On Fri, 27 May 2011 23:17:30 -0400, Home Guy wrote:

"hr(bob) " used improper usenet message composition style
by full-quoting:

You may be lucky and only have an error of 10 - 20%, but
I wouldn't be surprised if your error was as much as 25%.

When you speak of this error, do you mean

a) an error between (what the billing meter is reading in terms of KWh
for some arbitrarily short time span) AND (what my amp-meter derived
watt-measurement would give for the same arbitrarily short time span)

An amp meter measures amps, not watts. Even if you assume 120V, you're still
calculating volt-amps, not watts.

or

b) an error between (the monthly power use of the office derived from
several different measurements with various devices and appliances
turned on or off as per time-of day and day-of-week) AND (the actual or
real pattern of device usage over the course of a real month).

What is the error you speak of?

c) The error between Volt-Ampere-hours and Watt-hours.

The errors you mention above will likely be even bigger, though. Surprisingly,
a watt-hour meter (it's already there) is the real way to measure watt-hours.
;-)