# Help figuring out electric usage

Discussion in 'General Electronics Chat' started by eels1212, Jul 25, 2008.

1. ### eels1212 Thread Starter New Member

Feb 21, 2008
7
0
Hi
Can some body help me figure usage of my ups back up system? The system out put is about 90amps on each phase of 277v ( 3phases) that is a continuos load. I need to convert that to watt hours so I can give our IT dept the cost of their electric.
Thanks,
Ed

2. ### theamber AAC Fanatic!

Jun 13, 2008
321
0
If you want to know how much your device is consuming you will need to know first, In houses watt-hour meters will show actual power used.
In industrial settings Electrical companies have VAR meters in order to detect PF and if it falls too low below 80-85% they will get penalized.

The average real or actual power, P. Real power is the energy that flows to the load. It is what the electric company bills home users for.

For a three phase you can use this :
ExIxPFx1.73/1000 will give you KW.(you can omit the PF to get an aproximation)
http://ltc.cit.cornell.edu/courses/ee476/FinalProjects/s2008/cj72_xg37/cj72_xg37/index.html

3. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
The system provides 24.93 kW, based on the numbers given. That would be 598.32 kWh per day. Seems like a lot, yes? I suspect the values given are maximum the UPS can provide, rather than what is actually being used.

Also, the UPS is not 100% efficient. There is some energy lost as heat. Actual cost will be more than what is computed based on output. One needs the values for input.

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Is it a "true UPS" or a "standby UPS"?

A true UPS is always running from the batteries, which are constantly being recharged.

A standby UPS is running from line current, but switches over to batteries in the event of a fault.

True UPS's are the best protection your equipment can get. They are "always on", so there is no such thing as a "wait time" or other nonsense between fault detection and the UPS being on line. It's always online, and your equipment is never at risk, which saves a bundle on maintenance, and jacks reliability way up.

The downside is the efficiency overhead. You'll have to consult your UPS manufacturer for those figures.

Backup UPS's (the kind that are only engaged when there is a power fault) are not as reliable, however they are more cost efficient from the standpoint of efficiency. If there is a mains power glitch, you may not know of a problem with your UPS until it's too late - unless you regularly run the diagnostics.

When it all boils down to it, what you want to report is the efficency rating of your UPS. If the efficiency is 85%, then everything that's plugged into it will have a 15% "efficiency tax" on top of it's power rating.