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DC power inverter newbie questions

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Posted by ballypinballs on September 2, 2005, 12:13 pm

Thinking about buying a power inverter for back up power house/camper.
I guess my main question is input amps.Using deep cycle battery/s like
for my trolling motor.How many can I/should I use to run a 2000/4000w
inverter. I guess by putting a couple in parallel or more would hold
12v and multiply amps.But how high can you supply for input and not
damage the inverter?
thanks for any help

Posted by Anthony Matonak on September 2, 2005, 12:29 pm

ballypinballs@yahoo.com wrote:

The inverter will draw power from the batteries. It's not a question
of how many amps can you pump into it but rather how many amps the
inverter will require from the batteries.

Watts = Volts x Amps

This means that if you have a 4000 watt inverter (and you were to
max it out) that it would need 333 Amps at 12V. Actually, it would
require more because of loss inside the inverter. Generally folks
don't max out their inverters so you'll be using less power and
therefor it would require fewer amps.

The way most folks design backup power systems to is figure the
maximum wattage that would ever be needed at any one moment. This
would be the size of the inverter required. Then they figure out
how many watt-hours would be required over the period of time
they would need to use this before recharging the batteries. This
would determine the size of the batteries, generally by sizing
the batteries with at least 20% more.


Posted by ballypinballs on September 2, 2005, 1:05 pm

Excellent info, whats the math for watt hours....15 amp draw for 1
hour..I guess the watts would be 1800 but what would it take for
battery/s to maintain that for one hour?....In my mind what I want to
accomplish is run my oil furnance and frig. if we loss power(winter)  I
need the formula to do the math :)  Most items cycle on and off so the
power need would not be needed 100% of the time. But like a  A/C unit
thanks keith

Posted by Anthony Matonak on September 2, 2005, 2:43 pm

ballypinballs@yahoo.com wrote:

Watt-hours is exactly what it sounds like, Watts x Hours.
If you have something that takes 100 Watts and you run it
for 1 hour then it consumes 100 Watt-hours. Run it for
1.5 hours and it consumes 150 Watt-hours.

There are also Amp-hours. Amps x Hours. If you know your
batteries need to supply 100 amps for 2 hour then your
batteries need 200 Amp-hours.

You would have to measure, in some fashion, how much your
furnace and fridge consume. They actually sell devices to
do just this but you might be able to rent or borrow one.
You can also use your power companies meter to measure as


Posted by Christian M. Mericle on September 2, 2005, 4:02 pm

On 2 Sep 2005 05:13:10 -0700, ballypinballs@yahoo.com wrote:

I think the main thing I learned is, for the longevity of the
battery's power supply, use as much stuff made for 12V DC as possible
and only use the inverter when you have to.

-- Christian

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