UPS and runtime

I have an APC BackUPs ES700 and am intrigued by the claimed run time. Supposedly the 700 refers to a claimed 700VA capacity and it has a 9 Ah battery in it.

I tested it this evening by plugging a TP-Link network switch into it. The power adaptor claims 0.9A. According to the PowerChute software with mains removed the unit lost 20% of battery over 4 minutes. So that seems to be 20 minutes in total. The software only said shutdown in 7 minutes but that's with a 5 minute buffer.

With my low power server (~20 W under heavy load and it's only a private mail server), router and HG612 plugged in the software seems to be claiming 10 minutes of run time.

Now fair enough I only really have the UPS to guard against short glitches and don't expect it to run the server for an hour but this performance seems to be way less than most articles suggest. Reviewers of this unit suggest it can power a desktop PC and a monitor for 15 minutes.

So I'm curious if anyone knows enough about electrics to explain these figures.

The battery in theory, disregarding internal losses can hold 9Ax12V = circa108VA

There will be other losses due to internal circuitry and a cut out before the battery is completely discharged.

I do not know how 700VA is reached.

In reply to a post by Brunel:

I do not know how 700VA is reached.

There are 8 surge protected outlets but only 4 of them are battery backed up,.

I presume 700VA is the max for all 8 outlets.

In reply to a post by Brunel:

The battery in theory, disregarding internal losses can hold 9Ax12V = circa108VA

Afaik, APC use 24v batteries (mine did), so that would take it to ~220VA. Also the 700VA refers to the maximum permitted connected load, say around 500W at typical power factors.

So, with everything at 100% efficiency, it could run that 500W (700VA) for about 18 minutes.

But at that high a discharge rate the battery is not going to have 9AH capacity, so (at a guess) more like <10 minutes.

Add in the current it takes just to run the inverter, take the efficiency down to a more likely value ~75% and running a PC and monitor for ~15 minutes sounds about right.


eta- zaggie tells me that it does use a 12v battery, so that 15-minute time for a PC & monitor sounds pretty good

Edited by billford (Mon 30-Dec-19 23:07:05)

In reply to a post by zaggie:

I presume 700VA is the max for all 8 outlets.

On mine (also APC but different model) it was for the backed-up outlets only.

The battery in the ES700 is 12v 9Ah

In reply to a post by zaggie:

The battery in the ES700 is 12v 9Ah

Ah, OK, thanks. Mine was an RS800 and used 2x12v in series.

(I don't have it now- it packed in and circumstances had changed so I no longer needed a UPS)

In reply to a post by Andrue:

Reviewers of this unit suggest it can power a desktop PC and a monitor for 15 minutes.

It's a subjective statement.

Some PCs use as little as 150W, and many modern monitors use 10-30w, depending on size and model. So say 175-200W as a ballpark figure for a low spec machine, so the statement isn't a lie.

My own system has a draw of around 500W, depending on load and efficiency, and it's pretty much a high-end system (or it was last year). Having said that, it's used for gaming for the most part, and in the event of a power failure, the router pops too, and it's a disconnect from the game server. I'd sooner have the router on the UPS, than the PC, and maintain a stable connection if more likely to have short-term power drops.

That's the one good thing about laptops and docking stations for business situations. You get all the benefits of a proper monitor, mouse and KB when at the desk, and should the power go, just pop the laptop off the station and carry on for as long as your laptop is up for it. Probably cheaper than a PC and UPS solution too.

Thanks for the replies, guys. I tested the wattage draw using a test plug and it claims 29w. Now it's not an electrician's tester so accuracy is going to be debatable but it makes sense at least.

Green PC, router and modem are probably all around 10w.

I've done a bit more research and come up with a couple more possible causes:

* Low power draw - a couple of articles implied that inverter efficiency is proportional to load so perhaps at 30 w I'm losing power there.

* Power factor - I don't know what the PF of any of the adaptors are but reading online they could be as low as .7. I'd like to think that the green PC had a better PF but it may only be green in the sense of keeping my bills down and since PF is not currently used in domestic charging it may have a standard adaptor. Interestingly a 'proper' PC probably has a better PF because the EU apparently mandates that for equipment that draws more than 75 w.

Anyway it's got me curious and I'm learning things which is always good