Best UPS for a NAS: Watts, Waveform and Real Runtime
Size a UPS on watts, not VA: consumer units publish a power factor near 0.6, so a 1500 VA model carries 900 W. For a NAS with an active PFC power supply, buy pure sine wave. The CyberPower CP1350PFCLCD at $219.95 is the best UPS for most NAS builds, delivering 810 real watts and holding a 46 W four-bay NAS for close to three hours, far more than the 10 to 15 minutes a clean shutdown needs.
Two specifications decide this purchase and the big number on the box is neither of them. The first is the real watt rating, which is the VA figure times a power factor of roughly 0.6 on consumer units, so a 1500 VA model typically carries 900 W. The second is the output waveform, because a modern NAS power supply expects a real sine wave and a stepped approximation can make it shut down at the exact moment it needed protecting.
Three picks below, then the arithmetic that shows why the runtime question answers itself, then the wider field with real watts and computed runtime for every unit.
Best UPS for a NAS by build size
APC BE650G1 Back-UPS 650VA / 390W
Enough runtime for a two-bay NAS to see the outage and shut itself down cleanly, which is the entire job.
Best for: A two-bay NAS and a router, nothing else on the battery outlets.
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CyberPower CP1350PFCLCD PFC Sinewave UPS
Pure sine wave output, which matters because power supplies with active PFC can refuse to run on the stepped waveform cheaper units produce.
Best for: A four-bay or six-bay NAS plus network gear, with real runtime margin.
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APC BR1500MS2 Back-UPS Pro 1500VA / 900W Pure Sine Wave
900 W of capacity is far more than a home NAS needs. You are buying runtime and a replaceable battery pack, not wattage.
Best for: A rack with several devices, or anyone who wants to ride out short outages entirely.
Check pricePrices change often, confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.
What size UPS does a NAS actually need?
Smaller than you think, because home server loads are tiny relative to any unit you would buy. A four-bay NAS draws about 46 W in normal operation. On an 810 W unit that is under 6 percent of the rating, which is the region where lead acid batteries behave well and runtime is generous.
The load figures below are drives plus board and fans, using 6 to 9 W per active mechanical drive and 10 to 25 W for the board, which is the same model the power draw calculator uses. The spin-up column is the one people miss: drives pull 25 to 30 W each for a few seconds when they start, so a restart while on battery is by far the largest instantaneous load a UPS ever sees from a NAS.
| Build | Idle | Active | Spin-up surge | Runtime on 173 Wh | Runtime on 216 Wh |
|---|---|---|---|---|---|
| Mini PC with one external drive | 14 W | 22 W | 110 W | 4 h 49 min | 6 h 1 min |
| Two-bay NAS | 22 W | 32 W | 120 W | 3 h 18 min | 4 h 7 min |
| Four-bay NAS | 32 W | 46 W | 134 W | 2 h 16 min | 2 h 51 min |
| Four-bay NAS plus switch and router | 48 W | 64 W | 240 W | 1 h 37 min | 2 h 2 min |
| Six-bay NAS | 42 W | 60 W | 148 W | 1 h 44 min | 2 h 10 min |
| Eight-bay NAS | 52 W | 74 W | 250 W | 83 min | 1 h 45 min |
| Eight-drive DIY tower | 78 W | 111 W | 287 W | 54 min | 68 min |
| Small rack, NAS plus server plus network | 105 W | 140 W | 316 W | 42 min | 53 min |
Runtime is computed with the same model as the UPS runtime calculator: roughly 62 percent of nameplate watt-hours reach the load after inverter losses and usable depth of discharge, with a further derating as load rises. The 173 Wh column is the CyberPower CP1350PFCLCD and the 216 Wh column is the APC BR1500MS2.
Read those runtime columns and then stop shopping on runtime. Every unit in this class holds a normal home NAS for well over an hour, and a NAS needs 2 to 5 minutes to shut down cleanly. Runtime is not the constraint. Waveform, the watt rating and whether the battery is replaceable are the constraints.
Why does the waveform matter more than the size?
Active power factor correction is a circuit in modern power supplies that shapes the current drawn from the mains into a clean sine. It is standard on 80 Plus rated ATX supplies, on the internal supplies of Synology and QNAP units, and on the external bricks that come with mini PCs. It is also the reason cheap UPS units are a problem.
While on battery a line-interactive unit synthesises its output. Cheaper models produce a stepped approximation of a sine wave, and the abrupt voltage transitions look like a fault to an active PFC input stage. The outcomes range from loud buzzing, through the power supply shutting itself off, to the UPS reading an overload and cutting the load entirely. All three happen at the exact moment the equipment needed protecting.
The cheapest true sine wave unit here is the CyberPower CP1350PFCLCD PFC Sinewave UPS at $219.95, and a sine wave model tends to publish a higher power factor as well, so some of the premium comes back as real watt capacity.
How do the units compare on real watts?
| Unit | VA | Real watts | Power factor | Waveform | Battery | Four-bay runtime | Eight-drive runtime | Cost per watt |
|---|---|---|---|---|---|---|---|---|
| APC BE650G1 Back-UPS 650VA / 390W | 650 | 390 W | 0.60 | Stepped approximation | 86 Wh | 66 min | 25 min | $0.28 |
| CyberPower CP900AVR UPS | 900 | 480 W | 0.53 | Stepped approximation | 108 Wh | 84 min | 32 min | $0.33 |
| APC BX1500M Back-UPS 1500VA / 900W | 1500 | 900 W | 0.60 | Stepped approximation | 216 Wh | 2 h 51 min | 68 min | $0.21 |
| CyberPower CP1350PFCLCD PFC Sinewave UPS | 1350 | 810 W | 0.60 | Pure sine wave | 173 Wh | 2 h 16 min | 54 min | $0.27 |
| CyberPower CP1500PFCLCD PFC Sinewave UPS | 1500 | 1000 W | 0.67 | Pure sine wave | 216 Wh | 2 h 51 min | 69 min | $0.24 |
| APC BR1500MS2 Back-UPS Pro 1500VA / 900W Pure Sine Wave | 1500 | 900 W | 0.60 | Pure sine wave | 216 Wh | 2 h 51 min | 68 min | $0.33 |
Power factor is calculated from the manufacturer's own watt and VA figures rather than assumed. Battery capacity is the nominal pack size for the unit class. Runtime figures use a 46 W four-bay load and a 111 W eight-drive tower load. Prices change often, confirm on Amazon.
Two rows are worth reading against each other. The APC BX1500M and the APC BR1500MS2 both carry 1500 VA and both carry 900 real watts. Identical headline figure, identical capacity, and one produces a pure sine wave while the other produces a stepped approximation. Nothing on the front of either box tells you which is which. Then compare both to the CyberPower CP1500PFCLCD, where a higher power factor buys 100 W more real load from the same nominal size.
What should go on the battery outlets?
Every consumer UPS splits its sockets into two banks, and only one of them is backed by the battery. Everything you put in the battery bank spends your runtime, so sorting the equipment is free capacity.
- Battery bank: the NAS, the network switch and the router. The switch and router cost perhaps 15 W between them, and without them a NAS being told to shut down by another machine never receives the message.
- Surge only: monitors, desktop workstations, anything you are not trying to keep alive. A headless server does not need a screen to power down.
- Neither: laser printers, kettles, space heaters. A laser printer pulls over a kilowatt during fusing and will trip the unit.
The full breakdown with typical draw per device is on the UPS runtime calculator, which will also tell you the load percentage you are running at and warn you when the headroom is gone.
What makes a UPS actually protect the array?
The data cable. A UPS with no connection to the server is a delay rather than a protection: it holds the array up for twenty minutes, then drops it mid-write exactly as the outage would have, just later. Every unit here ships with a USB cable and every serious NAS platform reads it natively.
Four settings decide whether it works when it matters:
- Trigger on remaining battery capacity, not elapsed time. A pack fades as it ages, so "shut down at 40 percent remaining" stays correct for years while "shut down after 10 minutes" quietly stops being safe.
- Set an initial delay of around 60 seconds so a four second blink does not power the house down.
- Decide what happens when power returns mid-shutdown. The safe answer is to finish the shutdown and power back on afterwards rather than aborting halfway through stopping services.
- Test it once, deliberately. Pull the plug with the array idle and watch the sequence run. This is the only way to discover the cable was in the wrong port before an outage teaches you.
Which features are worth paying for and which are not?
Worth paying for: pure sine wave output, a user-replaceable battery pack, and automatic voltage regulation. That last one corrects mild brownouts by switching transformer taps without touching the battery, which matters if your supply sags when a compressor starts, and every cycle avoided is a cycle off the pack's life.
Not worth paying for on a home NAS: large battery capacity beyond a comfortable shutdown margin, online double-conversion topology, and rack mounting unless you already have a rack. Online units eliminate the transfer time entirely and cost several times more to solve a problem a NAS power supply already rides through.
One thing that is easy to overlook: a sealed unit with no replaceable pack is a unit you throw away in four years. Check that before buying, not after. The APC BE650G1 is the honest budget entry here for a two-bay box or a mini PC, and it is a sealed stepped-waveform unit, which is the trade you are making at that price.
Who should not buy the expert pick?
The APC BR1500MS2 at $299.99 is the most capable unit here, with 900 real watts, a pure sine wave and a replaceable pack. Most four-bay owners do not need it, because the CyberPower CP1350PFCLCD already delivers 810 W and already holds a four-bay NAS for hours.
The case for stepping up is a genuinely larger load: an eight-drive tower, a NAS plus a virtualisation host, or a small rack where the total is well over 100 W and the spin-up surge from eight drives is 200 W or more on its own. If that describes your build, the extra battery and the extra headroom are both doing real work.
And remember what the UPS is not protecting you from. It stops a power event reaching the array mid-write. It does nothing about a drive dying, which is what parity is for, and nothing about deletion, ransomware, theft or fire, which is what a real backup is for. Those three protections are independent and a complete build needs all of them.
Related reading
- UPS runtime calculator to size a unit against your own load
- Power draw calculator for the load figure everything here depends on
- Home server power consumption for what the array costs to run
- Best NAS for home for the box the UPS is protecting
- Power consumption chart for watts by component
Deciding on a specific model? We review the APC BR1500MS2 1500VA UPS review in full.
Frequently asked questions
What size UPS do I need for a NAS?
Size against the watt rating rather than the VA figure, and against your active load rather than idle. A four-bay NAS draws about 46 W in normal use, so almost any unit above 400 real watts carries it comfortably. The number that actually decides your purchase is battery capacity, because runtime is watt-hours divided by watts, and a target of 10 to 15 minutes at your real load is generous for a clean shutdown.
Do I really need a pure sine wave UPS?
Yes if the power supply uses active power factor correction, which most modern NAS units, ATX supplies and mini PC bricks do. An active PFC supply reads the stepped waveform cheaper units produce as a fault and can buzz loudly, shut down, or refuse to transfer to battery at the exact moment it was needed. A sine wave model costs roughly $60 to $100 more and removes the entire class of problem.
Is VA the same as watts?
No. VA is apparent power and watts is real power, and the ratio between them is the power factor. Consumer line-interactive units publish a power factor around 0.6, so a 1500 VA model is often rated 900 W. Buying by the VA number alone overstates what the unit can carry by roughly 40 percent. The watt figure is always printed on the specification page, and it is the one to size against.
How long should a UPS keep a NAS running?
Long enough to shut down cleanly, which is not long. A NAS needs roughly 2 to 5 minutes to stop services, flush write caches, unmount volumes and power off, so 10 to 15 minutes at your real load gives comfortable margin including the delay you configure before shutdown triggers. Buying enough battery to ride out an hour costs several times more and protects nothing the shutdown would not.
Does the UPS need a data cable to the NAS?
Yes, and it is the part most often left in the box. Without the USB or serial connection the UPS simply delays the problem: it holds the array up for twenty minutes and then drops it mid-write exactly as the outage would have. Every serious NAS platform reads it natively, and network UPS tools let one machine with the cable signal every other machine on the network.
How often do UPS batteries need replacing?
Sealed lead acid packs last roughly three to five years and shorter in a warm cupboard, and they fail by quietly losing capacity rather than dying outright. A unit that genuinely gave 40 minutes when new can be down to 4 with every light still green. Run the self test twice a year and treat a sharp drop in reported runtime as the replacement signal. A user-replaceable pack turns that into a cheap job.
How we choose: we compare published manufacturer specifications, drive datasheets, published reliability statistics and verified owner reviews. We do not test hardware in person, and we are not tied to any NAS vendor. Capacity and power figures here are researched guidance, not a warranty. RAID protects against drive failure, not against deletion, ransomware, fire or theft, so keep verified backups regardless of what any calculator tells you.
Working out your own cost per usable terabyte? The Home Server Build Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.