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Old 09-07-2012, 18:06   #1
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8 Amps from a 400w Inverter ?

I have a small 400w inverter running a TV, airport base station, Western Digital media center and 2 hard drives.

now, I have not done any *actual* research or anything, but I was guessing the TV was about 100 watts, (32" LED) and then maybe 25 for the other 4, 200 watts total.

But The inverter does not like it when I add a laptop to the mix and shuts down. My Macbook air should be what, like 65?

I'm just starting to try and figure out what I need to live on a daily basis, so I just got a new Blue Seas Clamp DC Ammeter. and here's where it gets weird.

just the media center and hard drives, the inverter is pulling 8 amps DC. Which I *thought* was like 800 watts ac, not 400.

is that even possible for the inverter to be running 50% efficient without being insanely hot?
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Old 09-07-2012, 18:10   #2
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Re: 8 amps from a 400w inverter?

Simple version.

Watts = Volts X Amps

So 8 A at 12 V DC = 96 Watts
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Old 09-07-2012, 18:25   #3
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Re: 8 amps from a 400w inverter?

OK, thanks. It's difficult being as stupid as I am. I was using an online converter to try and figure it.

But I'm still confused; @ 400 watts the thing would be pulling 32a DC out of a 15 amp cigarette lighter plug!?
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Old 09-07-2012, 18:41   #4
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Re: 8 amps from a 400w inverter?

Yup, and not a good thing, as you surmised. You're not so dumb after all, are you? ;-)
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Old 09-07-2012, 18:45   #5
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Re: 8 amps from a 400w inverter?

You have to separate the AC and DC draws. If it is a 400W inverter and you are pulling max power (and for simplicity ignore inefficiencies and AC power factor)

400W AC @ 125V = 3.2 A AC output

to make that 400 W AC you need 400 W DC which would be 33 A DC. Could suck you batteries down pretty quickly.

PS
Just noticed the comment about the lighter plug. No not a good idea.
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Old 09-07-2012, 18:52   #6
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Re: 8 amps from a 400w inverter?

Eight amps DC at a nominal 12 volts is in fact 96 watts. The inverter is rated in Watts AC which is roughly 3.3 amps at 120V. A laptop computer with a discharged battery can probably pull an amp and a half. The TV is probably about an amp to an amp and a half. Then add to that the media center pulling just a bit less than an amp and your over the maximum rating of your device. All Electric appliances must be labeled with their maximum current draw. If your units have "Bricks" or wall adapters the ratings will be listed on those. Typically these are the maximum ratings and the normal ratings are a bit lower. In any case add up the ratings of everything you have plugged in and you probably find that you are definitely in trouble at the maximum current draw. Also you'll probably find that most of these inverters can only support their maximum rating for a limited amount of time.
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Old 09-07-2012, 19:01   #7
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Re: 8 amps from a 400w inverter?

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Originally Posted by Captain Bill View Post
Also you'll probably find that most of these inverters can only support their maximum rating for a limited amount of time.
Yep. When you get close to the max rating, the problem is more often heat than wattage.
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Old 09-07-2012, 19:09   #8
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Re: 8 amps from a 400w inverter?

You don't need to guess at the current draw, all your devices should be marked with their voltage and current requirements. These are maximum requirements, and may not be consistent with their actual run time operation.

Some devices pull more power at startup, and therefore do not draw their max current all the time. Your TV might be like that. Your hard drives surely draw far less power when they are not accessing data.

Unless all your current values are in the same voltage, you cannot just add up the currents. You have to convert them to watts before you can add them up to get a complete current draw. For each device, calculate Amps x Voltage. Add them up to get the maximum current draw for all your devices together.

Your smaller electronic devices probably show their internal voltage/current, which is being drawn from the wall transformer. The wall transformer is not perfectly efficient, and has an additional current draw of it's own. Very cheap wall transformers do not always have this information stamped on them.

Each laptop I've owned has pulled more power than the last one, and for no good reason that I can really determine. My last Dell pulled 96W all by itself when charging the battery, which I really thought was excessive.

Your laptop will charge more efficiently from a DC charger than from the AC charger.

If you are planning for future cruising, look for devices that operate on 12VDC or a 12VDC adapter. They will operate more efficiently than those which require stepup power or AC conversion. They will also simplify your life in foreign ports, where 120VAC might not be available.
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Old 09-07-2012, 19:21   #9
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Re: 8 amps from a 400w inverter?

Regarding the current draw at the 12V outlet:

400W @ 12V = 33 Amps, as you've stated.

If your inverter is overloading, then you should be blowing a fuse.

Several possibilities:
1) Your outlet might be more 15a. "Power Outlets" may commonly be fused at this level - I'm not sure of that. A true "cigarette lighter" is a 30a source
2) Your inverter might not really be operating at 400W. That may be a "surge" value, or some made up marketing number, or your inverter might be defective.
3) You might be overloading your circuit. If the fuse is not correctly installed, or is somehow defective, or is the wrong size then you may be gradually cooking your internal wiring, possible creating a short or fire in the future.
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Old 09-07-2012, 22:29   #10
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Re: 8 amps from a 400w inverter?

I have been stomped on this before but I will try again. I have a 400w inverter. It has a cigarette lighter plug.

If - 400w / 12vdc = We presume 33 amps are going through the lighter plug - This is ridiculous and ain't happening. You would smoke every lighter plug I have ever seen.

What is really happening is this:

400w / 110vac = 3.6amps

3.6 amps X 12vdc = 43.7w

Many are presuming the input and output watts are constant. That is not the case. The amps are constant. watts are a calculated product measuring "power". only voltage and amperage are "real"

If we were putting in 12vdc X 33amps = 396 we would get 110 X 33 = 3630 on the output.

Don't beleive it? Plug a hair dryer or kettle into your 400w inverter and see what happens.

The product is rated at 400w @ 110vac = 3.6amps. That's what it pulls from the 12vdc batteries (losses ignored)
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Old 09-07-2012, 23:52   #11
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Re: 8 amps from a 400w inverter?

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Originally Posted by Ex-Calif View Post
I have been stomped on this before but I will try again. I have a 400w inverter. It has a cigarette lighter plug.

If - 400w / 12vdc = We presume 33 amps are going through the lighter plug - This is ridiculous and ain't happening. You would smoke every lighter plug I have ever seen.

What is really happening is this:

400w / 110vac = 3.6amps

3.6 amps X 12vdc = 43.7w

Many are presuming the input and output watts are constant. That is not the case. The amps are constant. watts are a calculated product measuring "power". only voltage and amperage are "real"

If we were putting in 12vdc X 33amps = 396 we would get 110 X 33 = 3630 on the output.

Don't beleive it? Plug a hair dryer or kettle into your 400w inverter and see what happens.

The product is rated at 400w @ 110vac = 3.6amps. That's what it pulls from the 12vdc batteries (losses ignored)
So.... why did I measure 8 amps DC with only the airport and WD TV on it?
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Old 10-07-2012, 02:50   #12
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Re: 8 amps from a 400w inverter?

Quote:
Originally Posted by Ex-Calif View Post
I have been stomped on this before but I will try again. I have a 400w inverter. It has a cigarette lighter plug.

If - 400w / 12vdc = We presume 33 amps are going through the lighter plug - This is ridiculous and ain't happening. You would smoke every lighter plug I have ever seen.

What is really happening is this:

400w / 110vac = 3.6amps

3.6 amps X 12vdc = 43.7w

Many are presuming the input and output watts are constant. That is not the case. The amps are constant. watts are a calculated product measuring "power". only voltage and amperage are "real"

If we were putting in 12vdc X 33amps = 396 we would get 110 X 33 = 3630 on the output.

Don't beleive it? Plug a hair dryer or kettle into your 400w inverter and see what happens.

The product is rated at 400w @ 110vac = 3.6amps. That's what it pulls from the 12vdc batteries (losses ignored)
I hate to "stomp", as you put it, but this is very wrong. Don't feel bad, thought - I don't know the first thing about biology, other than where my kids came from (the Stork, of course :-))

The thing that's constant on both sides of the inverter is wattage. If 400W is going out, then 400W has to be going in. It's physics, basic electrical theory, conservation of energy, and all that sort of earthly science. In practice, more watts are going in that coming out with the difference dissipated as heat inside the inverter, but let's set that subtlety aside for now.

What the inverter does is trade off volts for amps while all the time maintaining wattage. It also converts DC to AC along the way, but that's another subtlety that we can ignore for now.

The calculations that other have posted are correct, and the basic formula is P = I x V, aka power (watts) equals current (amps) times voltage (volts).

The inverter conversion, in a perfect world, would be:

400W input @12V, 33A
400W output @120V, 3.3A

Yes, a sustained 400W through a lighter plug is too much, and represents sloppy, potentially dangerous engineering of a product, but what the heck plenty of people buy them anyway. I guess manufacturers are counting on the 400W load never being more than a brief start up load and never a sustained load. And also relying on the vehicle manufacturer to have fused the lighter socket so you will blame the crappy car when the inverter blows the fuse.....

Most lighter sockets are rate at 10A, or 120W (there's that math again). An inverter larger than that should be hard wired to the battery with a suitable fuse or breaker.
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Old 10-07-2012, 02:59   #13
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Re: 8 amps from a 400w inverter?

By the way, the only way to ever GAIN watts through a device is if it's a generator of some sort. A mechanical generator, a solar panel, etc. Otherwise watts (power) is ALWAYS lost in ALL devices. That includes wires too. Engineers strive to minimize those losses, but they are always greater than zero. It's that "conservation of energy" thing in physics.

Oh, and even the watts that are gained in that generator or solar panel only occur because something else "gave up" that energy. In a motor driven generator is comes from burning diesel (or whatever), and in a solar panel it comes from the solar radiation. It always comes from somewhere, and you always lose some (sometimes a lot) along the way.
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Old 10-07-2012, 03:10   #14
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Re: 8 amps from a 400w inverter?

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Originally Posted by xymotic View Post
But The inverter does not like it when I add a laptop to the mix and shuts down. My Macbook air should be what, like 65?
The reason your inverter shuts down is the cigarette lighter plug. Cig plugs and their associated wiring will only supply 10A, if you are lucky, without excessive voltage drop.
When the input voltage drops too low your inverter will shut down.

So the most your 400w inverter will supply when powered by via the cig lighter plug is 10x12=120w ( not even that because at 10 A the input voltage will have dropped to 11v or lower.

Connect the inverter via sutable cabling and fusing and the inverter will go back to giving its full power.

Ps when drawing this sort of current the fuse or CB to cig lighter plug should have blown you need to check its rating.
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Old 10-07-2012, 04:31   #15
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Re: 8 amps from a 400w inverter?

See the several iterations of Ohm’s Law
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