Home Server Battery Backup Guide

Q: How do I know when to replace the battery?

NUT reports battery health. When `battery.charge` no longer reaches 100% or runtime drops significantly, replace the battery. Most UPS units also have a "replace battery" LED/alarm.

Quick Recommendation

For a mini PC server (50-100W load): CyberPower CP685AVRG (~$75, 685VA/390W, ~~15 min runtime). For a NAS + switch + server (200-400W load): APC Back-UPS Pro 1500S (~~$200, 1500VA/900W, ~10 min runtime). Pair either with NUT (Network UPS Tools) for automatic shutdown when battery gets low.

Why Your Server Needs a UPS

A power outage without a UPS means:

Data corruption — file writes interrupted mid-operation corrupt filesystems, databases, and Docker volumes
ZFS/Btrfs risk — copy-on-write filesystems are resilient but not immune to dirty shutdowns
Hardware damage — power surges when electricity returns can fry PSUs and drives
Downtime — server won’t restart automatically if BIOS isn’t configured for it

A UPS gives you 5-30 minutes of battery power — enough for an orderly shutdown that closes databases, stops containers, and unmounts filesystems cleanly.

UPS Types

Type	How It Works	Switchover Time	Best For
Standby	Battery kicks in when power drops	5-12ms	Basic protection, low loads
Line-Interactive	AVR regulates voltage; battery for outages	2-4ms	Home servers (recommended)
Online (Double Conversion)	Battery always inline	0ms	Critical infrastructure

Line-interactive is the sweet spot. The Automatic Voltage Regulation (AVR) handles brown-outs and voltage sags without switching to battery — extending battery life. Standby UPS units drain battery on every minor fluctuation. Online UPS is overkill and expensive for home use.

Sizing Your UPS

Step 1: Calculate Your Load

Measure or estimate the total power draw of everything on the UPS:

Device	Typical Draw
Mini PC (N100)	8-25W
Desktop server (Ryzen/Intel)	50-120W
Enterprise rack server	150-350W
4-bay NAS (Synology/QNAP)	30-50W
Managed switch	10-20W
Router	10-15W
PoE switch (with APs powered)	30-80W

Step 2: Size the UPS

Rule of thumb: UPS VA rating should be 1.5-2x your total load in watts. This ensures adequate runtime and doesn’t stress the battery.

Total Load	Minimum UPS	Recommended UPS
50-100W	500VA	750-1000VA
100-200W	750VA	1000-1500VA
200-400W	1000VA	1500-2200VA
400-600W	1500VA	2200-3000VA

VA vs Watts: UPS capacity is rated in VA (volt-amperes). Watts = VA × power factor. Most UPS units have a 0.6 power factor, so a 1500VA UPS delivers 900W. Always check the watt rating, not just VA.

Step 3: Estimate Runtime

Runtime depends on load and battery capacity. Manufacturer specs are optimistic — expect 70-80% of advertised runtime.

UPS	Rated VA/W	Runtime @ 100W	Runtime @ 200W	Runtime @ 400W
CyberPower CP685AVRG	685/390W	15 min	6 min	—
APC Back-UPS Pro 1000	1000/600W	25 min	11 min	4 min
APC Back-UPS Pro 1500S	1500/900W	40 min	18 min	8 min
CyberPower PR1500LCD	1500/1050W	35 min	15 min	7 min

You don’t need 30+ minutes of runtime. The purpose is to shut down cleanly, not ride out the outage. 5-10 minutes is enough for a graceful shutdown. Save money on a smaller UPS and invest in a proper shutdown script.

Top UPS Picks

CyberPower CP685AVRG — Best for Mini PC Servers

Spec	Value
Type	Line-interactive
Capacity	685VA / 390W
Battery outlets	4
Surge-only outlets	4
Interface	USB (HID-compatible)
Runtime @ 50W	~30 min
Runtime @ 100W	~15 min
Price	~$75

Perfect for an Intel N100 mini PC + switch + router. The 50W load gives 30 minutes — more than enough.

APC Back-UPS Pro 1500S (BR1500MS2) — Best Overall

Spec	Value
Type	Line-interactive (with AVR)
Capacity	1500VA / 900W
Battery outlets	6
Surge-only outlets	4
Interface	USB (HID-compatible)
Runtime @ 100W	~40 min
Runtime @ 200W	~18 min
Runtime @ 400W	~8 min
Price	~$200

The homelab standard. Handles a server + NAS + networking gear comfortably. The pure sine wave output is safe for active PFC power supplies (which most modern PSUs use).

CyberPower PR1500LCD — Best for Rack Mount

Spec	Value
Type	Line-interactive
Capacity	1500VA / 1050W
Form factor	2U rack-mountable
Interface	USB + serial
Runtime @ 200W	~15 min
Price	~$280

Fits in a server rack next to your enterprise hardware. 2U form factor, front LCD display.

Automatic Shutdown with NUT

NUT (Network UPS Tools) monitors your UPS via USB and triggers a clean shutdown when battery runs low. It’s the standard for Linux UPS management.

Install NUT

sudo apt install nut

Configure NUT

# /etc/nut/ups.conf
[myups]
    driver = usbhid-ups
    port = auto
    desc = "APC Back-UPS Pro 1500S"

# /etc/nut/upsd.conf
LISTEN 0.0.0.0 3493

# /etc/nut/upsd.users
[admin]
    password = changeme
    actions = SET
    instcmds = ALL

[upsmon]
    password = changeme
    upsmon master

# /etc/nut/upsmon.conf
MONITOR myups@localhost 1 upsmon changeme master
SHUTDOWNCMD "/sbin/shutdown -h now"

# /etc/nut/nut.conf
MODE=standalone

# Start NUT
sudo systemctl enable nut-server nut-monitor
sudo systemctl start nut-server nut-monitor

# Test
upsc myups

NUT as Docker Container

If you prefer running NUT in Docker alongside your other services:

services:
  nut-upsd:
    image: instantlinux/nut-upsd:2.8.2
    ports:
      - "3493:3493"
    devices:
      - /dev/bus/usb:/dev/bus/usb
    environment:
      - API_USER=admin
      - API_PASSWORD=changeme
      - DESCRIPTION=Home Server UPS
      - DRIVER=usbhid-ups
      - PORT=auto
    restart: unless-stopped
    privileged: true

Shutdown Script

Create a script that gracefully stops Docker containers before system shutdown:

#!/bin/bash
# /usr/local/bin/graceful-shutdown.sh

# Stop Docker containers gracefully
docker stop $(docker ps -q) --time 30

# Sync filesystems
sync

# Shutdown
shutdown -h now

Configure NUT to call this script:

# In /etc/nut/upsmon.conf
SHUTDOWNCMD "/usr/local/bin/graceful-shutdown.sh"

Multi-Server Shutdown

NUT supports a master/slave topology. The server connected to the UPS via USB is the master. Other servers on the same UPS monitor the master over the network:

On slave servers:

# /etc/nut/upsmon.conf
MONITOR myups@master-server-ip 1 upsmon changeme slave
SHUTDOWNCMD "/sbin/shutdown -h now"

The master waits for all slaves to shut down before shutting itself down.

Monitoring UPS in Your Dashboard

With Uptime Kuma

Uptime Kuma can monitor NUT status:

Add a TCP monitor for localhost:3493
Or use the NUT monitor type if your version supports it

With Grafana + Prometheus

Use the nut_exporter to expose UPS metrics to Prometheus:

services:
  nut-exporter:
    image: druggeri/nut_exporter:3.1.1
    ports:
      - "9199:9199"
    environment:
      - NUT_EXPORTER_SERVER=nut-upsd
    restart: unless-stopped

Track battery charge, load percentage, input voltage, and runtime remaining on your Grafana dashboard.

Battery Maintenance

Task	Frequency	Why
Check battery health (self-test)	Monthly	Catches failing batteries early
Replace battery	Every 3-5 years	Lead-acid batteries degrade with age
Clean contacts	Annually	Corrosion increases resistance
Test shutdown script	Quarterly	Confirm the script still works

Self-test:

# Trigger UPS self-test via NUT
upscmd myups test.battery.start.quick
# Check results
upsc myups ups.test.result

Battery replacement: Most UPS batteries are standard sealed lead-acid (SLA). Replacement batteries cost $20-50 — much cheaper than a new UPS. APC uses RBC-numbered batteries; CyberPower uses standard 12V 7Ah or 9Ah SLA.

FAQ

Do I need a pure sine wave UPS?

If your server has an active PFC power supply (most modern ATX PSUs do), yes. Simulated sine wave can cause active PFC supplies to shut down or malfunction during battery operation. All APC Back-UPS Pro and CyberPower PFC-compatible models output pure sine wave on battery.

Can I put my UPS in a server rack?

Yes — buy a rack-mount UPS (2U form factor). The CyberPower PR1500LCD and APC SMT1500RM2U are popular rack-mount options. Tower UPS units can also sit on a rack shelf.

How do I know when to replace the battery?

NUT reports battery health. When battery.charge no longer reaches 100% or runtime drops significantly, replace the battery. Most UPS units also have a “replace battery” LED/alarm.

Should I put my modem/router on the UPS?

Yes. If the server stays up but your network goes down, remote management (SSH, iDRAC) won’t work, and network-dependent services fail. Put your modem, router, and primary switch on the UPS.