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Runtime is the most expensive thing you can buy in a UPS. Wattage capacity is cheap; battery energy is heavy and wears out. So the useful question is not "how much runtime can I get" but "what am I going to do with the minutes." Answer that, and the number usually turns out smaller than people expect.
Three strategies, three different numbers
| Strategy | Goal | Typical runtime target | Main risk |
|---|---|---|---|
| Ride-through | Survive flickers, sags and short outages without anything restarting | Seconds to a few minutes | A longer outage ends in a hard power loss if no shutdown is configured |
| Graceful shutdown | Ride through short events, then save work and power down cleanly | About 5 to 15 minutes at end of battery life | Shutdown takes longer than budgeted, or the signal never reaches the device |
| Bridge to generator | Keep equipment running until a generator takes over | From under a minute (automatic standby) to 15 minutes or more (manual portable) | The generator fails to start or its power is rejected by the UPS |
| Long runtime | Keep small, critical gear online for hours | 1 to 8+ hours, small loads only | Cost and weight of battery; inverter losses at light load |
Most homes and small offices combine two of these: ride-through plus graceful shutdown for computers, and long runtime for the internet connection.
What outages actually look like
Utility reliability statistics can mislead here. Under IEEE 1366, the standard most US utilities use for reliability reporting, an interruption lasting five minutes or less counts as momentary and is excluded from the headline outage-duration figures. The events that hurt computers most (sub-second dropouts, recloser operations during storms, sags when a large motor starts nearby) are exactly the ones those numbers leave out.
A common field observation follows: in many places the UPS switches to battery far more often for brief events than for real outages. That makes ride-through the most frequently used feature of any UPS, and it costs almost nothing in battery capacity. You can see your own pattern in the event log that most UPS software keeps; see UPS software overview.
Budgeting a graceful shutdown
A clean shutdown has four parts, and runtime must cover all of them:
- Ride-through delay: how long you wait before deciding the outage is real. One to five minutes is common.
- Shutdown duration: how long the device takes from the shutdown command to powered off. Measure it; do not guess.
- Power-off margin: time for the UPS to receive the "shutdown complete" state, or to turn its outlets off on a timer if it supports that.
- Safety factor: covers load spikes, a cold battery and estimation error. We use 1.5.
Worked example: a NAS and its network switch
A four-bay NAS, a small switch and a router together draw 85 W. You time a manual shutdown of the NAS: 3 minutes 20 seconds, rounded up to 4 minutes because a scrub or backup job could be running. You choose a 3 minute ride-through delay.
- Delay + shutdown + margin: 3 + 4 + 1 = 8 minutes
- With a 1.5 safety factor: 8 x 1.5 = 12 minutes
- That must hold with an aged battery at 80% capacity, so on a new battery the chart should show at least 12 / 0.8 = 15 minutes at 85 W.
Almost any 600 to 900 VA class line-interactive UPS shows far more than 15 minutes at 85 W on its runtime chart, so the requirement is easily met. The harder part is the plumbing: the NAS must actually get the signal. Over USB it usually works out of the box; over the network, the switch carrying the signal must be on battery too. See UPS for a NAS and Network UPS Tools for multi-device setups.
Our analysis: shutdown on a low-battery signal is a trap with old batteries
Many setups wait for the UPS's "low battery" signal before shutting down, to maximize ride-through. That works with a new battery. With an aged battery, the remaining runtime when low battery triggers can shrink to a minute or less, and the firmware's estimate may not have caught up. A fixed timer (for example, shut down after 3 minutes on battery) plus low battery as a backup trigger is more robust, because the timer does not depend on the battery's health model.
Bridging to a generator
If you have a generator, the UPS only has to cover the gap between the outage and stable generator power. How long that gap is depends almost entirely on how the generator is started and connected:
| Setup | Typical gap | Runtime to plan for |
|---|---|---|
| Standby generator, automatic transfer switch | Often around 10 to 60 seconds, depending on start and transfer delays | A few minutes, to allow a failed first start attempt |
| Portable generator, manual transfer switch or interlock | As long as it takes to get outside, start it and switch over: realistically 5 to 20 minutes | Graceful shutdown runtime; treat the generator as a bonus |
| Portable generator with extension cords | Similar, plus time to reroute cords | Graceful shutdown runtime |
The bigger problem with generators is usually power quality rather than time. Many line-interactive UPS units reject the frequency and voltage swings of smaller generators and stay on battery, draining it while the generator runs. See using a UPS with a generator and UPS not working with a generator before you rely on this plan.
Long runtime: usually only for the network
Keeping a desktop PC running for an hour is expensive. Keeping a modem, router and a phone charger running for four hours is not, as long as you pick the right tool.
Consider a 20 W load (modem plus router) for 4 hours. That is 80 Wh at the load. With a typical inverter at a very light load, overhead can easily push the battery draw to 30 W or more, making it 120 Wh or more from the battery, and the AC UPS's runtime chart may not even list loads that small. A DC mini UPS that feeds 12 V directly to the equipment skips the inverter altogether, and a small portable power station holds several hundred watt-hours. See DC mini UPS vs AC UPS and UPS vs portable power station.
Split the load by strategy
The most cost-effective home setup is often two devices: a mid-size UPS that gives computers a clean shutdown, and a separate small battery source that keeps internet equipment alive for hours. If everything shares one UPS, the computers' shutdown also determines how long your Wi-Fi survives.
Runtime worksheet
Fill this in for each group of equipment that shares a UPS:
| Line | Question | Example answer |
|---|---|---|
| 1 | Measured load during an outage (after any load shedding), W | 85 |
| 2 | Strategy: ride-through, shutdown, generator bridge, long runtime | Shutdown |
| 3 | Ride-through delay before acting, min | 3 |
| 4 | Measured shutdown or generator transfer time, min | 4 |
| 5 | Margin for signaling and power-off, min | 1 |
| 6 | (Line 3 + 4 + 5) x 1.5, min | 12 |
| 7 | Line 6 / 0.8 = minimum runtime on a new battery at line 1 load, min | 15 |
Take line 7 and line 1 to the manufacturer's runtime chart, or plug them into the runtime calculator. If the target is out of reach, cut the load before buying a bigger UPS: a second monitor, speakers and a printer can be a large share of a desk's draw and add nothing during an outage.
Situations that need more than the formula
- Medical devices. A CPAP or oxygen concentrator plan should come from the device maker and your clinician. See UPS for CPAP and UPS for medical equipment.
- Sump pumps and furnaces. These need hours, not minutes, and have motor starting surges. See UPS for a sump pump and UPS for a gas furnace.
- Remote sites. If nobody can reach the equipment, plan for automatic shutdown and automatic restart when power returns, which depends on BIOS or firmware "restore on AC power loss" settings.
- Frequent outages. If the UPS goes to battery many times a week, cycle wear matters more than runtime; see UPS battery lifespan.
Frequently asked questions
Is 5 minutes of UPS runtime enough?
For a single PC that hibernates or shuts down automatically, often yes, provided those 5 minutes are what an aged battery still delivers at your real load. It is tight for a NAS or server that takes several minutes to stop services, and too short if you want to ride out outages of a few minutes before deciding to shut down.
How long will a UPS keep my Wi-Fi running?
A modem and router together typically draw 10 to 30 W. A mid-size AC UPS at that light load can often run them for one to several hours, but check the manufacturer's runtime chart at the lowest listed load, because inverter overhead makes light-load runtime less impressive than a simple Wh calculation suggests. DC mini UPS units avoid that overhead.
Should I set my computer to shut down immediately when the power goes out?
Usually not. Many interruptions last seconds, and an immediate shutdown turns every blip into a forced restart. A short delay of one to five minutes, followed by an automatic shutdown if power has not returned, balances availability and safety. The delay must fit inside your runtime budget with margin.
Can a UPS run during a long blackout of several hours?
Only for small loads or with substantial added battery. Multi-hour runtime for a desktop PC requires extended battery packs or a different tool, such as a portable power station or a generator. For most homes the practical plan is to shut down computers and keep only internet and phones powered for the long haul.
Does runtime need to cover the generator warm-up time?
It needs to cover the time from outage to stable generator power, including any start delay and transfer time, plus margin in case the first start attempt fails. Some installers also configure a short warm-up before transfer, so ask what your system does rather than assuming the minimum.
Sources and further reading
- IEEE Std 1366, Guide for Electric Power Distribution Reliability Indices (defines momentary vs sustained interruptions)
- U.S. Energy Information Administration: Annual Electric Power Industry Report (EIA-861), reliability data
- NFPA 110, Standard for Emergency and Standby Power Systems
- IEEE Std 446 (Orange Book), Recommended Practice for Emergency and Standby Power Systems