From: jeffh@nonsuch.EBay.Sun.COM (Jeff Huntington)
Newsgroups: rec.boats
Subject: Re: inverter and battery use
Date: 4 Jan 93 19:21:59 GMT
Reply-To: jeffh@nonsuch.EBay.Sun.COM
Organization: Sun Microsystems, Inc.
NNTP-Posting-Host: nonsuch.ebay.sun.com

In article 725765468@genghis.borland.com, mshulman@genghis.borland.com (Michael Shulman) writes:
> Hello, all:
> 
> My brother just took delivery of a Catalina 36 with 3 deep cycle 900 A/H 
> batteries and an inverter. Since he's interested in "Creature Comforts", 
> he's outfitted the boat with
>    color tv
>    vcr
>    microwave oven
>    AC track lights
> etc.
> 
> I wanted to help him be able to estimate the amount of life he can expect 
> from his batteries. I told him that since P = I * E
> 	where P is power in watts
> 	and I is current in amps
>         and E is voltage,
> 
> that his running his 600W microwave for 1 hr at 110V (which takes approx 6A/
> H, right), that if we estimate the efficiency of his inverter at 50% (pretty 
> conservative), that he should be able to run his microwave for 37 hours, 
> even using only half the charge of his batteries:
> 
>          microwave takes: 6A/H
>          inverter 50% efficiency: 12A/H
>          battery (50% of total): 450A/H
>          so, you can get 37 hours.
> 
> But then I thought, that can't be right. Intuitively, it seems like it 
> should be closer to 3 or 4 hours.
> 
> What am I doing wrong in my calculations?
> 
> And what items are going to be really expensive to run off the inverter?
> 


I am not absolutely sure about this, but I think I am right (and my experience 
seems to bear it out).  You are mixing apples and oranges (or rather AC and DC)

I did some similar calculations over the holidays on my system, that went something
like this.  My 600 watt microwave actually consumes about 825 watts (allowing for
losses, power for the turntable, and light.  Throwing in a ball-park figure for
the inefficiency of my inverter,  I figure that when I am running the microwave, I
am drawing about 1000 watts from the battery.  Since everything was so ball-park 
thus far, I figured that if my battery was 10 volts (it's over 12, but 10 is a
nice number for calculating) my microwave would be drawing 100 amps from the battery.
I don't think it is that bad, but it is not that far off either.

In your calculations, you used the calculated AC amps as the current supplied
by your battery.  You must get into the DC world before your calculate your
battery draw.  I think the watts can be the common denominator (for these rough
calculation purposes).

I think of the items listed, the microwave will be the biggest killer for the
batteries.  In addition to the high wattage needed for the microwave, they like
a nice sine wave for their input.  Inverters don't produce a nice sine wave, so
the microwave is not as efficient.  Some inverters produce better sine waves than
others, so how inefficient the microwave will be will depend on how many square-
wave steps the inverters uses to produce the AC output.

I can tell you that I used our microwave for the first time off the battery a couple
of days age.  Using it for just under a half hour really drained the battery (below
12 volts).  Of course, as the voltage drops, the power output drops.  So the 
microwave must be run longer to cook the food.  None of this was really a surprise
to me.  I just let my wife go ahead and do what she wanted to do, and I observed 
what happened with the 'trons.

One last thing, that should be obvious.  If you have an AC water heater, be sure 
the circuit breaker is OFF before you disconnect shore power.  Powering a water
heater from your battery (through your inverter) will really drain a battery. :-)

Hope this helps some.

Jeff Huntington