UPS gives backup power for mere minutes, Solar Inverters can give backup for hours and even days, depending on your budged, I have done a few server rooms and can tell you, there is very little if any type of UPS that can match a solar inverter.
Depends on the UPS.
I have advised clients to get UPS’s, the same size as solar inverters, with battery banks as large as any solar inverter, can take, for their businesses or server rooms.
The reason I justify the cost of a solar inverter is that the next plan is to get solar panels for daytime consumption grid-tied, to further save on Eskom, and if the bank is properly specced, to even power the Critical Loads 24/7/365, weather permitting.
Some of the big boys:
20kVA 16kW Transformer Based Online UPS
40kVA 36kW Transformerless Online UPS - just imagine, 80 x 12V 105Ah batteries. 
Ps. runtimes quoted are at max power …
One UPS specialist company: https://www.cooperpower.co.za/index.php/products/ups/online/3-3-phase
And if you want, you can go modified sine wave with a UPS-PG2000-6 i.e. 2000VA/1400W with 4 x 12V 105Ah or 2 x 200ah, or 4 x 200ah, as the charger is, I think, max 24v 20amps.
Will work perfectly for servers, modified sine wave, for ±6-8 hours at a time.
https://www.cooperpower.co.za/index.php/products/ups/offline/inverex-intellipower
Perhaps but I have never seen (or experienced) any warnings that a regular UPS cannot be operated continuously. If this were the case there would be some mention of it.
If cost is an issue then inverters that are perfectly good that have been thrown out by the high flying IT industry can be picked up for a song. If required I can point you to a company in Kyalami that specialises in these.
Believe it or not: Cost. As Jaco also said, they are designed to carry a heavy load for just long enough that the backup generator can start, and they are marketed accordingly, which means that their continuous capabilities is often half of the number on the box, which means that if you want to use it to run things for long periods of time you need to double up on the size, at which point the price is suddenly a whole lot less appealing.
They do, but it is a bit less (peak-wise) than a fridge, but it’s longer in duration. A fridge can draw 4 or 5 times its nominal power for a split second in order to start up (so a 250W compressor spikes to 1.2kW, a 1200W air conditioner spikes to 5kW+, and so forth), while a household-type laser printer draws up to 2kW when heating the fuser unit, probably depending on model.
One can probably argue that it is unlikely that all ten of them will print at the same time, but ten of them gives pretty good odds that two of them might be going at the same time…
This is one of my pet peeves. The cookie-cutter cheap-inverter with some Lead acid batteries solution, without realising that the batteries (not the inverter) is the important part.
So you want to stick to C/10 if you can and definitely no more than C/5. So a 200Ah battery can give you maybe 35A if we thumbsuck a bit, yell Peukert into the air a few times and wave our hands, and 35*50 (to make the math easy) is like 1700W. That’s badly under-batteried for a 5kVA.
I beg to differ on the time the UPS can run for. Yes, the time is limited due to the size of batteries fitted but see the manual on a respectable UPS (attached)
There is an battery connection on the rear that allows you to add external batteries.
Also the unit has forced ventilation (with fan failure detection).
So the unit might have been designed for short backup times but no reason one has to stick to this…Manual_UPS_GAIA_1_3kVA_en_us.pdf (1.3 MB)
APC actually make some really nice rackmount lithium UPS’s now. Nicely suited for constant load-shedding as the standard lead is better designed for rare emergencies vs constant cycling.
A lot of them don’t…
My dell server here takes only 12V on the motherboard, I had to rig up a special adapter cable to make it work from a regular ATX power supply.
Of course I’m using it with regular SATA drives, and the cables for that run all 3 voltages, however it is well known that no SATA drives actually use 3.3V (A lot of SATA power adapters don’t even bother with the 3.3V wire).
So this particular server uses only 2 voltages 12v and 5v (the second actually being the drives and not the server itself), and does not use ATX. This is not that uncommon a lot of servers do their own 5v conversion on their drive controller card or on the motherboard (I just happen to not have the relevant card for this server). There even exist some attempts at standards (ATX12V) for this.
That said for a regular company an inverter is going to be the path of least resistance, rather than trying to switch to different power supplies, in large data centers its a different story though, especially the very large ones. (e.g. see things like https://www.datacenterknowledge.com/archives/2016/03/09/google-contributes-48v-shallow-data-center-rack-to-ocp)
For sure. From reading the original post I thought it was the backup of a server rack that they were interested in. A few posts later I realised that wasn’t the case and an inverter setup would definitely be more suited for this use case.
This actually has not been true for a long long time, modern desktop computers power things like the CPU exclusively from the 12v rail and they have their own VRMs to do step down to required voltages from there. For a variety of design reasons it just works better this way.
5v Is used almost exclusively by peripherals like drives (as a secondary voltage, they too use 12v more), and 3.3V is mostly an antiquated thing that people wish would just go away, I think cheaper motherboards still use it for RAM, but most motherboards (AFAIK) also just use their own VRMs for that now.
A quick look at the specs of any modern ATX PSU will confirm this, as you will quickly see that the 12V rail has by far the largest amperage available, and that the other rails are generally just step downs off the 12V rail.
This is why we see an increasing push to replace ATX with ATX12VO and so on.
I believe the same is true for graphics cards. Have VRMs for both core and memory, so I believe they would also draw 12V over the PCI-E slot as well as through the PCI-E power cables.
The 12VO standard will be nice, but my suspicion is that motherboards will get more expensive (because it will need some additional circuitry), and PSUs will probably not get cheaper. Also, there’s now more “parts” in the MOBO that can fail…
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Will be interesting to see how it pans out. Everybody instantly assumes that 5v will have to be on the motherboard, but I guess an alternative scenario is that theres a little “drive controller” card that plugs into the motherboard, is easily replaceable/re-usable across boards, and that 5v VRM goes on that.
Taking it more extreme maybe all the VRMs become little card/stick things or something, so that VRMs are easily replaceable/swappable everywhere on the computer.
Alternatively maybe drives make the leap to 12v only or the VRM goes on the drive.
Maybe thats optimistic though and more expensive motherboards just become the norm instead, even then though I can’t actually recall the last time I had a motherboard fail on me, its been ages so I’m not sure failure rate is all that high.
The bigger downside I guess is that the motherboard is more expensive, and you are paying for that same part (the VRM) on every upgrade even though it could be re-used.
Which is why I said “traditionally speaking” 
The original AT standard had 4 rails (12V, 5V, -5V, -12V) with varying capabilities, then came ATX which added 3.3V, but as you say, nowadays a lot of the equipment have their own SMPS to make things even simpler.
I do suspect however that in a server room, you will have enough equipment of varying types and ages that converting the place to 48VDC isn’t really cost effective or feasible. If you design the place from the get-go with that in mind, that may be an option, but not to retrofit.
Besides, with all respect to our friend Richard, I think the hatred of inverters is a tad yesteryear. By all means, ask yourself if an inverter is really necessary (because it will save a lot of money if you don’t have to buy one), but there is no reason to avoid it as if it’s made of asbestos.
The technology is very well settled, and if you opt for an HF design the efficiency is 95% and up (which is basically on par with any switch mode power supply). Of course, in that case you are not looking at a Multiplus. They are LF designs with somewhat lower efficiency (closer to 88%).
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Sure inverters are not evil, and theres a good reason they are the default/standard way of doing things. And certainly for something like in this thread I wouldn’t suggest anything else.
Still they are expensive, there is no real avoiding that, as are batteries for that matter. In cases that they could be reasonably avoided (and efficiency gained as a result) there is potentially a lot of money to be saved.
For a system like mine not having an inverter shaves off probably 1/3 of the overall system cost; still I wouldn’t recommend it to most people…
As you yourself say inverters are a relatively stable technology now, unlike panels their price is also pretty stable, it seems unlikely that there will be any major gains to be had in this part of the system going forward, sure they get smarter and more flexible and gain more features with each iteration etc. but in terms of actual price/performance what we have now is pretty much it IMO.
We live in an increasingly DC world and for those of us who are interested in the future (as opposed to just the present), on how solar could be made increasingly more affordable, there is always going to be a large allure to looking into the DC side of things, as this is where the potential to largely reduce system cost lies; even if not currently practical for most people, and a lot of barriers certainly exist to reaching a point where it could be practical.
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I think for drives, which has low power usage, it would be a great solution, but there is a reason the CPU VRMs and GPU VRMs are so close to the cores and even distributed all around the core (on high end boards)… They need the power delivery to be extremely stable to all parts of the core (even the voltage drop from the one end to the other end of the core could cause instability).
While the CPU is busy chewing some 150W @ 100A (assuming 1.5V which is already very high), you want that wire (or path on the PCB) to be very short.
But if we want to go all out modular… Why have a mainboard at all! We are starting to see some interconnects that is super fast. Maybe the future is a fully modular PC with a CPU board, GPU board etc. with a big bus board connecting it all… and all the driver issues that would probably come with it. 
Though the future seems much more system on a chip than modular PCs…
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Investing in a backup power system is a smart idea especially here in Africa. Yes you can install the bare minimum with a UPS to ride through power outages but as a business you have way better options: Since most businesses operate during the daytime one should utilise the solar power that is available. PV panels deliver power day after day with a typical lifespan of 20 years (with a warranty nogal!) This will lighten the load on your batteries so they will last longer.
A well though out system will also save you on electricity costs and as a business you are entitled to write off the costs in your business.
I am glad you posted this manual. Here is a screenshot taken from this manual.
Now the problems we have experienced with these units is the recharge times. During periods of multiple load shedding’s per day, the batteries dont recharge before the next load shedding. Adding extra batteries, increases the recharge time. The quotes we received from different reputable UPS suppliers were between 8 and 14K per “30min” addons. In these cases a Solar inverter with lithium’s to handle 4 hour outages made a lot more sense.
If it takes 6 Hours to recharge a “5Min” bank, just imagine how long it would take to recharge only one “30min” addon unit. lets not even mention 8 of them to reach 4 hours backup. Roughly calculated, that should take appr, 288 hours or 12 days.
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Did you also get them the prices to do so, and what kind of recharge times did you allow?
I have made the comparisons a few times and not once did the UPS cost less than the “Solar” type battery backup.
Please do yourself a favor, get a quote on a online (not Line interactive) 3kva UPS with enough addon batteries to keep a 2kwh server room running for 4 hours. Even you would see that a Inverter with Lithium’s makes a lot more sense.
But you know me, I am passionate about my inverters and the new technology, maybe I asked the wrong company’s for prices…
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It’s no secret that these UPSs are designed for short power outages. (Go check the CB in the board and reset it.)
It’s not difficult to add a charger to assist charging of the extra battery bank…