How Long a UPS Actually Holds a Rack Up
An uninterruptible power supply is sold on two numbers that say nothing about backup time. Volt-amps and watts describe how much load the inverter can carry; how long it can carry it comes from the battery bank, and that is an energy question. Add up the bank in watt-hours — volts times ampere-hours, per battery — and you have the quantity that actually runs out. Restating it in kilojoules puts it on the same SI scale as everything else in a thermal or electrical model, and makes the comparison with a load measured in watts completely direct, because a watt is a joule every second.
Why the Ideal Number Is Always Too Optimistic
VA and Watts Count Different Things
The Inverter Takes Its Cut First
A VRLA Bank Is Never Emptied
Runtime Curves Are Not Straight Lines
Working a Battery Bank Through to a Runtime Figure
The sequence is the same whether you are checking a vendor's claim or sizing a replacement: total the bank, convert it, apply the derating, then divide by the load actually measured at the outlet rather than the load anyone estimated.
Total the bank in watt-hours
Multiply each battery's voltage by its ampere-hour rating and add them up — series or parallel makes no difference to the energy. Four 12 V 9 Ah blocks are 432 Wh whether they are wired as 48 V or as 24 V with two strings.
Enter that total and read the kilojoules
Type the watt-hour figure into the left box and the kilojoule value builds alongside it. With the load in watts and the energy in kilojoules, dividing gives seconds directly, with no hidden hour-to-second step to get wrong.
Reverse it when the datasheet leads with kilojoules
Lithium UPS modules and some European submittals quote stored energy in kilojoules rather than watt-hours. The swap arrows put kilojoules on the left; alternatively type into the right-hand box, since the calculation runs in whichever direction you feed it.
Copy the figure into the capacity record
The copy control above either box returns digits with no unit, and Ctrl+C in the field does the same. A change record, a capacity spreadsheet or a monitoring threshold wants the bare value, not a string that has to be cleaned up first.
UPS Classes by Battery Bank, Stored Energy and Runtime
Runtimes below apply the derating from the note above to each bank: stored watt-hours, times 0.9, times the usable fraction, divided by the load. The lithium row uses 0.85 and 0.90 because a LiFePO4 bank can be taken far deeper without damage.
| Class | VA / W rating | Battery bank | Stored (Wh) | Stored (kJ) | Full load | Half load |
|---|---|---|---|---|---|---|
| Desk or home office | 650 VA / 390 W | 1 × 12 V 7 Ah | 84 | 302.4 | 4.9 min | 13.5 min |
| Small server or NAS | 1 000 VA / 600 W | 2 × 12 V 7 Ah | 168 | 604.8 | 6.4 min | 17.5 min |
| Rack line-interactive | 1 500 VA / 900 W | 2 × 12 V 9 Ah | 216 | 777.6 | 5.4 min | 15.0 min |
| Rack online, 3 kVA | 3 000 VA / 2 700 W | 6 × 12 V 9 Ah | 648 | 2 332.8 | 5.4 min | 15.0 min |
| Lithium rack unit | 3 000 VA / 3 000 W | 48 V 20 Ah LiFePO4 | 960 | 3 456 | 14.7 min | 31.1 min |
| Large online, 6 kVA | 6 000 VA / 6 000 W | 16 × 12 V 9 Ah | 1 728 | 6 220.8 | 6.5 min | 18.0 min |
| 10 kVA with battery cabinet | 10 000 VA / 10 000 W | 40 × 12 V 9 Ah | 4 320 | 15 552 | 9.8 min | 27.1 min |
Two patterns come out of that column of figures. The first is how narrow the range is: from a 650 VA desk unit to a 6 kVA rack machine, full-load runtime barely moves, because manufacturers scale the bank with the rating and everybody targets roughly five minutes. The second is that buying runtime means buying batteries, not volt-amps — the 10 kVA row only reaches ten minutes because a cabinet adds 4 320 Wh, and the lithium row triples the full-load figure on a bank that is barely half the size, purely by being dischargeable.
What the Converter Adds to a Capacity Review
Bank Sizes Tried One Cabinet at a Time
Overtyping the watt-hour total recalculates on the spot, which suits the way an extension is actually specified — two blocks, then four, then a full cabinet, watching where the runtime finally clears the shutdown window.
Runtime Figures Clean Enough for the Change Record
Copying gives the number by itself, so a converted bank energy can be pasted into a capacity sheet, a maintenance ticket or a monitoring threshold without a unit string to tidy up afterwards.
Megajoule and kWh Views for a Whole Room of Racks
Search either dropdown for the megajoule or the kilowatt-hour and a whole-room battery total stops being an unreadable string of digits, which is where a single cabinet's kilojoules stop being convenient.
Comma Decimals From a European Submittal
A value written 777,6 is read exactly as 777.6, and spaces inside the number are ignored, so figures lifted from a continental datasheet or a distributor's quotation need no reformatting first.
Runtime Questions Before the Next Power Cut
My UPS has two 12 V 9 Ah batteries — how many minutes is that?
Start from 216 Wh, or 777.6 kJ. At a 450 W load the ideal division gives 28.8 minutes; multiply by 0.9 for the inverter and by roughly 0.58 for what a lead-acid bank actually surrenders at that rate and you land near 15 minutes, which is where most vendor curves for this class sit. Push the same bank to a 900 W full load and the usable fraction falls to about 0.42, giving a little over five minutes. Treat those as a sanity check on the published curve rather than a replacement for it, and confirm with a real load test.
Why does doubling the load more than halve the runtime?
Because a lead-acid battery yields less total charge when you take it quickly. That is the Peukert effect: at high current more of the plate area is unreachable before the terminal voltage collapses, so the bank is not merely emptied faster, it is emptied less completely. On the 216 Wh example the usable fraction drops from around 0.58 at 450 W to around 0.42 at 900 W, turning a fifteen-minute runtime into five and a half rather than seven and a half. Internal resistance also wastes more as heat at higher current. The lesson for capacity planning is that headroom on a UPS buys disproportionate runtime.
Why does the same box carry both a VA number and a smaller watt number?
They limit different things. The volt-amp figure is what the inverter's switches, transformer and wiring can pass, since those are stressed by current regardless of whether it is in phase with the voltage. The watt figure is real power — what the load consumes and what the batteries must supply. Their ratio is the power factor: older units were built around 0.6, so a 1 000 VA machine handled only 600 W, while modern designs run 0.9 to 1.0. A UPS is overloaded when either limit is exceeded, but only the watt number is any use for runtime, because only real power drains the bank.
How many minutes does a graceful shutdown really need?
Less than most people fear, provided the automation exists. A NAS flushing caches wants one to three minutes; a hypervisor host suspending or shutting down a handful of guests wants three to five, sometimes ten if a database has to close cleanly. Then add the delay before shutdown is triggered, because nobody wants a ten-second flicker to take the rack down. Five to ten minutes of derated runtime is usually enough, and the real failure mode is not a short bank but a shutdown agent nobody has tested since the last operating-system upgrade.
The unit is three years old — how much of the stated runtime is left?
Plan on somewhere near three quarters, and less in a warm rack. Sealed lead-acid life halves for roughly every 8 °C above 25, so a bank sitting in 35 °C exhaust air ages at more than double the rate on the specification sheet. The 216 Wh example at 75 % of capacity is 162 Wh, or 583.2 kJ, which drags a fifteen-minute half-load figure down to about eleven. Self-test routines check that the batteries respond, not how much energy they still hold, so schedule a genuine timed discharge and treat three to five years as the replacement window.
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