How much power does an inverter use?

[jornvango]

In our Casita travel trailer, I have one 12V outlet. I have two inexpensive inverters (Walmart) that plug into the 12V outlet: one inverter is 100W, the other 400W.

My laptop needs 90W to charge. Does it matter if I plug it into the 100W inverter or the 400W inverter?
Am I correct to assume that the 400W inverter will only draw down the RV battery based on what I plug into it? As in, if I plug in my laptop which requires 90W, will the 400W inverter only draw 90W from the RV battery or does it always pull 400W?

I'm electrically challenged and am trying to figure out what I need.

Thanks!

Jorn

[Lwiddis]

I agree with the comments about “not pushing” the cigarette adaptor. Running a small inverter to power my 50 watt usage TV is the max I demand.

[BobsYourUncle]

A word of caution in using the 12V power source. They are fine for light duty use, charging a phone and other small devices.

These outlets rely on a 2 contact plug to get power. The tip typically has a little spring loaded contact that touches a metal plate for connection. It is iffy at best, ok for small stuff.

The heavier the load, the more demand placed on the surface to surface contact point. Over time, a bit of surface corrosion can build up and increase resistance. Increased resistance = increased heat, which can lead to failure of the socket, and possibly fire.

The original design of these things was to power a cigarette lighter, which does draw hard, but only for a few seconds.

Running heavier draw devices for long periods of time is risky. Be careful. A good plan would be to hard wire an inverter in there.

I had a triple plug that operates off a single outlet in my truck. I ran 3 things in there, all light duty, GPS, dashcam, phone charger. The unit started giving me trouble, and I smelled something burning. The triple socket got so hot that the plastic was melting. So much for that....

[2oldman]

rdhetrick wrote:
Since nobody answered your actual question, Yes, the inverter will only draw from the batter what is needed - plus its "dead" load.

Correct. Every inverter uses a small amount of current to run itself.

[JimK-NY]

The situation is even worse than described. Your laptop runs on dc battery power so you are losing efficiency by inverting to A/C current and then the laptop needs to convert that to dc of the correct voltage. Both processes lose efficiency.

A better solution is to use a "car" charger without the need to invert the current. With a bit of checking you should find this for your laptop. I do the same for other charging when possible. That includes items such as charging camera batteries.

Actually I am surprised that an inverter is over 80% efficient. Even small ones put out a lot of heat and have built in cooling fans.

[wa8yxm]

What PianoTuna said... they are about 88% efficient (up to 90)
I figure the estimated 12 volt load by dividing the 120 volt load in watts by 10 and it's remarkably close to the panel ammeter reading.

There is some "Idle" load (overhead to operate the inverter's computer parts)
But basically the load the inverter presents to the batteries is dependent on what it is powering

A 100 watt light bulb. about 10-11 amps
a 1000 watt microwave about 100-105 amps. Note that 1000 watts on the microwave is input (about 750 cooking power)

a 1500 watt teakettle (I have one) 150-160 amps

[rdhetrick]

jornvango wrote:

Am I correct to assume that the 400W inverter will only draw down the RV battery based on what I plug into it?
Jorn

Since nobody answered your actual question, Yes, the inverter will only draw from the batter what is needed - plus it's "dead" load.

The 400w probably larger dead load than the 100w, but that is not certain and would depend on efficiencies.

And as others have pointed out, the ratings are peak, but so is the 90w charger.

[BFL13]

The 100 and 400 are less than that continuous. The 400 will be about 320, so use that.

meanwhile the RV 12v socket might only do 8 amps so your laptop wanting 9 amps is pushing it. You want the 400 on a battery with wires and clamps (it should have come with a set) to do that job.

[pianotuna]

Jorn,

Inverters are about 88% efficient when operating at full load.

So loss is about 12%.

Use the inverter that is the smallest that will power the load.