UPS standby power consumption - Page 1
Forum section: electronics / general

author: james_s
author_rank: Super Contributor
timestamp: July 25, 2020, 06:23:12 pm
content: Has anyone ever come across reviews or charts of the standby consumption of uninteruptible power supplies or made any measurements yourself? I've noticed that this data seems to almost never be published, and yet in most applications a UPS will spend at least 99.999% of the time just sitting there burning power waiting for something to do so this is one of the most important characteristics to me.I recently acquired a handful of small secondhand UPS units and took some measurements after installing a good fresh battery and allowing 24 hours to fully charge and observed a significant variation.APC Back-UPS ES 550 - 2.0WTripp-Lite Internet750U - 3.1WEaton 3S 550 - 4.4WAPC Smart-UPS 1500 - 23.5WThe little Back-UPS was the champion at a paltry 2W while the big Smart-UPS is of the line-interactive type and is the massive guzzler of the bunch, I won't be putting that one into service here. All of the rest have various advantages and disadvantages, the Back-UPS lacks wall mounting slots and uses an iron transformer in the inverter that buzzes when on battery. The Tripp-Lite is more bulky, draws more power in standby, requires opening up the whole unit to change the battery and the mounting slots are only on one end so it can't be hung horizontally. Both of those use a ridiculous RJ45 connector for the USB interface. The Eaton is a tidy little unit, has a far more pleasant sounding alarm than the obnoxiously loud high pitched screech of the others, has mounting slots that support horizontal and vertical and a proper USB connector but it is hobbled by a small 5Ah battery and the power consumption is the highest of the standby types. All three of the small units are of the "modified sine wave" type, ie a square wave with dead time. The Smart-UPS is a true-sine unit however.Line-interactive is useless to me, I've never seen my mains voltage dip lower than 118V or higher than 123V for any significant length of time so that feature adds no value, only additional power consumption. In the event that the voltage goes out of range momentarily I'd rather it just switch to battery mode.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 29, 2020, 05:50:23 pm
content: Your measurements are consistent with my observations.The "line interactive" UPS offers a feature often labeled "AVR" (automatic voltage regulation). This design requires a large iron-core inverter transformer operating at 50/60Hz. This transformer is typically as large and heavy as the power transformer found in a large stereo receiver or amplifier. The increased standby power consumption is caused by hysteresis loss inside the iron core of this large 60Hz inverter transformer.What is "AVR?"Answer: If the incoming AC mains voltage increases or decreases by >12V, the "AVR" feature switches in a 12V buck/boost winding on the inverter transformer to raise or lower the UPS output voltage. The inverter transformer then operates as an autotransformer to raise or lower the incoming AC mains voltage by +/- 12V. This feature requires the primary of the inverter transformer to be energized at all times while the UPS main output receptacles are switched on. The APC "smart-UPS" series which was introduced in the 1990's is an example of this type. APC "smart-UPS" models with automatic voltage regulation include their 1000, 1100, 1400, 2500, and 3000VA models.Another feature of these APC models with "AVR" is that they use the large 60Hz inverter transformer as the source of charging current for the storage batteries.In contrast, a UPS ***without*** "AVR" often simply connects the incoming AC mains directly to the output receptacles except when there has been a major dropout or brownout of the incoming AC mains. The standby power consumption for this type of UPS will often be < 5W. The inverter transformer isn't powered up except when the UPS is generating power from the batteries.Some models do ***not*** contain a large 60Hz iron-core inverter transformer. Rather they use a "switching type" DC-DC converter to change 12V DC (or 24V DC) from the batteries to 160V DC (or 320V DC), and then they use an "H bridge" of MOSFET transistors to convert 160V DC to 120V AC (or 320V DC to 240V AC). This is a very efficient system, but it is seen mostly in UPS units with less than 1000VA (1000W) of power output. Also, these "2 step" units don't produce anything resembling a true sine wave when operating from the batteries. Their output waveform is a "square wave with dead zones." In contrast, some (but not all) UPS units with a large iron-core 60Hz inverter transformer actually produce a true sine wave.-E

author: james_s
author_rank: Super Contributor
timestamp: July 29, 2020, 06:07:41 pm
content: Yeah I'm familiar with how they work, AVR could be useful in a rural or industrial area where the line voltage is quite high or low for significant periods of time but here I have never seen the voltage more than a few percent from nominal for any sustained length of time so AVR offers no value.I have a mix of topologies here, all except for one of them are the standby type, some with an iron transformer for the inverter and others with the ferrite transformer high frequency inverter followed by an H bridge. All of mine with the exception of the big old Smart-UPS produce a "modified sine wave" which is nothing more than a square wave with some dead time. For most things that works fine though many dimmable LED bulbs do not behave well on them and iron transformers buzz. I have not seen a pure-sine UPS under 1kVA, or of the standby type, they all seem to be line-interactive.Another friend gave me a couple more UPS's the other day so I have some more quiescent draw data points. APC seems to be the overall winner so far in terms of standby efficiency. I'd like to get some more samples of other brands but even though used UPS's are worth essentially $0 shipping is not free.APC Back-UPS CS 500 - 2.3WAPC Back-UPS XS 1500 - 5W

author: tom66
author_rank: Super Contributor
timestamp: July 29, 2020, 07:07:44 pm
content: Question, maybe it's silly, but with most devices now having universal input ranges with active PFC (my computer ATX supply is rated 90V ~ 265V AC); is there much need for a line-interactive UPS for many devices nowadays? With older, non-power factor corrected SMPSes, or equipment with 50/60Hz transformers within, I could see the need, but that's a small snapshot of kit these days.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 29, 2020, 08:36:10 pm
content: Quote from: tom66 on July 29, 2020, 07:07:44 pmI agree: Most of today's UPS applications don't need automatic voltage regulation.Also, if the powered device has active power-factor correction (PFC) then the waveshape of the UPS output doesn't matter very much either.-E

author: james_s
author_rank: Super Contributor
timestamp: July 29, 2020, 10:15:35 pm
content: Quote from: tom66 on July 29, 2020, 07:07:44 pmMost devices in general don't need any kind of line voltage regulation, they will operate without problems over at least as wide a range as most of the AVR features can compensate for. Maybe some equipment from the 80s was sensitive enough to matter but I can't think of any specific examples.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 30, 2020, 03:00:28 am
content: Old school linear power supplies can suffer from line voltage variations. There is a trade-off between the voltage dipping too low for regulation vs. excessive heat generation at the high end. An AVR system can help a marginal linear supply be happier. Filament bulbs lifetime will be decreased at the high end of voltage range. Neither of these loads are commonly used on UPSs. Even non PFC, non universal voltage switch mode supplies should not have any problem with the range of voltages AVR can correct. I have run into exactly one piece of European industrial equipment specified for 230VAC that doesn't run well on US 208V depending on where in the tolerance band you are but that is extremely rare and better fixed with a fixed boost transformer.