Re: Power Consumption

Charles Scott (cscott@freeway.net)
Wed, 21 May 1997 03:04:53 -0400 (EDT)

Rick:
I'm sure it was a simple error because you obviously had the math
correct, but it's VOLTAGE * AMPERAGE = WATTS (E*I=P).
Also, I believe that VA is actually just the voltage times the current
being delivered to a load. That sounds the same as the above equation
but that equation assumes a power factor of 1. In reality, most loads are
reactive and so the current and voltage are not in phase. This means
that VA will generally be higher than the real power consumed. This is
also why you can reduce your electic bill if you correct the power factor.
In Stavros's question number 2 he asks if power dissapation refers to heat.
The word "dissapation" does imply heat and in most cases is very close to the
actual power consumed in this type of equipment. The reason is that the
equipment does not deliver any significant final product other than heat
(unlike an electric motor or power amplifier). Remember however that the
amount of actual energy consumed will be equal to the VA times the Power
Factor which is less by some margin than VA.
One final comment regarding the use of an "on-line" UPS. Since they
always produce the AC output from the batteries while the batteries are
being charged by the incoming AC line, there is no switching time between
loss of the AC line and delivery of UPS power and therfore no risk that
your equipment could be affected by a momentary loss of power.

Chuck Scott

On Tue, 20 May 1997 falken@htonline.com wrote:

> Doesn't anyone remember Ohms Law???
> VOLTAGE / AMPERAGE = WATTS???
>
> 120VAC / 1 AMP = 120 WATTS
> 120VAC / 2 AMP = 240 WATTS
>
> get the picture....now after you figure out all your wattage, then
> you select the correct UPS. I normally recommend using a UPS that is
> double the size of your maximum draw. At the very least 1.5x your
> max draw.
>
> One note here, American Power amoung others have started to label
> their UPS's in Volt Amperes as well as Watts. Make sure you are
> looking at the Watt rating. 1400 VA does not equal 1400 watts.
> Actually VA = about 2/3 of Watts. (Anyone care to give the
> formula?).
>
> Also, don't bother with the backup ups's I only buy Online UPS. That
> way there's no drop in voltage (however so slight), when the UPS
> kicks). Also, you don't have to worry about sags with an Online UPS.
> Backup UPS's only kick in when power goes below 10+/- of 120VAC, (or
> thereabouts). So, with some backup ups's you can actually be running
> on 105 volts!!! Online supplies continous power at 120VAC 60hz
> regardless of input power range.
>
> This is especially important now that summer is upon us and we will
> be getting those sags and spikes when the power companies switch
> grids to meet demand of afternoon cooling, etc.
>
>
> Regards
> Rick Schryvers
>
> > Date: Tue, 20 May 1997 19:08:42 -0500
> > From: Michael Hart <michaelh@bluebonnet.net>
> > To: Stavros Patiniotis <stavros@esc.net.au>
> > Cc: "portmaster-users@livingston.com" <portmaster-users@livingston.com>
> > Subject: Re: Power Consumption
> > Reply-to: Michael Hart <michaelh@bluebonnet.net>
>
> > I have have a Cisco Router, Adtran TSU, 2 US Robotics MP/16 modem Racks,
> > PM2e30, 2 computers and 1 monitor on an APC Smart UPS 1400 and it will
> > run for between 30 and 45 minutes.
> >
> > Stavros Patiniotis wrote:
> > >
> > > Slightly off topic (but its to do with my pm's):
> > >
> > > (1) Can someone remind me how to convert from amps and volts to Watts.
> > >
> > > (2) Also Power Dissipation is reffering to heat not electricity used,
> > > correct?
> > >
> > > (3) What is a VA (Volt-Amp)?
> > >
> > > (4) How many watts does a average computer use (bloody computer manuals
> > > don't have anything in them!) ?
> > >
> > > I ask this as I'm wanting to buy some UPS's for my pm's and a couple of
> > > computers, and in the PM's manual there is only Voltage and Current. (oh
> > > and power dissipation)
> > >
> > > TIA
> > >
> > > Regards,
> > >
> > > Stavros Patiniotis
> > > ------------------------------------------------------------------------------
> > > -System Administrator / Network Manager Escape.Net -
> > > - 465b South Rd -
> > > -email: stavros@esc.net.au Keswick SA 5035 -
> > > -URL: http://www.esc.net.au Ph 82932526 Fax 82932949-
> > > ------------------------------------------------------------------------------
> >
> >
>