UPS switchover: what the millisecond figure buys you

What actually happens in those milliseconds

When a power station is plugged into the wall with an appliance plugged into it, the mains normally passes straight through. The inverter is idle and a relay connects the input socket to the output socket. This is called pass-through, or bypass.

When the grid fails, three things happen in sequence. The unit's monitoring circuit has to detect the loss, which it cannot do instantly because a healthy mains voltage passes through zero twice in every cycle anyway, so the circuit must wait long enough to be sure the absence is real. Then the relay has to physically move, which is a mechanical contact with mass and a spring. Then the inverter has to come up to full output and, ideally, in phase with where the mains was.

The published switchover figure is the total of those three, and the mechanical middle step is the one that sets the floor. That is why the numbers across four brands cluster so tightly rather than spreading out: they are all limited by roughly the same physics.

UPS, EPS, and what the words are hiding

Strictly, what a portable power station provides is an emergency power supply, an EPS: the load runs on the grid until the grid disappears, then transfers to the battery. Many makers label it UPS anyway, and the site records whatever their specification table states.

A true online, double conversion UPS works differently. The load is always fed from the inverter, which is always fed from the battery, which is always being recharged from the mains. There is no transfer, because nothing switches. The grid failing simply means the charger stops. Its switchover time is genuinely zero, not a small number.

That architecture costs efficiency, because everything runs through two conversions permanently, and it costs cell life, because the pack is being worked continuously. It belongs in server rooms. A power station that idles on bypass and transfers in a few milliseconds is the right design for a device that spends most of the year doing nothing, but it is a different machine and should not be described as an online UPS.

One figure in this catalog needs care. BLUETTI Apex 300 publishes its switchover as No break. That is the maker's own claim, printed as given, and the site shows it literally rather than converting it to a zero that would read as missing data.

What a computer power supply really tolerates

Inside every desktop power supply is a bank of capacitors on the high voltage side, charged to the peak of the incoming wave. Their job during normal operation is to smooth the supply between mains peaks. Their side effect is that they keep the outputs alive for a short time after the input disappears. That interval is called hold-up time.

The long-standing design target for that hold-up is around one full mains cycle, which at sixty hertz is a little under seventeen milliseconds, and supplies are specified at full load. Most desktops do not run at full load, so the stored energy lasts proportionally longer: a machine drawing a third of its supply's rating rides through appreciably more.

This is the number every switchover figure should be read against. A transfer completing in 10 ms finishes with the capacitors barely discharged. A transfer in 20 ms still sits inside the window for a lightly loaded machine. Below that window the computer never knows anything happened, and going lower changes nothing it can perceive.

Why a smaller number stops being worth paying for

Once the transfer is faster than the load's own ride-through, the specification has done its job and further reductions are invisible. There is no prize for a switchover twice as fast as necessary, because nothing downstream was going to notice the slower one either.

The devices people actually plug in have generous ride-through. A desktop and its monitor run on capacitors as described. A router, a modem and a network switch run from small external adapters whose output capacitors carry them easily. A CPAP machine is a low-draw device with a smoothing stage of its own. A games console behaves like a small desktop. A television will usually blink at worst.

What this means in practice: treat the switchover figure as a pass or fail test rather than a ranking. If a unit publishes a figure in this range at all, it will carry the equipment above. Choose between units on capacity, output and cycle life instead, because those differences are real and permanent, while a few milliseconds of transfer time are not.

The exception is a desktop that is already borderline. A machine with a heavily loaded supply, an old supply with tired capacitors, or a power-hungry graphics card has less margin than the figures above assume. If a cheap plug-in tester matters to you, the honest check is your own: pull the plug on the power station's input and see whether the machine survives.

What a switchover does not protect against

Anything that happens while the grid is present. In pass-through the appliance is connected to the mains, faults and all. Sags, spikes, brownouts and switching transients pass through with it. A unit that transfers only on total loss does nothing about a voltage that is merely bad, and none of the makers here publish an automatic voltage regulation specification.

Loads above the pass-through limit. The bypass path has its own current rating, which is not always the same as the inverter's continuous output. Exceed it and the unit will not simply transfer, it will trip.

Units with no transfer function at all. Several models in the catalog have none. Anker SOLIX C200 DC reads No UPS function, as do the other DC-only units, which have no AC output to transfer. EcoFlow DELTA 2 reads No UPS function as well, despite being a full AC unit, because the maker does not state a switchover figure. Absence of a published number is not the same as absence of a feature, and this site will not guess which it is.

A long outage. Switchover decides whether the transition is seamless. Capacity decides how long the lights stay on afterwards, and that is a different calculation entirely.

What breaks in an outage, and what to plug in first

Ranked by how much the switchover figure matters:

• A desktop computer with unsaved work. The only common household case where milliseconds genuinely decide the outcome. Anything in this catalog with a published figure will carry it.
• Networking gear. A router and modem reboot on a cut and take a minute or two to come back. The interruption is an annoyance rather than a loss, but it is the cheapest thing to protect and the one people miss.
• A CPAP machine. Worth protecting for comfort rather than safety, and a light enough load that it barely touches the pack overnight.
• A fridge. Almost indifferent to switchover, and often better off with a short gap: many compressors will not restart against residual pressure and need a few minutes anyway. What a fridge needs is capacity and a unit that tolerates its start-up spike, which the run time calculator and the inverter page cover.
• Lighting and charging. Entirely indifferent. Plug them in afterwards.

If the list above is your use case, almost any unit on the comparisons index that publishes a switchover figure meets the requirement, and the decision lies elsewhere.