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I asked my dealer for a 3 kVA UPS for my PC-class computer with a 12V * 8 * 42 Ah battery bank. However, he provided me with a 6 kVA UPS with the same battery bank.

He claimed no extra money for the increased capacity. Will the larger UPS add to my power consumption?

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Pragyaditya Das
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5 Answers5

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In your case, there are two important parameters for a UPS.

Power (kVA) and Energy (VAh).

Power is a measure of how much instantaneous power the UPS can source. This is important based on how much power the attached devices need to draw.

Energy is important because it dictates how long the UPS can continue to run. This is largely dictated by the amount of the energy storage (batteries) in the device.

So, if the Battery Bank (energy capacity) is the same size, but the available power is higher than the devices you need to connect, then this device is equivalent for your needs.

There is a chance that the electronics that allow it to deliver more power, may make it less efficient in your use case, it is unlikely to be significant.

Stephen Rauch
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    “There is a chance that the electronics that allow it to deliver more power, may make it less efficient in your use case, it is unlikely to be significant.” Shouldn’t the data sheet of the device have information about the efficiency at different load levels? If the UPS is used for a single PC with maybe <300W the 3kW UPS would already be overkill. I don’t know about UPSes specifically but usually boost/bucket converters have terrible efficiency below ~10% load. Of course this should only shorten the battery runtime, not normal power consumption and is probably still relatively insignificant. – Michael Feb 26 '18 at 08:50
  • @Michael Depends on the UPS type. There are UPS units which always run the load from the battery. This reduces the chance that power problems on the input side are going to affect the devices connected to it. If OP has such UPS, then, the efficiency might matter. – AndrejaKo Feb 26 '18 at 09:36
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    I've found the best way to describe the difference between power and energy to people (since the terms are so often used incorrectly) is to say this: If energy is how much money you have, power is how fast you spend it. – Samuel Feb 26 '18 at 20:28
  • Note that instantaneous power is almost certainty higher than the listed power rating, because most electronics are rating for higher instantaneous current than sustained current – cat40 Feb 26 '18 at 22:22
  • Actually, if you're under 20% or so load, it's *very* significant. For example, take a look at http://www.apc.com/products/efficiency_loss_chart.cfm?base_sku=SRT6KRMXLI-6W&chartSize=large — somewhere around 15% it dips below 90% efficient, and falls off a cliff. Down to 60% efficient at the left edge of the chart. (Off-line, as opposed to on-line double-conversion, should not have this problem.) [Of course, you could say that an extra 100W won't make that huge a difference on a power bill, which is probably true... Unless you have a bunch of 'em.] – derobert Feb 28 '18 at 18:01
  • For more background (and how much it can matter in large deployments), see e.g., http://www.apc.com/salestools/VAVR-6LJV7V/VAVR-6LJV7V_R3_EN.pdf [keep in mind that is APC sales material]. – derobert Feb 28 '18 at 18:24
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You probably got a good deal. It is possible that the bigger UPS will use marginally more or less power because its running at a different point in the efficiency curve - its impossible to say without knowing precise details of the unit, and its unlikely to be consequential.

That said, if you have a 6kva unit, it means that the components are rated for higher current, which means they will be more reliable. It also means you can have a higher maximum power draw from them (but this will, of-course, deplete the batteries faster)

This does, of-course, assume that the UPS's are "like-for-like", ie they are otherwise similar models.

davidgo
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Not at all (probably). But, you also do not have larger capacity.

kVA (kilo Volt-Ampere) is a measure of apparent power just like W (Watt). In fact, they are the same (W = VA) but when you read "W" this is a kind of theoretical figure which doesn't have too much in common with a real attached consumer whereas VA accounts for inductive losses and such, so it's much closer to a "real" device.

This has nothing to do with capacity (ampere hours) at all. Capacity is exactly the same.

Now, does it affect your power consumption? In theory, yes. While the battery is the same, the inverter is more powerful, so it operates somewhere in the lower range rather than somewhere in the upper range of its ability. Operating things (inverters, lightbulbs, motors, no matter what) in their upper range usually yields a greatly reduced lifetime, but best efficiency. Operating things (no matter what) in the low-mid range of their ability usually yields best lifetime but inferior efficiency.

In principle, this means that you are burning extra energy because your device is not operating in its best efficiency range.

In practice, as it happens, the load-efficiency curve on every no-shit inverter nowadays is ultra-steep. Anything at or above 20% you're pretty much at 90+%, and it doesn't get much better than that. Indeed, above approx. 80% the curve tends to go down again.

So, in practice, there's no real difference, but your UPS will probably last longer, just be happy.

Damon
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To add to the other answers, just make sure you have the infrastructure to be able to power it.

A 6kVA UPS will need (in North America) either an L6-30R twist-lock (or L14-30, or similar - not sure what's used in India for ~30A circuits? Probably a commando type) :

enter image description here

receptacle or a hardwire mains supply on a circuit with 30A capacity (for 208V-230V input).

In India, a 3kVA unit will probably just use a standard power plug (M-Plug to IEC 320 C20) - a 6kVA unit will not.

J...
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  • Only if the UPS charges so fast that the charging current plus the load current will be above 3kVA. Most likely, that will not be the case. In the interest of safety, though, make sure that a 3kVA capable outlet is fused if using that kind of setup on it! – rackandboneman Feb 26 '18 at 21:01
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    @rackandboneman There's no "if" - you have to supply it with circuit that matches its rated maximum input draw. Anything less is really a hack job and, depending on where you are, probably some degree of illegal (or at least having legal and liability consequences). – J... Feb 26 '18 at 21:10
  • Normally, I'd agree. Can't tell for sure without knowing the exact model of UPS, and if it specifies the maximum charging power used. The unit might actually be configurable for that kind of feed for all we know. – rackandboneman Feb 26 '18 at 21:42
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    @rackandboneman Nope. 6kVA is 6kVA. If all you want is the Ah and not the power handling you buy a different unit. It can't possibly know that it's on a circuit that can't deliver enough current. – J... Feb 27 '18 at 00:37
  • Unless you tell it to by setting a DIP switch, putting a different fuse, or whatever the manual says you can do. WIthout type and manual known, all is conjecture. – rackandboneman Feb 27 '18 at 11:11
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    @rackandboneman It's not conjecture to say that a 6kVA UPS *requires* a branch circuit with wiring and a breaker suitable to carry that load. It **is** conjecture to hypothesize about a mythical "adjustable" UPS that has a magic switch to let you connect it to an inadequate mains supply. Have you ever seen such a thing? I haven't. – J... Feb 27 '18 at 12:37
  • There is no safety issue here. If the UPS really tries to pull more amps than the circuit can supply, there's already a breaker in the panel that will trip and prevent it from doing so. The issue is that your breaker would keep tripping. In reality the 6kVA rating almost surely just means that you can put that much load on it (in which case you'd need an appropriate circuit for it) if you want to, not that it will charge the battery at such an extreme draw. – R.. GitHub STOP HELPING ICE Feb 27 '18 at 21:23
  • @R.. `The issue is that your breaker would keep tripping.` - Unless breaker was defective, and permits loads greater then what it should. Then your wires may get too hot, and start a fire, or fail in other unpleasant ways. It is never good to trust any single component to protect everything. – Zoredache Feb 27 '18 at 22:12
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He not increased the capacity, that is same. He increased the maximum power load.

To fully charge the bank costs same, but you can use higher load (of course, it will drain faster).

uDev
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