Guide to Uninterruptible Power Supplies (UPS)

Eaton_1500VA_smallThe UPS is a misunderstood beast, so we have written this guide to clear up misconceptions and provide information to help you work out if you need one.

At its core, a UPS is a battery that sits between the mains power supply and your equipment.  When the power drops out, the battery is there to keep your gear running long enough to save your work and shut it down normally.

Avoiding an abrupt power cut to a PC or server is always a good thing.  A power cut can damage your files, OS, and occasionally hardware.  Complex systems and those under load, such as servers, are at particular risk.  For example, if a database write is in progress when the power is cut then your database will likely be damaged.

A UPS with large battery capacity can also be used to keep critical systems running for hours.  Many people assume a long uptime is the main reason to buy a UPS, but in fact that is a relatively uncommon, secondary purpose.  If you need extended runtime, a UPS can be configured for that purpose, but you might also want to look at generators in combination with a UPS.

Another benefit provided by most UPS units is built in power protection, from basic surge protection to more advanced power conditioning.  A UPS does not necessarily replace a good surge protector, and in fact using a surge protector behind a UPS may add a better quality of power protection compared to entry level UPS protection, and will at least protect the UPS itself.

Types of UPS Technology

There are three technologies typically used on consumer and SMB level UPS devices.

Standby UPS for Backup Power

Standby_UPS_CircuitEntry level UPS units use a “standby” design where power flows directly from the mains to your devices when mains power is available.  At the same time, the battery is charged using mains power.

When mains power is cut off, battery power is automatically switched on through an inverter to power your devices.  It takes a small but appreciable period of time to switch to the battery, and that time can cause issues for some sensitive equipment.  For example, some modern active PFC power supplies might pull excessive current for a moment as it comes back online if the switch takes too long, overloading the UPS.

Most of these units employ a basic surge protection in the circuit to protect against surges and brown outs (not shown in diagram).

The signal output tends to be a simulated sine wave, which is fine for most gear but can cause issues with some power supplies.

Line Interactive UPS with Power Conditioning

Line_Interactive_UPS_CircuitA line interactive UPS is often assumed to be an “online” system where the battery is always feeding power directly to your equipment.  Not true.

A line interactive UPS is similar to a standby UPS but adds an additional component able to regulate voltage.  When the mains voltage goes a little above or below an acceptable level then this additional component can adjust the voltage sent to your equipment, and so the battery does not need to be drained to handle it.  The UPS will usually click when this kicks in.  If voltages vary too much, then the battery will take over as the power source, same as for a standby UPS.

The output signal from a line interactive UPS may be a simulated or pure sine wave output, depending on the model.

Line interactive UPSs are relatively cheap and are the best value type of UPS for many circumstances.

Online Double Conversion UPS: Clean and Continuous Power

Double_Conversion_UPS_CircuitDouble conversion UPS models are the high end option in the for consumer and SMB market.

This technology keeps the inverter online at all times so any power interruption does not require switching and the output power quality is consistent, clean, with a true sine wave output at all times.

Because the battery and inverter are always online, additional stress is placed on these components and there is some loss of efficiency with associated waste heat.  Its not a serious problem, but is a reason why you need to ensure the UPS is well ventilated and personally I prefer to stick a decent surge protector behind these units.

A similar but more recent technology called an online delta conversion UPS is very similar to a double conversion unit but adds components to improve efficiency and reduce issues with the traditional online UPS.  This type of technology tends to be found in only large UPS units, over 5kVA but a worthwhile upgrade for higher end commercial demands.

Online double conversion systems are the best choice for critical systems with modest sized installs, but do cost significantly more than line interactive or standby UPS models.

Whats with Simulated vrs Pure Sine Wave??

Mains AC power is supplied in the form of a sine wave that smoothly alternates between positive and negative values.  Recreating that form from a DC source at the other end of a UPS battery can be expensive.  Very basic equipment will produce a square wave where the voltage jumps straight from positive to negative 240V.  Harsh.  Most cheapish UPSs will do a better job with a modified square wave form, also called a simulated sine wave that is closeish to the real thing, but not a smooth curve, just some steps.  Better units including all online UPS units will produce a nice smooth sine wave that works best.

squarewave modsquarewavesinewave

Modern, efficient computer power supplies (anything with the 80+ certification) feature active power factor correction (PFC) and they do not always play well with simulated sine waves, let alone the even more horrible square waves.

If you find yourself with  an active PFC power supply and anything less than a pure sine wave output, you may find your PC randomly rebooting and/or your power supply struggling and dropping efficiency when it switches to battery.  Many PSs will handle it, and as long as it works for the short time you are on battery you can get away with a cheaper unit, but its not ideal.

Note there are simulated sine waves, and then there are simulated sine waves.  In other words some of the modem UPS units do a pretty good job of producing a nearly accurate simulated curve, others not so much.  You tend to get what you pay for in that regard and closer to a pure sine wave, the better.

In general, try to get a UPS with a pure sine wave output but if the cost is excessive for your use, its likely a simulated wave for good brand name UPS units will be OK.

The much misunderstood “VA” vrs Watts

Once you decide on the UPS technology you want, you need to match it up with the battery specifications.

UPS units are usually quoted with “VA” specification, often in their model name.  VA stands for Volts x Amps, and if you remember your high school science you will know Volt*Amps= Watts so you would reasonably expect that VA figure to represent the output capacity in watts.  But then you might notice that a UPS will also specify maximum output in Watts, and the number will be different, and lower, than the VA number.  So whats up with that?


The reason for the difference is because the VA represents the maximum theoretical output, called apparent output, but available power will be less and convention says we should not use more than 60% of the apparent output, meaning the watt rating will be about 60% of the VA.  A 1000VA UPS should be rated at 600W.

Some manufacturers will use a relatively high maximum wattage quoted, maybe 70% or more of VA.  There may be some justification to do that in some cases, though I think more often it’s the marketing people sticking their nose in.  Personally, I prefer to look at the VA and work out wattage based on 60% of that.

That convention, and the variance it implements, brings up a good point.  Do not overload your UPS.  A reasonable target to run the UPS at about half the rated load, so if your equipment uses around a 300W and may burst up a bit past that, then go for a UPS rated to handle about 600W, or 1/0.6 * 600W = 1000VA.  Its usually OK if you run it up to that 60% figure, and potentially a little past that for short periods, but lower is safer and will give you better runtime.

Another issue many fail to consider is the behaviour of active PFC power with switching UPS units supplies (ie, all new PC power supplies nowadays).  Where the switchover to battery takes a while, the power supply may try to slurp up a lot of power to catch up in the moment that power is switched back on, and that can overload the UPS.  Consider the nominal rating of the power supply, plus a bit, against the UPS rating and aim for a UPS that is rated a little higher than the power supply to avoid this problem.  For example, a 750W PS that might normally use just 150W would probably be OK with a 1000VA line interactive UPS but might fail with a 500VA unit, even though 500VA can supply much more than 150W.

Take into account that a battery performance and capacity will be reduced over time, till they need replacing.  Also consider that the runtime will improve as your reduce load on the battery, so loading it up to near maximum may not give you long enough to shut down for your equipment.

UPS Runtime

The VA figure relates to output at a given point at time.  Many people assume that a high VA means high run time.  In fact that’s not true and the two specifications are not directly related.

runningIt is quite possible for a certain model 1000VA UPS to run for say 10 min at half load, and a different 1000VA model to run for an hour at half load.  The figure that matters for a battery is how much energy in watt hours that it can store.

Most UPS units are built to allow time for shutdown but not much more than that, so if you need a long runtime after the power goes out, you need to look at long runtime batteries, add additional batteries, or run a UPS at a fraction of its maximum output.

UPS specification normally quote expected runtime at half load.  If you run it at full load, then roughly halve that time.  If you run at a quarter load, double it, and so on.  Also again consider that those numbers will tend to reduce as the battery ages and you always want to factor in some extra buffer time.

Beware of outlets with weird plugs and no battery protection

Many entry level UPS units now have standard wall power point style plugs available to make it easier to plug in any gear you want.

Eaton_550VABe aware that some may be only wired up to surge protection and not battery backup.  For example, you might have a PC plugged into one plug and a laser printer into the other for convenience.  You don’t want the laser printer to switch to battery; just make sure you know which plug is which!

You can plug in a double adapter or power board into a UPS outlet, but take care not to overload it with excessive demand.  It is not the number of devices that matter, but their total load.

UPS_CableOlder and larger UPS units will have a number of three prong power leads, or “jug leads” as I call them.  That’s the standard power lead you see running into your PC.  They are fine when plugging into a PC power supply, but can be a pain if you want to plug in other gear and you may need to buy separate cabled that convert to a wall plug.  Just make sure when you buy at UPS that you end up with the cables you need to plug in your gear.

Equipment you should never plug into a UPS

Some equipment can draw a heavy load of power for a short time, and these can damage the UPS and any other equipment attached.

Laser printers are notorious for it.  Don’t plug in anything that needs that big pulse of power that spike up beyond what your UPS can handle.

Set up Automatic Shutdown

For most buyers, the purpose of a UPS is allow for a clean shutdown of your equipment during a power outage.  To meet this goal, don’t forgot to install and setup compatible UPS management software on your devices so they shut down automatically when unattended.

LansafeThe UPS will be typically connected to your server or PC with a  serial or USB cable, with the option to use a network cable in higher end models.  Those cables are essential to let the UPS talk to your gear and let them know the power just went out.

I personally prefer a simple serial cable when available and in small scale deployments, as they tend to have less issues with drivers and avoid the situation using a network cable where if the switch fails or loses power, signal will be lost (so if using the network cable, make sure the switch/s are plugged into a UPS!).  USB cables will do the job if that’s your only option, but in my experience, tend to be a little less reliable.

I personally don’t trust the automatic setting with the software.  Take a look (and test) the runtime you are getting out of your UPS, and gain an estimate of how long your machine takes to shut down when the shutdown signal is sent.  The software will let you set a delay after the power goes out to start shutting down your server/PC. Hopefully during that time it will come back up, if not, the signal is sent and then your PC will start shutting down.  You may also have the option to turning the UPS itself off before the battery is entirely drained, using that function or not is a matter of design and personal choice.

Leave plenty of leeway.  If your UPS is estimating 30 mins of runtime and your critical server might take 5-10 mins to shutdown once the signal is sent, you might send the shutdown signal after just 5 mins to give it plenty of time to properly shutdown.  You do not want the battery to run or UPS turning off out during the shutdown process!

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