Date: Sat, 25 Oct 2003 15:51:27 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Microwave oven in Westy?
In-Reply-To: <3F99B8DB.1050709@communitybuilders.info>
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At 07:42 PM 10/24/2003, Craig Oda wrote:
>well with only 2 deep cycle batteries.
Rule of thumb, have amp-hour capacity equal to four times your max draw for
conventional batteries, three times for gel/AGM batteries like Optima.
> Also, this looks like it might
>require a step down voltage converter from 120VAC to 12VDC if I was on a
>grid.
Sure would. If you're thinking that way, an AC-powered oven and inverter
would be probably cheaper and definitely more versatile.
> 65Amps? Isn't that a whopping big amperage?
No...or yes, depending on how you look at it. Useful rule of thumb,
whatever amperage a device needs at 120 volts, multiply by ten and you'll
be in the ballpark for running through an inverter -- probably a touch on
the low side because of inverter efficiency. If it's a non-sine-wave
inverter there may be additional drain or lowered performance because the
peak voltage is not as high on a stepped-wave inverter. This reduces the
available torque of power tools, for example.
The fact that power delivered is the product of volts and amps (with some
fudging collectively known as Power Factor if we're talking about AC*) but
the size of the necessary wires depends strictly on amps is the reason why
we now have 12-volt cars instead of six, and larger vehicles use 24 and 48
volts (and we use 110/220 at home, a few thousand to cross the city, and in
the range of a million volts to cross really long distances). Double the
voltage, halve your cost for copper.
*If you look at the rating for an inverter or UPS, you'll see it has two,
one for wattage and one for VA or volt-amperes. Appliances will have one
or the orther or both. Most practical devices that work on AC draw
noticeably more current than you would expect from the wattage rating,
never mind why for now. Motors are notorious in this regard, which is why
Sears will boast about "thirteen-amp motor" on a tool without saying much
about what the actual wattage is. Motors are also notorious for huge
startup currents, which is why you have time-delay fuses and breakers at
home, and why you care about the peak (five-second) rating of an
inverter. This whole issue is a huge problem for the power company,
because they're selling you power, but they have to sustain the extra
losses in their lines when you suck up extra current. For industrial
outfits they tack on a surcharge to cover this.
> I think I'm running
>15 to 30 amp fuses and I didn't think I would max those out in general use.
15 amps will get you about 150 watts...you're looking at approaching 1,000.
750 watts (ok, 742) is equal to one horsepower. Generating it on the fly
takes about three horsepower-worth of fuel because of engine
inefficiencies. Alternator and battery and inverter inefficiencies add
maybe another 25 per cent. Tanstaafl...**
david
*"There ain't no such thing as a free lunch." Heinlein, _The Moon is a
Harsh Mistress_
--
David Beierl -- dbeierl@attglobal.net
