Many power stations can sit plugged into the wall with your devices plugged into them. Wall power passes straight through to the devices and keeps the battery topped up. When the wall power fails, the station switches the devices over to its battery. Makers call this UPS (uninterruptible power supply) or EPS (emergency power supply) mode.
The catch is the switchover time: the gap, in milliseconds (ms), between the wall power dropping and the battery taking over.
Why a few milliseconds matter
Lights, fans, fridges and phone chargers don’t notice a gap of 20 or 30 ms. Desktop computers, network storage, some medical devices and some home network gear can. A computer’s power supply only has enough stored energy to ride through a short gap: the ATX power supply standard required 17 ms of hold-up time at full load, and its 3.1 revision cut that to 12 ms. A switchover longer than the power supply’s hold-up time means a sudden reboot and possibly lost work.
That is why the UPS ranking only includes stations rated for 10 ms or less: fast enough to stay inside the hold-up time of a power supply built to the current standard, with some margin. Conventional standby UPS units for computers can take up to about 25 ms, so a 20 ms power station is no worse than one of those, but it’s a gamble with a PC.
What the data shows
Of the 121 stations ranked here, 100 offer a UPS or pass-through mode, and 96 publish a switchover time:
- 38 switch over in 10 ms or less, including 3 that claim 0 ms, such as the EcoFlow DELTA Pro Ultra.
- 58 take longer than 10 ms, and 37 of those 20 ms or more.
Switchover times are the makers’ own figures. Few have been independently tested, so treat a borderline figure with caution.
Other things to check
- Pass-through wattage. Some stations limit what they can pass through from the wall in UPS mode, below their normal AC output. It’s listed in each product’s UPS notes where published.
- Battery wear. Most stations stop charging once full in UPS mode, but a station kept plugged in all year will see more calendar ageing. A LiFePO4 battery (see LiFePO4 vs NMC) tolerates that best.
- It isn’t a surge protector. A station’s UPS mode doesn’t replace surge protection for sensitive equipment unless the maker says so.
How the rankings use it
UPS support adds to the features factor in how we rank, and each product page shows the switchover time where the maker publishes one.
Sources
- Wikipedia, ATX: ATX 3.1 reduced the required power supply hold-up time from 17 ms to 12 ms.
- Wikipedia, Uninterruptible power supply: a standby UPS’s switchover can take as long as 25 ms; a double-conversion (online) UPS has no transfer gap.
- Switchover times: each product’s manufacturer specifications, linked from its page.