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Old June 28th 05, 03:55 PM
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On Tue, 28 Jun 2005 13:59:42 +1000, George Ghio

wmbjk wrote:
On Tue, 28 Jun 2005 09:36:36 +1000, George Ghio
wmbjk wrote:

Here's a sample day's work (yesterday's) in an off-grid workshop. The
project was building the last 2 of 5 scissor trusses for a friend's
aircraft hangar. All material was scrap - chords of 2" and 1.5"
schedule 40 pipe. Long braces from T posts, shorter ones from 5/8"
sucker rod. Most of the material was cut a few days ago. I wore out 3
chop saw blades (medium quality ones) which should give you an idea of
run time. The trusses are 40' wide, but built in halves. Each half
took about 30 minutes to lay out and prep, including about 15 minutes
use of a 4" side grinder. Then 26 welds per side, flip, another 26
welds. Layout, prep, and welding - about one hour total per half, with
a long break between each one to cool off. I built 4 halves, about 100
minutes welding for the day. I didn't check the current draw, but the
machine was set at 280 inches per second, 18 Volts, with .035" solid
wire. Perhaps 5kW input. Charging rate was medium, there was good sun,
but almost no wind, which was nice because I could leave the shop
doors open. When I quit for the day, my wife commented that battery
state of charge had dropped 6 Amp hours, and that she'd used the AC in
the office for a couple of hours. On this project, like most others
here, there wasn't any penalty for being off-grid, which is way cool


the Blunder From Down Under wrote

Ah Wayne, it looks like your saying 5kW for 1.6 hours and claiming that
it equates to 6 amp hours.

Would you like to refrase that amd perhaps look at the maths.

You use of units may be suspect. And your numbers would suggest that you
can only work when the sun is out.

5Kw input for 1.6 hours at 24V looks suspiciously like 333Ah.

I started at about 8AM, and I finished up at about 4PM. During that
time, 2000 Watts of tracked PV was doing its job, along with a tiny
bit of help from 1300 Watts of wind generator in the AM. Duh!

5kW input wonderful. Yes I made a mistake, I used 100 minutes when in
fact you meant 8 hours. Sorry. So this would be 5kW for 8 hours which is
of course 40kWh which is 1666.66 Amp hours. Is this your wonderful two
days autonomy at work.

No you Baron of Blunders, the largest part of the shop consumption was
(about) 5kW for 100 minutes. You had the demand part nearly right the
first time. You simply neglected to account for the balance of the
demands (all the house loads, about 4 kWhrs), and the *entire* supply
side (about 12 kWhrs). Why you're now trying to multiply the short
term shop power demand by 8 hours is between you and your therapist.

Face it man, you're busted. If there's a responsible authority
controlling solar installers in your area, reading such a fundamental
lapse in critical thinking by someone in their charge, then the writer
would at the very least be called onto the carpet for remedial
training. Not that such would do much good if he hadn't gotten the
basics down after 20 years.

The biggest blunder - YOURS.

Sure George, a system that can supply a day's use of workshop and all
the house loads as well, and come out with a 150 Whr deficit is
obviously just one giant blunder.

Get help man.