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UPS Uptime on Smaller Devices

J

Jerec

Weaksauce
Joined
Oct 22, 2009
Messages
109
I'm looking into a UPS purchase to power a few smaller devices. They'd be powering a cable modem, a router, and a VoIP ATA.

This 550VA UPS seems to be the top selling and it looks like it can keep a PC online for a few minutes (3.2 minutes at 330W, it says). I just wanted to get an estimate on how long it could run the three little devices I mentioned above.


Thanks!
 
Well, I'd guess those devices probably consume ~10W, assuming you're talking about standard consumer kit. Given 3.2 minutes at 330W, a linear estimate gets you 105 minutes. I'd derate that a bit since the UPS will probably not be very efficient at such a low load. I'd say you should expect at least an hour.

APC provides runtime vs. load graphs for their UPSs that might help you out.
 
we have a 250VA that powers a Linksys Router and cable modem last for about 30 min
 
Don't start from the 100% load figure, use the 50% or 25% (if you can find it) load rating.

Chemical characteristics of the batteries used make the runtime non-linear based on load. However, with a load as light as yours, inefficiency of the UPS itself will probably be the largest factor. (eg. a UPS with no loads plugged in won't last forever).

Don't oversize the inverter section too much and you won't waste money. The lower the inverter output capacity, the less waste, if ones wishes to use generalizations. The trick is that most UPS manufacturers directly scale battery capacity with inverter output capacity. You don't usually find a UPS on the market with huge battery and a small, efficient inverter intended just to keep a cable modem/router online for 24+ hours, for example.

See if you can find a 50% load rating for a ~550VA and ~300VA unit, then extrapolate to your real load from that to determine which to purchase.
 
I had a very small APC unit back the States (APC Back-UPS ES 350 UPS - 200 Watt) that kept my cable modem and wireless router up for at least 20 minutes, but it was worthless for everything else.
 
i guess it would be nice to have a backup battery that could run your router and modem for 2 hours or so (thats how long your laptop will last), but youre going to need a 1100va at least or better ups.... $$$!
 
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