DC to AC Amperage Calculation Question (120 VAC x .5A = 12 VDC X 5A?)

[zcapozzi]

Hi all,

Wanted to confirm my understanding of DC -> AC and how that impacts amperage specifically.

Device: Modem - 100-240VAC and .5A (but I live in the US so 120V)
AC: 120 VAC x .5A = 60W
DC: 12 VDC X 5A = 60W

Do I have this right? More information is below.


I have a modem that is 120-240 VAC and is listed as .5A (500mA). These are the specs on the actual device. According to some online calculators, 120VAC x 500 mA = 60W. The order of connections is as follows:

1. Modem is plugged into a surge protector
2. Surge protector is plugged into a 400W 12V Cobra inverter
3. Inverter is connected to my 12V battery

I've been measuring my energy usage from the battery (and subsequently the amount of electricity I've used that came from the sun) against the 12V battery, but I'd also been using the .5A listing from above. This would suggest I'm using 6W. I realized that I probably couldn't ignore the DC to AC change, so now I'm thinking that it's probably really 60W.

On the one hand, this is great because I thought I'd used about .5 cents of power overnight, when really it was closer to 5 cents. On the other hand, it changes some of my calculations about how long I can run things.

[Sunking]

Hi all,

Wanted to confirm my understanding of DC -> AC and how that impacts amperage specifically.

Device: Modem - 100-240VAC and .5A (but I live in the US so 120V)
AC: 120 VAC x .5A = 60W
DC: 12 VDC X 5A = 60W

Do I have this right?

Yes maybe. You are right as watts is the common denominator, and if 60 watts is true you got it right. However you do not know how power the unit really uses. When you see a spec of 100-240 VAC @ .5 amps the range is 60 to 120 watts. Look and see if it has VA rating, or output spec.

I've been measuring my energy usage from the battery (and subsequently the amount of electricity I've used that came from the sun) against the 12V battery, but I'd also been using the .5A listing

Then you have not been measuring anything. Voltage is only half of the equation. You have to monitor both voltage and current to determine how much power you are using. The input power rating to the power brick is meaningless.

[zcapozzi]

Yes maybe. You are right as watts is the common denominator, and if 60 watts is true you got it right. However you do not know how power the unit really uses. When you see a spec of 100-240 VAC @ .5 amps the range is 60 to 120 watts. Look and see if it has VA rating, or output spec.

Then you have not been measuring anything. Voltage is only half of the equation. You have to monitor both voltage and current to determine how much power you are using. The input power rating to the power brick is meaningless.

Thanks. Unfortunately, this device doesn't have a power brick so the input output isn't there. Some of the others do, which is very helpful.

Also, I know that I need both the current and voltage, but I haven't been able to get my current sensor working yet, so I'm stuck with just voltage. I've been tracking the current demands of the various things I've plugged in and how long they've been plugged in for until I can get the current sensor working. Once I have that, then I can just measure current and voltage on the wire coming from the battery and I won't need to worry about DC to AC conversion.

Until then, I'll just have to stick to guessing at the current demands and use the 120 VAC to 12 VDC conversion of 10x.

[SunEagle]

Thanks. Unfortunately, this device doesn't have a power brick so the input output isn't there. Some of the others do, which is very helpful.

Also, I know that I need both the current and voltage, but I haven't been able to get my current sensor working yet, so I'm stuck with just voltage. I've been tracking the current demands of the various things I've plugged in and how long they've been plugged in for until I can get the current sensor working. Once I have that, then I can just measure current and voltage on the wire coming from the battery and I won't need to worry about DC to AC conversion.

Until then, I'll just have to stick to guessing at the current demands and use the 120 VAC to 12 VDC conversion of 10x.

Those "kill a watt" meters are pretty accurate when it comes to 120volt appliances. Easy to use and really good data concerning watts used.

[zcapozzi]

Those "kill a watt" meters are pretty accurate when it comes to 120volt appliances. Easy to use and really good data concerning watts used.

I've seen a few youtube videos about those, and I saw a guys who has his meter tweeting energy usage. Pretty cool. Unfortunately, the DIY-er in me can't submit to buying something to do this for me. Just gotta get that current sensor working ( I think it might be busted actually ) and then I can start tweeting out energy stats my self. Thanks for the tip though. If I can't figure this current sensor out, then I may go that direction.

[inetdog]

Hi all,

Wanted to confirm my understanding of DC -> AC and how that impacts amperage specifically.

Device: Modem - 100-240VAC and .5A (but I live in the US so 120V)
AC: 120 VAC x .5A = 60W
DC: 12 VDC X 5A = 60W

Do I have this right? More information is below.

You have two major factors to consider when trying to determine power with an ammeter (and voltmeter).
Not even counting Dereck's warning that the same current into your inverter will correspond to different amounts of power depending on the state of charge (and therefore voltage) of your battery.
The other two are:

1. You need to allow for the efficiency of the inverter. This will typically come out to be approximated as a fixed wattage loss whenever the inverter is running plus some percentage of the actual load. For a poor inverter, this can easily turn out to be a 20% or greater loss when operating at the low end of the inverter output capability.
2. On the AC side, even knowing current and voltage will not be enough to give you the power at that point. The reason of for this is that an AC load can have a Power Factor (PF) associated with it which makes the actual power delivered less than the product of the current and the voltage. Look up Power Factor for more details.