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Re: [Shop-talk] Remote electric meter readout

To: shop-talk@autox.team.net
Subject: Re: [Shop-talk] Remote electric meter readout
From: "John T. Blair" <jblair1948@cox.net>
Date: Mon, 22 Oct 2007 08:41:03 -0400
At 05:28 PM 10/21/2007 -0700, Doug Braun wrote:

>I always have electricity bills higher than I expect,
>and I've always wanted something that would tell me
>how much electricity my house was using at any given
>moment.  (Then I could tell if my air compressor or
>other shop tools were a significant contributor).

That's not hard to get a good estimate of how much an item is
costing to run.  But you'll have to do a little math and possibly
know a couple of conversion factors.

For starters:

1. 1KW = 1,000 watts

2. Watts = Voltage (volts) * Current (amps)

3. You need to know the cost of 1kw of electricity.  Here in VA I pay
   9 cents in the summer and 7 cents in the winter for 1kw.

So if I have a 100 watt light bulb I can run it for 10 hrs for about 7 to 
9 cents.  Call it 10 cents or it costs 1 cent per hour of use.

You mentioned an air compressor.  So lets use mine.  It's a 5 hp 220V
So the 1st thing I need to know is the conversion form HP to KW.  You
can look that up many places or search for it on the web.  1 HP is 
about 750 watts, so 5 hp is 3750 watts or 3.75 KW.  Now cost = KW * $,
so 3.75 * 7 cents is about 25 cents / hr to run.  But the compressor
doesn't run for an hour unless I'm doing something like sandblasting or
sanding.  It usually runs for less than 5 minutes then is off for quite
some time, then fires for a few minutes.

You can do the same with any shop tool.  Say you have a table saw with
a 10A motor.  All you need to know is the saw 110 or 220 V?  Remember
Power (watts) = Voltage * Current, so if we assume it's a 110V saw, then
P = 110 * 10 or about 1.1KW, and Cost = KW * $, again at 7cents/kw hr,
I get 1.1 * 7 or about 10 cents per hour of usage.

Now if you are really into geek, then fine buy one of those fancy do dads.

But as someone else said, you get get a cheap clamp on Am meter for under
$30.  You take the current reading from the meter, multiply by 220V for
the service to your house, then by the cost per KW hr and you know how
much your house is costing you to run.

John


John T. Blair  WA4OHZ     email:  jblair1948@cox.net
Va. Beach, Va             
Phone:  (757) 495-8229

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