server, NAS, router, switch) on the battery outlets. Non-critical items (monitors, desk lamps) go on surge-only outlets.

Automated Shutdown with NUT

Network UPS Tools (NUT) monitors the UPS and triggers a clean shutdown when the battery gets low.

Install NUT

sudo apt install nut

Configure NUT for a USB-connected UPS

Edit /etc/nut/ups.conf:

[myups]
    driver = usbhid-ups
    port = auto
    desc = "APC Back-UPS"

Edit /etc/nut/upsd.conf:

LISTEN 0.0.0.0 3493

Edit /etc/nut/upsd.users:

[admin]
    password = your_secure_password
    upsmon master

Edit /etc/nut/upsmon.conf:

MONITOR myups@localhost 1 admin your_secure_password master
SHUTDOWNCMD "/sbin/shutdown -h now"
POWERDOWNFLAG /etc/killpower

Edit /etc/nut/nut.conf:

MODE=standalone

Start and enable:

sudo systemctl enable nut-server nut-monitor
sudo systemctl start nut-server nut-monitor

Check UPS status

upsc myups

Key fields:

battery.charge: Current battery percentage
battery.runtime: Estimated runtime in seconds
ups.status: OL (online/normal), OB (on battery), LB (low battery)

NUT with Docker

Run NUT in a container and share the USB device:

services:
  nut:
    image: instantlinux/nut-upsd:2.8.1
    restart: unless-stopped
    devices:
      - /dev/bus/usb:/dev/bus/usb
    ports:
      - "3493:3493"
    environment:
      - API_USER=admin
      - API_PASSWORD=your_secure_password
    volumes:
      - ./nut-config:/etc/nut

Shutting down multiple machines

If your NAS and server are separate machines, run NUT in client mode on the secondary machine:

On the NAS (/etc/nut/nut.conf):

MODE=netclient

/etc/nut/upsmon.conf:

MONITOR myups@server_ip 1 admin your_secure_password slave
SHUTDOWNCMD "/sbin/shutdown -h now"

When the UPS hits low battery, both machines shut down cleanly.

Battery Maintenance

Battery lifespan

Typical UPS batteries (sealed lead-acid) last 3–5 years. Replacement batteries cost $20–50 for most consumer UPS units.

Signs the battery needs replacement:

UPS beeps constantly or shows a “replace battery” indicator
Runtime has dropped significantly from when it was new
The UPS fails self-test

Temperature matters

Battery lifespan halves for every 10°C above 25°C (77°F). Keep the UPS in a cool, ventilated location — not inside a closed cabinet or next to a heat-generating server.

Self-test

Most UPS units run a periodic self-test (weekly by default). You can also trigger one manually:

upscmd myups test.battery.start

FAQ

Can I use a UPS for my router and modem too?

Yes, and you should. A UPS on your router + modem keeps your internet connection alive during brief outages, which means your server stays accessible remotely and monitoring alerts still work.

How long should a UPS last before shutdown?

10 minutes is the practical minimum. This gives NUT time to detect the outage, wait for brief outages to resolve, and execute a clean shutdown. 15–20 minutes provides comfortable margin.

Does the UPS need to power everything?

No. Only connect equipment that needs a clean shutdown (servers, NAS, network gear). Monitors, printers, and non-critical peripherals don’t need battery backup.

Should I buy APC or CyberPower?

Both are reliable. APC has broader NUT compatibility and a longer track record. CyberPower offers better value at the 1000–1500 VA range. Check NUT’s hardware compatibility list before buying a less common brand.

Can I use a car battery or lithium UPS?

LiFePO4 (lithium) UPS units exist and offer 2–3x longer battery life than lead-acid. They’re more expensive upfront ($200–500) but may be cheaper over a 10-year lifespan. EcoFlow and Bluetti make home UPS units with lithium batteries.