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Old 11-29-2015, 08:50 AM   #1
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Inverter question

I'm power/electricity dumb I'm looking to buy an inverter that I would plug into the cigarette lighter adapter, that allows me to use certain electrical components while driving without using the generator.

I see tons of inverters and they are rated by watts. I'm trying to determine the right size I need to power the different items I have.

Now is when I get dumb. What's the right "math" that I need to analyze to understand the size inverter I need? Do I simply need to understand the amps a component continually uses? And if so, again...what's the right math that converts the amps used for 120v into a 12v/X watts inverter?

Does my question even make sense? Lol
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Old 11-29-2015, 09:12 AM   #2
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Your question makes sense but you don't need to go that far. Inverters are rated in watts and that's all you need to figure out what it will run. You always need to allow a little 'headroom' when you figure too. Example: you want to run a laptop off the inverter. The laptop power supply says 75 watts. That would work in a 100 watt inverter.

Here is the tricky part. You want to add a cell phone to charge also. Your cell phone charger you plug in the wall says it has 2 amp output. Phones charge at 5 volts. So 2 amps times 5 volts is 10 watts. 75 watts plus 10 watts is 85 watts. So you have enough inverter power to run both.


The last thing to figure out is how many amps the inverter will draw from your cigarette lighter plug. A 100 watt inverter running off of 12 volts is 8.3 amps. (watts divided by volts equals amps. Also: amps times volts equals watts) The trcky part for inverters is that they 'waste' a little bit of power. Inverters are about 90% efficient. Read here for all of the details. http://www.marxrv.com/12volt/12volta.htm
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Old 11-29-2015, 09:35 AM   #3
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Most likely your 12V aux plug is rated at 15 amps. Max inverter would be 15A x 12V = 180W
I wouldn't go that high though. Probably a 150W would carry all what you need. The other feature you're looking is output plugs to meet your intended needs.
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Old 11-29-2015, 10:46 AM   #4
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Quote:
Originally Posted by Still Kickin View Post
Most likely your 12V aux plug is rated at 15 amps. Max inverter would be 15A x 12V = 180W
I wouldn't go that high though. Probably a 150W would carry all what you need. The other feature you're looking is output plugs to meet your intended needs.
Are you saying any inverter above 180 watts running off 12v is a waste because that extra power could never be used anyway?
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Old 11-29-2015, 10:54 AM   #5
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Quote:
Originally Posted by Evereddie View Post
Your question makes sense but you don't need to go that far. Inverters are rated in watts and that's all you need to figure out what it will run. You always need to allow a little 'headroom' when you figure too. Example: you want to run a laptop off the inverter. The laptop power supply says 75 watts. That would work in a 100 watt inverter.

Here is the tricky part. You want to add a cell phone to charge also. Your cell phone charger you plug in the wall says it has 2 amp output. Phones charge at 5 volts. So 2 amps times 5 volts is 10 watts. 75 watts plus 10 watts is 85 watts. So you have enough inverter power to run both.


The last thing to figure out is how many amps the inverter will draw from your cigarette lighter plug. A 100 watt inverter running off of 12 volts is 8.3 amps. (watts divided by volts equals amps. Also: amps times volts equals watts) The trcky part for inverters is that they 'waste' a little bit of power. Inverters are about 90% efficient. Read here for all of the details. http://www.marxrv.com/12volt/12volta.htm
I looked at some of the components and they have an input and output rating. Your cell phone example is the output. So I don't need to care about the input part? Just output?
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Old 11-30-2015, 09:27 AM   #6
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Larger inverters will only draw as much as the load attached to them. Do not get one that is just large enough because most are listed by max (short-term) wattage. A 100W inverter most likely will supply 100W for a short amount of time. You have to look at the full specs to get what you want.
It's best to find a pure sine wave inverter since the cheaper inverters are hard on sensitive electronics. Cheaper inverters have a stepped output similar to switching a lamp on and off. A sine wave is similar to sliding a dimmer on and off, much smoother and closer to what the power company provides.
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Old 11-30-2015, 09:39 AM   #7
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Just a word of warning. Most cigar/12volt outlets are not fed by that large of a wire and fused by 5 to 15 amp fuses. You may not draw enough power to pop the fuse, but enough to over heat the wire. Don't ask me how I know. Turned out ok!
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Old 11-30-2015, 09:49 AM   #8
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Beware here as cigarette lighter terminals are notoriously bad for mechanical connection. Of course the lighter only had to work for 15 seconds, but your inverter will draw power for hours. I have blown the fuse with a very small plug in inverter that was charging a Macbook Pro laptop as Apple is prone to drawing a lot of current, especially when the laptops are very low on battery power. You can use something like this:



But even this may blow your fuse and anything bigger should be hard wired to 12V.
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Old 11-30-2015, 09:51 AM   #9
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X2!
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Old 11-30-2015, 10:52 AM   #10
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Here's another possible solution for you. I purchased one of those portable jump starting units and travel with it. It has a 110V outlet on it as well as a 12V outlet and you can use it without worrying about killing a battery, etc. I originally bought it to run my cpap if the power was out during and after storms here in Florida (good old hurricanes!). Works great.
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