Hi All,
I have been confused about power inverters for a while now and can't seem to find a good answer anywhere that's not advertising to buy some particular power inverter.

I would like to know how to calculate the required power for the desired power. In other words - how many amps on 12volt dc does it take to create 100watts on 110v ac?

Is there a formula for this ?

Thank You,
John

 

Power in = power out if the inverter is 100% efficient. In practice the efficiency may be around 85%.
So, for 100W out, you'd need, say, 115W in. If the input voltage is 12V then the input current would be 115W/12V = 9.6A.

 

Hi John. Yes there is and it is precisely the same calculation that you would use for a switch mode power supply. Here goes:

"The power out of any conversion scheme will always be less than, sometimes much less than the power in."

-- Papabravo​

The only thing you have to do is pick a target efficiency for whatever conversion scheme you are using. In the case of AC inputs or outputs you need to us RMS values. Let us try your example with an inverter that is 80% efficient.

\(\text{Output Power = 100 watts}\)
\(\text{Input Power = 100 watts/0.8 = 125 watts}\)
\(\text{Input Current = 125 watts / 12 Volts }\approx\text{ 10.42 Amperes}\)

Q:That's great but what if my inverter is more or less efficient?
A:You run the calculation for a couple of values and draw a graph

Q:What happens if I need more or less power
A:Same answer, compute a few values and draw another graph and you'll end up with a set of performance curves.

You can use the same calculations to evaluate a commercial unit. With a fixed load you can determine the output power and the input power and if the actual efficiency is anywhere close to you target.

 

Hi Guys,
Thank you both for the fast and very informative responses. I guess I'm still a bit confused.

Putting aside the efficiency factor...

A power inverter claiming to output 350 watts of 110V ac power will also draw 350 watts of power on DC? How in the heck can this inverter then plug into and be powered from a lighter socket capable of just 15 amps? If I'm doing the math right... 350W / 12V = 29.16A.

Am I missing something here?

Thanks again,
John

 

Hi Guys,
Thank you both for the fast and very informative responses. I guess I'm still a bit confused.

Putting aside the efficiency factor...

A power inverter claiming to output 350 watts of 110V ac power will also draw 350 watts of power on DC? How in the heck can this inverter then plug into and be powered from a lighter socket capable of just 15 amps? If I'm doing the math right... 350W / 12V = 29.16A.

Am I missing something here?

Thanks again,
John

Yes you are missing several somethings.

1. No conversion scheme can be 100% efficient. That means for 350 Watts output the input WILL REQUIRE MORE POWER.
2. If you try to pull more current out of a voltage source than it can provide, then the voltage must drop. If it does not drop then your specifications are inaccurate.

Example:

\(\text{12 Volts @ 15 Amperes = 180 watts}\)
\(\text{11.25 Volts @ 16 Amperes = 180 watts}\)
\(\text{10.58 Volts @ 17 Amperes = 180 watts}\)

You cannot pull more than 180 watts out of the input without something else happening: fuse popping, explosion, fire, that faint burning sensation, and you won't be able to deliver 350 watts to the load. The most you will get is 180 watts times the efficiency factor, or 144 Watts in the case of an 80% efficient inverter.

Do you get the picture?

 

How in the heck can this inverter then plug into and be powered from a lighter socket capable of just 15 amps? If I'm doing the math right... 350W / 12V = 29.16A.

You math is correct.
The most power you can get for 12V @ 15A is 180W, ignoring efficiency.
For more than that the inverter must be connected directly to the battery.

 

Yeah. Just for simplicity in understanding I said forget the efficiency. I understand about the failure (fuse pop) too. Which is why I was asking again...

This seems to me like the manufacturers are telling false truths just like walmart audio amplifiers...

So thank you all very much.

 

You math is correct.
The most power you can get for 12V @ 15A is 180W, ignoring efficiency.
For more than that the inverter must be connected directly to the battery.

Even the battery has limits. The more output power you require from the inverter, the faster your battery will discharge. Fast discharging of an SLA battery will shorten it's life considerably. Marine Deep Cycle batteries are a better choice for continuous duty applications than standard automotive starter batteries.

 

Yeah. Just for simplicity in understanding I said forget the efficiency. I understand about the failure (fuse pop) too. Which is why I was asking again...

This seems to me like the manufacturers are telling false truths just like walmart audio amplifiers...

So thank you all very much.

You never have to do the math, when it comes to efficiency. It is only important that you recognize that you will always have to put more in than you get out. Alternatively that you will always get out less than you put in. Whichever way is easiest to remember, that is the only relevant understanding that is important when it comes to power.