A small UPS has a narrow job. It carries a desk or a network closet through flickers and short outages, and when the power stays off it buys enough minutes to save work and shut down cleanly, or to hold on until a generator takes the load. This calculator answers the two questions behind that: what rating you need, and how many minutes the battery gives at your load. It is built for home office staff, small office managers and the IT lead asked why the UPS under reception died after four minutes.
How the calculator works
List each device on the battery outlets with its watts and quantity. The tool adds them for the load, then sizes the unit:
- Minimum UPS rating (W) = load × 1.25
- Minimum UPS rating (VA) = minimum watts ÷ 0.6
The 1.25 factor keeps the load near 80 percent of the watt rating, with room for a spike and some growth. The 0.6 is a typical watt-to-VA ratio for consumer line-interactive units (1,500 VA for 900 W, say). Among the units listed below it runs from about 0.53 to 0.67, so check both numbers; the watt rating usually binds.
Runtime uses Peukert's law, because lead-acid batteries give up less of their labeled capacity when drained fast. First comes the battery current:
Battery current (A) = load W ÷ (battery volts × efficiency)
Then:
Runtime (hours) = 20 × (Ah ÷ (current × 20)) ^ k, capped at 20 hours
The 20 is there because sealed lead-acid capacity is quoted at the 20-hour rate: a 9 Ah block delivers 0.45 A for 20 hours, not 9 A for one. Battery voltage is 12 V for most small units, while 1,500 VA models usually carry two 12 V blocks in series (24 V). Amp-hours come from the battery label, and inverter efficiency is about 85 percent. The Peukert exponent k is typically 1.25 for small sealed lead-acid (AGM) blocks; 1.0 would be a perfect battery. The target is the minutes needed to save work and shut down, or for a generator to start.
Worked example
The default is a desk with a Desktop PC (150 W), a Monitor (25 W), a Modem (10 W) and a Router (12 W), on a 12 V 9 Ah battery at 85 percent efficiency, Peukert 1.25, with a 10 minute target.
- Load: 150 + 25 + 10 + 12 = 197 W.
- Minimum rating: 197 × 1.25 = 246.25, about 246 W.
- Minimum VA: 246.25 ÷ 0.6 = 410.4, about 410 VA.
- Battery current: 197 ÷ (12 × 0.85) = 19.3 A.
- Runtime: 20 × (9 ÷ (19.3 × 20)) ^ 1.25 = 0.18 hours, or 10.9 minutes. Call it about 11.
- The 10 minute target is met, with about a minute to spare.
The Peukert step is the one people skip. A plain energy sum (12 V × 9 Ah × 0.85 ÷ 197 W) promises 28 minutes, but at nearly 20 A the block yields only about 3.5 of its 9 rated amp-hours. It works the other way too: the same 197 W on a 24 V bank of two 9 Ah blocks halves the current and gives about 26 minutes, well over double. On a 12 V 7 Ah block, the example drops to about 8 minutes and misses the target.
Limits and judgment
- Battery age. Sealed lead-acid UPS batteries typically last three to five years, less in a warm closet. At 80 percent of original capacity the example falls to about 8 minutes.
- Light loads. At a few tens of watts the inverter's own consumption weighs more and the estimate runs long. For a closet of network gear, the maker's runtime chart is the better guide.
- Output waveform. Many lower-cost units put out a stepped, simulated sine wave on battery. Computers with active PFC power supplies, now common, can shut down at the moment of transfer on that waveform. Give them a pure sine wave model.
- Laser printers and heaters. A laser printer's fuser pulls a heavy current while it heats, enough to overload a small UPS. Put it, and any space heater, on the surge-only outlets or a separate strip.
- Unattended shutdown. Connect the USB cable and set up the maker's software (APC PowerChute or CyberPower PowerPanel) so the machine closes cleanly when nobody is at the desk.
Choosing a UPS in this size class
For a modem, router and switch, about 30 W in all, a 600 VA unit is plenty: on a 12 V 7 Ah block the estimate is about 1 h 24 min. A single desk like the worked example fits the 650 to 850 VA range, where 197 W sits at roughly 44 to 51 percent of the watt rating; take the larger end if a second monitor or a NAS may join later. Workstations, small servers and anything with an active PFC supply belong on a 1,500 VA pure sine wave unit with a 24 V battery. Printers and lamps that only need spike protection go on a plain surge strip.
We chose the units below from spec sheets and published reviews, without testing them in-house. Confirm the watt rating, battery size and output waveform on the maker's current page.
Readiness kit
| Category | Pick | Why it is on the list | Link |
|---|---|---|---|
| UPS, network closet | APC Back-UPS 600VA (BE600M1) | Keeps a modem, router and switch up through short outages, the usual first fix for a small office. | Shop on Amazon |
| UPS, single desk | APC Back-UPS 650VA (BE650G1) | One PC and a monitor ride out a flicker and get time for a clean shutdown. | Shop on Amazon |
| UPS, desk and network | APC Back-UPS 850VA (BE850G2) | A little more headroom for a workstation plus network gear, with USB charging ports. | Shop on Amazon |
| UPS, pure sine wave | CyberPower CP1500PFCLCD | Pure sine output for active PFC power supplies, sized for a workstation or a small server. | Shop on Amazon |
| UPS, pure sine wave | APC Back-UPS Pro 1500VA (BR1500MS2) | Pure sine output with an LCD and USB charging, for a NAS and the network kit around it. | Shop on Amazon |
| Surge protection | Tripp Lite TLP606B surge strip | A plain, well-built six-outlet strip for devices that do not need battery time. | Shop on Amazon |
A UPS bridges seconds to minutes. For hours, compare a portable power station or a generator. To decide how long a system may be down before it hurts, start with RTO vs. RPO.
Sources
- Peukert's law (Wilhelm Peukert, 1897), as set out in lead-acid battery makers' technical manuals and on Battery University.
- APC by Schneider Electric and CyberPower user manuals for Back-UPS and CP series units (battery versus surge-only outlets, laser printers, battery replacement).
- CyberPower technical notes on pure sine wave output and active PFC power supplies.
- Ready.gov: Power Outages
Frequently asked questions
What size UPS do I need for a desktop computer?
Add the watts of everything on the battery outlets and multiply by 1.25. For the example desk that gives about 246 W and 410 VA, so a 650 to 850 VA unit fits; check the watt rating as well as the VA figure.
Why is UPS runtime so much shorter than the battery's watt-hours suggest?
Lead-acid batteries deliver less of their labeled capacity when drained fast, the effect Peukert's law describes. At 19 A, a 12 V 9 Ah block gives up only about 40 percent of its rated amp-hours, so the example lasts about 11 minutes rather than 28.
Do I need a pure sine wave UPS?
If the computer has an active PFC power supply, a pure sine wave model is the safe choice. Simulated sine output can make those supplies shut off at the moment of transfer. Modems, routers and small adapters are usually fine on either type.
How often should UPS batteries be replaced?
Typically three to five years for sealed lead-acid UPS batteries, less in a hot room. Run the self-test a few times a year and replace the battery when the unit flags it or runtime falls below your target.