APC Back-UPS Pro BX1500M

Length x Width x Height

- Length: the longest side of the device

- Width: the shorter side of the device

- Height: the vertical dimension of the device

The physical weight of the unit, typically measured in kilograms (kg) or pounds (lb).

Almost all of a UPS's weight is its battery. Lead-acid units are heavy for their capacity, while lithium iron phosphate (LiFePO4) models store the same energy in roughly half the weight.

Whether the UPS is a tower that stands on the floor or a desk, a rack-mount chassis for a 19-inch cabinet (measured in rack units, 1U or 2U), a convertible that does both, or a compact unit sized for a modem and router.

Towers are the usual choice at home. Rack-mount and convertible models belong in a network cabinet next to a switch or a NAS, and compact models fit behind a TV stand or inside a media closet.

The apparent power the UPS can deliver, in volt-amperes (VA). It is the number in most model names (BR1500, CP1500).

VA is always higher than the watt rating, so size a UPS by its watts, not its VA. The VA figure is mainly useful to compare models of the same family.

The real power the UPS can deliver to connected equipment, in watts. This is the figure that has to exceed the combined draw of everything plugged into the battery outlets.

A modem, a router and a couple of access points draw well under 100 W, a NAS 30 to 60 W, and a desktop computer with a monitor 200 to 500 W. Leaving 20 to 30% headroom keeps runtime reasonable and the battery cooler.

How the UPS handles mains power. A standby (offline) UPS passes mains power straight through and switches to battery when it fails. A line-interactive UPS adds an autotransformer that corrects sags and surges without touching the battery. An online (double-conversion) UPS powers the load from its inverter all the time, so there is no transfer at all.

Line-interactive is the sweet spot at home. Online units give the cleanest power but run warmer, louder and cost more; standby units are fine for a modem and a router.

Whether the battery output is a true sine wave, like utility power, instead of a simulated (stepped) sine wave.

Computers, NAS units and monitors with active PFC power supplies can buzz, reboot or refuse to run on a simulated sine wave. Simple electronics such as modems and routers with external adapters do not care.

Whether the UPS uses a lithium iron phosphate (LiFePO4) battery instead of the usual sealed lead-acid (SLA) battery.

A lead-acid UPS battery typically needs replacing every 3 to 5 years. LiFePO4 batteries last thousands of cycles and around 10 years, weigh far less and recharge faster, which offsets their higher price over the life of the unit.

How long the UPS keeps running on battery with a load of half its watt rating, as published by the maker.

Runtime is not linear: at a quarter of the rated load a UPS usually lasts more than twice its half-load figure. A modem and router drawing 20 W can run for well over an hour on a 1500 VA unit that only lasts ten minutes at half load.

How long the UPS keeps running on battery at its full watt rating, as published by the maker.

Full-load runtime is short on every consumer UPS, usually a few minutes. It is the time you have to save work and shut a computer down, not to ride out an outage.

How long the battery takes to recharge to 90% after a full discharge, as published by the maker.

A short recharge time matters where outages come in clusters: a unit that needs 16 hours to recharge may not have much left for the second outage of a stormy day.

Whether the battery can be replaced by the owner, usually through a door on the front or the bottom of the unit.

With lead-acid batteries lasting 3 to 5 years, a replaceable battery is what keeps a UPS in service for a decade instead of turning it into e-waste.

Whether the battery can be replaced while the UPS keeps powering the connected equipment.

Hot swap lets you change a worn battery without shutting down the router, the NAS or the server the UPS protects.

Whether the UPS accepts external battery packs to extend its runtime.

Extra battery packs are the only way to stretch a UPS from minutes to hours without buying a larger unit.

The total number of AC outlets on the unit, counting both battery-backed and surge-only outlets.

The number of outlets that keep delivering power from the battery during an outage.

Only these outlets protect against an outage. Plug the router, the modem, the NAS or the computer here, and leave printers and speakers on the surge-only outlets.

The number of outlets that protect against surges but switch off during an outage.

The socket standard of the AC outlets: NEMA 5-15R (the standard North American socket), IEC C13 (the server and rack standard, used with C14 cables) or the local wall sockets in other regions (Schuko, BS 1363).

IEC C13 outlets need a C14 to device cable, which most computers and NAS units accept but wall adapters do not.

Whether the UPS has USB-A ports to charge phones and small devices.

Whether the UPS has USB-C ports to charge phones, tablets and some laptops.

Whether the UPS has an RJ-45 in and out pair that protects a network or phone line against surges.

Lightning surges travel through network and phone cables too, so dataline protection guards the modem and the router from the side the power outlets do not cover.

Whether the UPS has coaxial connectors that protect a cable modem or TV line against surges.

Automatic Voltage Regulation (AVR) corrects low and high mains voltage without switching to battery.

Without AVR a UPS jumps to battery on every brownout, which wears the battery out years sooner. It is standard on line-interactive units and absent on most standby ones.

How much surge energy the unit's protection can absorb, in joules, as published by the maker.

A higher rating means the surge protection survives more or larger spikes before it wears out. Most home units fall between 500 and 1,500 joules.

How long the UPS takes to switch from mains power to battery when power fails, in milliseconds.

Most computer power supplies ride through about 10 to 20 ms without power. Online double-conversion units have no transfer at all (0 ms).

Whether the UPS connects to your Wi-Fi network for monitoring in an app or a web dashboard.

A Wi-Fi UPS can send outage and low-battery alerts to your phone without a computer connected to it.

Whether the UPS has a built-in network management port, so it can be monitored and shut down over the network (web interface, SNMP or Network UPS Tools) without a computer attached by USB.

Whether the UPS can be monitored and configured from a phone over Bluetooth.

Whether the UPS has a USB communication port (normally USB HID) to report its status to a computer or a NAS.

This is what lets a NAS or a PC shut itself down safely before the battery runs out. Synology, QNAP, Unraid, TrueNAS, Windows, macOS and Home Assistant (through Network UPS Tools) all read a USB HID UPS.

Whether the UPS has a slot for an optional network management card that adds a web interface, SNMP and email alerts.

A slot lets you start with USB monitoring and add network management later, which matters when several machines have to shut down from one UPS.

Whether the maker offers desktop software (such as PowerChute or PowerPanel) to monitor the UPS and shut down connected computers automatically.

Whether the UPS can be monitored from a phone app, directly or through a connected computer or network card.

Whether the UPS has a screen showing load, battery level, estimated runtime and input voltage.

A display tells you at a glance how much the connected equipment draws and how long the battery will last, without installing any software.

Home Assistant is an open-source platform that allows for extensive customization and integration of a wide variety of smart devices.

UPS units reach Home Assistant through the Network UPS Tools (NUT) integration, which reads a USB HID or network-managed UPS, or through the maker's own integration. Home Assistant can then show battery level and load, alert you of an outage and run automations when the power goes out.

A mode that lowers the UPS's own power consumption, or cuts power to idle devices plugged into its outlets.

A UPS runs all day for years, so its own consumption adds up on the electricity bill.

Whether the UPS can be switched on from its battery when there is no mains power at all.

Cold start lets you power up the router or a computer during an outage that started while the UPS was off.