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From: pw (no email)
Date: Tue Jul 01 2003 - 12:52:07 EDT
Frank Wrote:
Is there any way to find out how many 12 volt amps it takes to make 1.5
amps
120 volts tru the inverter.
I have a small fridge that is rated at 1.5 ac 120 volts, seems that it draws
a
lot from the battery tru the inverter.
Anyone with similar experience?
Frank Marinic
F/V Mar Boa
Paul's reply:
"Volt Amps" are Watts. A given piece of equipment will always use [nearly]
the same number of watts (Volt Amps) whether it's running on 12 Volt DC or
120 Volt AC.
A quick way to keep the formula straight in your head is to draw a triangle
with a "T" in it touching the 3 sides - I'll try to draw it here with text
characters and hope the e-mail transfer doesn't screw it up.
(Place the 'W' above the T crossbar - place the 'V' to the left and the 'A'
to the right of the upright)
/ \
/ W \
/_____\
/ V | A \
/_________\
Remember that triangle and you will always be able to quickly work out any
situation, solving for the value you need:
Watts = Volts X Amps (they are beside each other)
Volts = Watts / Amps (Watts is over Amps)
Amps = Watts / Volts (Watts is over Volts)
You will need to allow for losses in your inverter - (I'd guess an allowance
of 10 - 20%) maybe Arild could give us a ballpark figure there.
So, in your example of 1.5 amps and 120 Volts - the power usage is 120 X 1.5
(Volts X Amps) = 180 Watts.
Now change the Voltage to 12 volts on the battery side of the inverter. Use
the formula again. The watts haven't changed - the new voltage is 12 - so
solve for the new Amps value.
The amps would be 180 / 12 (Watts divided by Volts) = 15 Amps. Add 10-20%
for losses and you have your answer.
Don't forget that when you are working with batteries, time becomes a factor
as well, because now you are talking about Ampere-hours. (1 amp flowing for
1 hour) As currents get higher the relationship to a battery's capacity is
not linear - i.e. The efficiency goes down - maybe Arild could refresh our
memory there also - he spoke about this in a post a week or so ago when
talking about batteries and heavier currents - I think it was "Peukert's
Phenomenon" [nothing to do with seasickness])
Cc: Arild, Frank
--
Regards,
paul whealy
_________
The trouble with the rat race is that
even if you win, you're still a rat.
-- Lily Thomlin
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