What an uninterruptible power supply does, how to size one, what to plug in, and what to keep off it.
Reviewed September 2026
The power flickers for half a second. Your router reboots. Your computer restarts. The file you were working on is gone. The video call drops. Twenty minutes later everything is back to normal, but the damage is done.
A UPS prevents that. It sits between the wall and your gear, its battery always charged, and the instant the power dips or drops, it switches your equipment onto battery in milliseconds. Your computer never reboots. Your router never loses its connection. You have time to save your work or, if the outage lasts, shut down cleanly.
This handbook covers what a UPS does and does not do, how to read the ratings, what belongs on one and what will damage it, and how to maintain it.
A UPS is a battery in a box with a charging circuit and an inverter. When wall power is normal, the UPS passes it through to your devices and keeps its battery topped off. When wall power drops, sags, or disappears entirely, the UPS switches your equipment onto its battery.
The critical word is "uninterruptible." The switch happens in milliseconds. A computer's power supply has enough energy stored in its capacitors to ride through that gap, so the machine never notices. Your monitor stays on. Your router stays connected. Your NAS keeps its drives spinning.
A UPS also conditions the power it passes through. Most consumer units include automatic voltage regulation (AVR), which raises low voltage and trims high voltage without touching the battery. This matters in areas with chronic brownouts or voltage sags, because it means the battery stays fresh for real outages rather than being drained by minor fluctuations.
The trade-off is time. A UPS is measured in minutes, not hours. A typical consumer unit carrying a normal load runs five to fifteen minutes on battery. That is enough to save your work, shut down your computer cleanly, and wait out a brief flicker. It is not enough to power through a four-hour storm outage. For that, you need a portable power station, a generator, or both.
Every UPS carries two power ratings: VA (volt-amperes) and watts. They are related but not interchangeable, and confusing them is the most common sizing mistake.
| Rating | What it measures | What it tells you |
|---|---|---|
| VA (volt-amperes) | Apparent power | The total electrical load the UPS can carry, including reactive power that does no useful work |
| Watts | Real power | The power that actually runs your devices and generates heat |
Consumer UPS units typically have a power factor between 0.6 and 0.7. That means a "1500 VA / 900 W" unit can deliver at most 900 watts of real power, not 1,500. Size your UPS by the watt rating, not the VA number. If a manufacturer only lists VA, multiply by 0.6 to estimate the worst-case watt capacity.
Some higher-end units carry a power factor closer to 1.0 (often marketed as "PFC compatible" or "pure sine wave"). These cost more but give you more usable watts per VA, and they are the only safe choice for computers with Active PFC power supplies.
UPS units come in three topologies. The name describes how the battery, inverter, and incoming power relate to each other.
The most common consumer type
Wall power goes straight to your devices. The battery and inverter sit idle until the power drops, then the UPS switches over. Switchover time is typically 5 to 12 milliseconds. Inexpensive and efficient. No voltage regulation while on wall power. Fine for a basic home computer or networking gear.
Typical price range: $40 to $100
The sweet spot for most households
Adds automatic voltage regulation (AVR) while on wall power. The transformer boosts low voltage and bucks high voltage without ever touching the battery. Switchover time is typically 2 to 4 milliseconds. Handles brownouts and voltage sags without draining the battery. The most common choice for home offices, network closets, and desktop workstations.
Typical price range: $100 to $300
Zero transfer time, continuous conditioning
Wall power charges the battery continuously, and the inverter runs your devices from the battery at all times. There is no switchover because the battery is always in the circuit. The cleanest possible output, zero transfer time, and complete isolation from line noise. Higher cost, more heat, shorter battery life due to constant cycling. Overkill for most homes but relevant for sensitive lab equipment or a serious home server.
Typical price range: $300 to $800+
For most households, a line-interactive unit with sine-wave output is the right balance. It handles brownouts without wasting battery, switches over fast enough for any consumer device, and costs a fraction of online double-conversion.
Most consumer UPS units have two groups of outlets: battery-backed (labeled "Battery + Surge") and surge-only. Only the battery-backed outlets stay powered during an outage. The surge-only outlets protect against voltage spikes but go dead when the power drops, just like a regular power strip.
Never plug these into a UPS
Consumer UPS manufacturers including CyberPower and APC/Schneider Electric warn against all of these in their user manuals. Plugging a prohibited device in can overload the unit, cause it to overheat, or damage internal components permanently. (Source: CyberPower CP1500PFCLCD user manual; APC Back-UPS community support, accessed September 2026)
Not for medical or life-support equipment
Consumer UPS manufacturers explicitly state: "Do not use for medical or life support equipment." A UPS delivers minutes, not hours. If a life depends on continuous power, build a dedicated plan with the medical provider, enroll in your utility's life-support registry for advance notice and priority restoration, and size real backup power (a generator or large battery system) to the device's actual draw and runtime. (Source: CyberPower CP1500PFCLCD user manual, accessed September 2026)
The most common failure. A UPS that has been plugged in for four years without testing may have a battery that can no longer hold a charge. The unit looks fine. The LEDs are green. Then the power drops and the UPS dies instantly. Test yours by pulling the plug with your gear running, once a year, and time how long it holds.
A cheap UPS with simulated (stepped) sine-wave output feeding a computer with an Active PFC power supply may cause the computer to shut down the instant the UPS switches to battery. The PFC circuit interprets the stepped waveform as a fault and shuts itself off. This defeats the purpose entirely. If your computer has an Active PFC power supply (most modern desktops do), buy a UPS with pure sine-wave output.
Adding devices over time without rechecking the watt total is gradual and invisible. The UPS may handle the load on wall power but overload and shut down on battery, exactly when you need it. Check the load meter on the front panel or in the monitoring software, and stay below 80 percent of the watt rating.
A UPS in a closed cabinet or near a heat vent can overheat. Most units shut down automatically above about 40 degrees Celsius (104 degrees Fahrenheit). Give the unit at least two inches of clearance on all sides and keep the ventilation grilles clear of dust.
Most consumer UPS units have both battery-backed and surge-only outlets. If your computer is on a surge-only outlet, it will die the instant the power drops, and the battery will sit there fully charged and useless. Check the outlet labels.
Add up the wattage of everything you plan to put on the battery-backed outlets. Multiply by 1.25 to leave a 20 to 25 percent buffer. Match the result to the UPS's watt rating, not the VA number.
| Setup | Typical draw | Minimum UPS size |
|---|---|---|
| Modem + router only | 20 to 40 W | 350 VA / 200 W |
| Desktop + monitor + router | 200 to 400 W | 850 VA / 500 W |
| Home office (desktop, two monitors, router, NAS) | 400 to 600 W | 1500 VA / 900 W |
More capacity means more runtime at the same load. A 1500 VA unit running a 40 W modem and router can hold them for an hour or more. The same unit at 600 W of load may last only five minutes. Both numbers are correct. The battery is the same; the draw is different.
The runtime printed on the box is a best case. Test your own by pulling the plug with your actual gear running and timing it.
Most consumer UPS units use sealed lead-acid (SLA) batteries. These are the same chemistry found in car batteries, scaled down and sealed so they do not require water top-ups. Typical lifespan is three to five years, faster in hot environments.
Replace the battery when the UPS signals a weak battery (usually a steady or flashing LED and a beep pattern) or when your timed runtime drops noticeably. Most consumer units take a user-replaceable battery pack that slides out from the front or bottom. No tools required on most models.
A spent sealed lead-acid battery is hazardous waste. It contains lead and sulfuric acid. Under federal EPA regulations (40 CFR Part 273), SLA batteries are classified as Universal Waste and are prohibited from household trash and curbside recycling.
Take spent SLA batteries to any of these:
SLA batteries are the most recycled consumer product in the United States, with a recycling rate of approximately 99 percent. The lead, polypropylene, and acid are all commercially valuable and infinitely reusable.
Sources: EPA Used Household Batteries (epa.gov, accessed September 2026); Mercury-Containing and Rechargeable Battery Management Act (Public Law 104-142)
A few recent consumer UPS models use lithium-ion batteries. These are lighter and last longer than SLA, but they cost more and require a different disposal path. Lithium-ion batteries also must never go in household trash or curbside recycling. Take them to a certified battery recycler or a household hazardous waste site. Tape the terminals before transport to prevent short circuits.
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A UPS is one layer in your power plan. These guides cover the rest.
Battery Power guide
The full picture from AA cells to whole-home battery backup
UPS Systems editorial guide
The deep dive on what a UPS does, switchover, VA math, and testing
Portable Power Stations handbook
When you need hours of runtime, not minutes
Battery Banks handbook
Portable USB power for phones and small devices
Individual comparison roundups. These will appear as we publish them.