Uninterruptible power supply output specifications

[david90]

Why do some UPSs have both Volt-amp and watt rating? According to APC, the connected load shouldn't exceed the UPS's VA and watt rating. Why? If I had a UPS with 1000VA and 500W rating, why can't it power a load consuming 1000VA @ 900W? If a UPS can output 1000VA, why does it care how much of that power is actual used by the load?

 

[Baluncore]

There is a difference between watts, VA and VAR.

Real power in watts is the voltage multiplied by the in-phase component of the current.

VAR is the product of the voltage and the quadrature component of the current, VAR is not real power, it is circulating energy.

VA is the product of voltage and current, ignoring phase.

 

[anorlunda]

Sometimes you see a rating such as 1000 W at 0.8 PF. That's the same as W and VA. The implication is that if the power factor is less than 0.8, you need to back off on the power; but it doesn't tell you how much.

1000VA and 500W implies to me that you could drive a reactive VAR load substantially higher than the real power load. Why would you want to? You probably don't want to.

 

[DaveE]

So, here's a pretty rough example. If you look at the inverter in your UPS, the DC current from the battery is determined by the real power that the downstream circuit losses & load require. There is no "reactive DC power" so the watt limit on the load, plus the designers knowledge of the converter circuit will determine the input currents on the primary side of the converter. OTOH, on the output, or secondary side, the current isn't just the real power content, it's limited in the specification by the VA rating, with the voltage (hopefully) regulated as a constant value.

Of course for the guy that designed the inverter it's a bit more complex. It mostly relates to the stress on the switching components. The point is they are talking about two different things. Theoretically, you could connect a capacitor to the output and have lots of VA, but no power.

 

[david90]

Assume the output rating is 1000VA / 500W. From the perspective of the UPS, what would happen if I connect a 1000W purely resistive load to the output? Which parts of the UPS would overheat? What I'm confused about is If the UPS can output 1000VA then why does it care that resistor is dissipating 1000W? According to APC, the load VA AND Watt should be less than the UPS rating.

Basically my question is why do UPS have a watt rating when the VA rating already describes how much the UPS can output?

 

[Baluncore]

Basically my question is why do UPS have a watt rating when the VA rating already describes how much the UPS can output?

The 500 watt rating is for real power.
The 1000 VA rating is for real power, plus the circulating energy due to load reactance.
https://en.wikipedia.org/wiki/Volt-ampere
https://en.wikipedia.org/wiki/Volt-ampere#Reactive

 

[Rive]

Basically my question is why do UPS have a watt rating when the VA rating already describes how much the UPS can output?

It's the opposite. You can take watt rating as the 'how much the UPS can output', and VA as 'with how bad loads can the UPS do that watt output'.

 

[david90]

With regards to the internals of the UPS, what are differences between a UPS with 1000VA / 500W rating and a UPS with 1000VA / 1000W rating? Restating the question in another form, why does the 1000VA / 500W UPS require 500VAR to go back to it while the 1000VA / 1000W UPS does not? I'm still trying to understand.

I understand the difference between VA, VAR and Watt. However I don't understand them in the context of UPS / power source.

 

[DaveE]

With regards to the internals of the UPS, what are differences between a UPS with 1000VA / 500W rating and a UPS with 1000VA / 1000W rating? I'm still trying to understand.

I understand the difference between VA, VAR and Watt. However I don't understand them in the context of UPS / power source.

One needs a battery and input circuitry to support a 1000W load, the other only needs half that amount. The output circuit requirements are similar since the VA spec is the same. One will supply more power to good loads (PF > 0.5). For worse loads the VAR limit is the same. Below 500W they are equivalent.

You're not going to get a great answer here about the "internals" because there are multiple topologies for inverter designs and all are fairly complex w.r.t. handling reactive loads. I think you'll have to study inverter design on your own to really know the details, then you still might not understand the limits on someone else's design.

Finally, a crude sketch I drew re. the load spec. You probably understand this, but if not, you'll need to.

 

[david90]

One needs a battery and input circuitry to support a 1000W load, the other only needs half that amount. The output circuit requirements are similar since the VA spec is the same. One will supply more power to good loads (PF > 0.5). For worse loads the VAR limit is the same. Below 500W they are equivalent.

You're not going to get a great answer here about the "internals" because there are multiple topologies for inverter designs and all are fairly complex w.r.t. handling reactive loads. I think you'll have to study inverter design on your own to really know the details, then you still might not understand the limits on someone else's design.

Finally, a crude sketch I drew re. the load spec. You probably understand this, but if not, you'll need to.

View attachment 314287

I think I understand now. The UPS can't supply 1000W but it can supply 1000VA because 866VAR quickly returns to the UPS. I can see how the size of the battery creates this limitation. Thanks

 

[rbelli1]

Historically the loads that were most likely to need a UPS had terrible power factor. This results in many of them having a large split between VA and W.

Modern electronics now need to follow stricter regulations around power factor. Many computers are certified to 80 Plus.

Better power factor should prompt UPS makers to narrow the split. This will be easier on our wallets and our backs.

BoB