


T&T
Ca.
New Member
Joined: 04/03/2004
View Profile

Could someone please explain to me about DC to AC amps. Like if I'm inverting 12 volts DC to 120 volt AC, would the current be 1:1? For example if I had 20 amps going into the inverter, would there be 20 amps on the AC side available? As you can see by my post I'm CONFUSED...Thanks for any help

garryp
AZ
Senior Member
Joined: 09/01/2003
View Profile

It is approximately 10:1. 10 amps at 12 volts converts to 1 amp at 120 volts. (think 12 x 10 = 120 x 1, power in = power out)
But there is the inefficiency of the inverter. Assuming 90% as kind of a worst case, then your 12 volt dc amps go to something like 11.1 amps (better efficiency is attainable with new units).
Travel Blog  Current Location  The Barth  Places Visited  2007 Travels
"Now, listen, you don't go any one special place. That's cornball style. You just go."
 Johnny in the movie The Wild One

Pushin40
Myrtle Beach, SC
Senior Member
Joined: 01/13/2004
View Profile

That ratio is a good rule of thumb. If you want to get more precise, figure out everything in terms of power (watts).
Basic electrical rule 1, 2 and 3:
voltage x current = power
or rearranged:
current = power divided by voltage
or rearranged:
voltage = power divided by current
For example, 12V X 2 amps = 24 watts.
or another example, 400 watts divided by 120 Volts = 3.33 amps
A 55W headlight that uses 12V would draw 55 /12 = 4.6 amps @ 12V
A 55 watt light bulb in a lamp at home would draw 55 / 120 = 0.46 amps @ 120V
As the previous post mentioned, inverters are not perfect when convertering 12V into 120V. If the converter consumes 1000W from the 12V battery, then a 90% effecient converter would generate 900W of 120V AC power best case. The other 100W is lost primarily as heat.
The other thing that gets tricky is that these ratings and the formula above are used for resistive loads, like light bulbs or hair dryers. Anything with a motor or transformer is considered an inductive load and can get much more tricky to calculate.
Consequently you need to give your self a safety margin when figuring out how big an inverter you need.
How does work in a practical sense?
Lets say you want an inverter for TV, DVD and Sat. Receiver. Look at the back of TV or in the manual. It should say how many watts it consumes. Lets say it is 400W. The DVD might be 100W and the Sat. receiver 50W  just as an example.
400 + 100 + 50 = 550 Watts. (just as an example)
You might think, well no problem, I'll use a 600 Watt inverter and have 50 watts left over. Depending on your inverter, that 600W might really be 600 x 90% effecient = 540 Watts of AC, less a 20% margin of error for the inductive transformers in the electronic of the TV, DVD and Sat. receiver 540  20% = 432 Watts.
Now you can see your 600 Watt inverter isn't big enough to do the job.
If we really need 550 watts of AC, add 10% to make up the effiency loss, then add a safety margin for inductive loads.
550 + 10% = 605 + 20% = 726 Watts.
Sounds more like an 800W inverter fits the job.
What does that mean in terms of wiring the 12V batteries to the inverter?
from the formula above:
current = power divided by voltage
In our example, we have an 800W inverter that runs on 12V
The current would thererfore be:
current = power divided by voltage
current = 800 watts divided by 12V
current = 66 amps.
That is important info because you can not use light gauge wire to carry 66 amps worth of 12V to the inverter nor could you use a 20A fuse to protect your inverter.
Now that's a lot of science for a guy who just wants to run a toaster on an inverter right?
800W / 120V = 6.66 amps
Using garryp's ratio 11:1, 6.66 x 11 = 73 amps.
That is a good ratio with a good safety margin.
This is all just MHO and should not taken as solid technical advise. In other words, don't blame me if you blow yourself up.
2004 GeorgieBoy Cruise Master 3755TS W22
Saturn SL2 toad
1 wife, 2 kids, 1 dog, 2 cats

Bubby's RV
CA
Senior Member
Joined: 07/22/2003
View Profile
Good Sam RV Club Member

Couldn't have said it better myself.
John, Winnebago Minnie 24V

carl66vw
Orlando, Florida, USA
Senior Member
Joined: 04/02/2004
View Profile
Good Sam RV Club Member
Offline

Good going Pushing40, Even I understood that one. I have always used this formula..Volts X Amps = Watts. First determine how many watts you need, then add in at least 10% more. Ie. 120V X 15 Amps = 1800 watts. This formula worked for me when I bought my inverter. I am currently using a 4000W inverter to run my refrigerator. No I don't have an RV refrigerator. Works great.
Have worked on cars most of my life but with these new ones I seem to be getting dumber and dumber. The more I learn, the more I need to learn. I was taught oldschool tactics and have many stories to tell. I also have what I think are good ideas.


WyoPapa
Cheyenne, WY
Senior Member
Joined: 11/16/2003
View Profile
Offline

Pushin40, great explanation. And totally accurate.
Dave & Pam, 2 Cocker Spaniels  Shelby & Summer
2005 Beaver 40' Quad slide DP

Ozlander
Rose Hill, Kansas
Senior Member
Joined: 04/23/2003
View Profile
Offline

What Pushin40 said.
Ozlander
06 Yukon XL
2001 TrailLite 7253

mr. ed
Tucson, AZ
Senior Member
Joined: 02/06/2002
View Profile

Color TV's employ what's called a degaussing coil during start up to eliminate any residual magnetism on the CRT (picture tube). This causes a small surge that some inverters can't handle unless they're rated high enough. My 400 watt inverter barely starts my color TV, but after the first surge is over, it handles it fine.

ebulman
Anywhere
Senior Member
Joined: 09/17/2003
View Profile

pushing40 did a good job!
I thought I new enough about inverters until the day I plugged the AC of my 150W AC/DC 12" colour TV into a 300 watt inverter power source. This combo, for whatever reason, instantly toasted the inverter, probably an inrush or (inrush being something bigger the the steady state operating power) something.




