Inverters 101: Understanding amps and volts

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Hi Mike,
Here’s a weird one regarding my 2017 F/R 34QS. Some information: I have a Magnum 2,000 watt pure sine wave inverter and four L16-6V 390Ah batteries along with three 160-watt solar panels and a 30-amp Go Power controller. The issue I have when I’m using the inverter (shore and generator not used) is this: My residential refrigerator says it uses 6 amps of current @ 115 volts. But as I look at the Go Power control panel while the fridge is on, it only says 5 amps. Shouldn’t it be more like 60 amps because watts divided by volts (700/12) = 58 amps?

I’ve contacted Magnum about it and all they give me is the formula for figuring it out, and I already know that. But the dang electric coffee pot that draws 900 watts shows about 75 amps, and according to the formula that’s correct. So, Mike, what have I missed, or have they done something to that refrigerator to make it super efficient?
 
Thanks for your time. —Dave Telenko
 
Dear Dave,
Like so many things in the engineering world, the devil is in the details. And while you have the basic formula correct, you’re applying it on the wrong side of the inverter equation, which is why you’re off by a factor of 10 or so. Let me explain….

The first thing to understand is that wattage is work, and watts are watts. (For a quick review of wattage read my recent article HERE). So if you already know the basic formula of volts time amperes equals watts it should be apparent that 12 volts times 10 amperes equals 120 watts, as well as 120 volts times 1 ampere equals the same 120 watts. In formula nomenclature it looks like 12 volts x 10 amps = 120 watts, as well as 120 volts x 1 amp = 120 watts. This could also be 1 volt x 120 amps = 120 watts or 1,200 volts x 0.1 amp = 120 watts. As you can see, there’s an infinite combination of voltage and current that can produce any amount of watts you want.

This relationship is all laid out in Ohm’s Law, essentially the unbreakable set of electrical rules that all appliances (and the entire universe) must abide by. See the chart above right for the complete set of Ohm’s Laws. For clarification, Amperes are represented by the letter “I”, Volts are represented by “E”, Watts are represented by “P”, and Ohms are represented by “R”. So the equation for watts is typically written as P = E x I.

Whatever way you write it, if you want more watts (power) in an electrical circuit, then you have to turn up amperage or the voltage or both. Conservation of energy says you can’t get something for nothing since that gets into perpetual motion machines and such. Been there, tried that when I was 8 years old, so I know that doesn’t work. (At the age of 8 I was trying to build a perpetual motion generator that would power my grandma’s house since she was complaining about her electric bill. Really…)

Inverters are really cool conversion devices that play with this voltage/amperage relationship to trade volts for amperage and vice versa. Say you want to store a bunch of power. Well, there’s no good (cheap) way to store 120-volts AC to run your microwave or whatever. So we use a 12-volt DC battery to store up a bunch of electrical power when it’s available from the grid or generator or solar panels. When we want to use it for a 120-volt appliance like our microwave to coffee maker, we convert it from 12 volts up to 120 volts. For now at least we’re going to ignore the AC/DC conversion thing since that’s not part of these basic calculations.

Here’s a diagram with a 12-volt battery, an inverter and a 1,200-watt microwave oven. Note that on the 12-volt side of the inverter you need 1,200 watts going in, which works out to 100 amps x 12 volts = 1,200 watts. But on the 120-volt side of the inverter you get 1,200 watts coming out, which works out to 10 amps x 120 volts = 1,200 watts. It works out to an approximate 10:1 or 1:10 conversion factor depending if you’re converting from 12 volts to 120 volts, or 120 volts to 12 volts. The easy way to think about this is that it takes 10 times as much current on the 12-volt battery side as comes out on the 120-volt inverter side. That’s also why the cables connecting your battery to the inverter are so large; in fact, they’re 10 times as large in cross-sectional area on the 12-volt DC side compared to the 120-volt AC side of things.

Now for you engineering types out there, yes, I’m using convenient voltages and ignoring losses in an inverter system (which hover around 95% efficiency in most cases). But this is Inverters 101, not a real design class where we would have to take into consideration all wiring voltage drops and inverter efficiencies, etc… 

Interestingly, this is also how AC transformers from the power companies work. By stepping up the voltage from the power generating plants to 250,000 volts or more, the power company can use much small diameter copper wire to carry the same amount of wattage to a city. Or think of it this way: If the 120 volts you get in your house started at 12,000 volts coming into your town, then the same diameter wire could carry 100 times as much power. And that’s exactly how it all works. Thank you Nicola Tesla for inventing 3-phase AC power and fighting Edison’s DC power, which would never have worked. 

And this is the same sort of voltage/current relationship that exists in all solar panel systems. But more on that later.   

Let’s play safe out there….

 

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

##RVT849

 

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14 Thoughts to “Inverters 101: Understanding amps and volts”

  1. Mlkirk

    Random inverter question. Having a Xantrex Freedom Xi 2000 inverter installed and technician says we need to choose a circuit. Isn’t it capable of powering all AC outlets in the rig?

    1. Yes, but in order to have an inverter power your entire RV via the circuit breaker panel you’ll need a separate circuit breaker in its own box to power the inverter. At least that’s what code suggests to me right now. I’m designing a similar installation right now and in contact with Xantrex engineering, so I’ll do an article on this soon. FYI, Xantrex is now an official sponsor of mine so I’m getting inverters to experiment with for seminars (a Freedom 2000 is arriving today). I also have their design and engineering support which is a great way for me to ramp up on this technology. More to learn and teach about….

  2. Dan

    I think your answer to Dave Telenko’s question went into too much detail. The average person would get “lost” after the first paragraph. To keep it simple just leave it at the following: Since the fridge is drawing from 120VAC the spec of 6 amps is AFTER the inverter. Therefore Dave’s equation should be 700 watts / 120 volts = 5.83333 amps (not 58 amps) . The manufacturer more than likely rounded up to 6 amp on the spec. So his coffee pot would be 900 watts / 120 volts = 7.5 amps (not 75).

    1. Mike Sokol

      I know my answer was more complicated than needed for his question, but I always use these as teaching moments. I probably have a few hundred emails like this, and it’s because the vast majority of consumers don’t understand the basic volts x amps = watts relationship, as you obviously do. In fact, most consumers don’t know the difference between voltage and current. So I like to keep illustrating electric principles as often as I can. I don’t just answer questions, sorry…

  3. PATRICK PETERMAN

    MIKE, WANTED TO GET BACK TO YOU; REGARDING MY PREVIOUS QUESTION REGARDING A DEAD OUTLET ON MY HOLIDAY RAMBLER; WELL I CALLED A SERVICE TECHNICIAN WHO ARRIVED TO AT MY HOUSE BRIGHT &EARLY- 5 HOURS LATTER HE STILL HAD NO CLUE-AT THIS POINT I ASKED HIM TO” KINDLY PACK IT IN” I WAS PRESENTED WITHA 450.00 DOLLAR BILL FOR BASICALLY LOCATING THE ELECTRICAL BOXES ON MY RV; I THEN REMEMBERED WHAT ROBERT ACQUINO OF PDS ELECTRONICS TOLD ME ABOUT MY “XANTREX INVERTER” MY INVERTER HAS A SINLE CURRENT IN AND DOUBLE CURRENT OUT- I NEEDED TO ONLY “FLIP THE BREAKER 20AMP SWITCH ON THE INVERTER ANS “WALLA!! POWER WAS RESTORED; I FELL LIKE AN IDIOT !! BUT I LEARNED A VALUABE LESSON; “KNOW YOUR EQUIPMENT”- THANK,S MIKE PATRICK.PETERMAN

    1. Chuck Woodbury

      Patrick, please, for the second time, do not type in CAPITAL LETTERS only. It’s considered “screaming” and it’s also harder for people to read.

    2. Mike Sokol

      Patrick,

      I’m glad to try to help, but please stop typing IN ALL CAPITAL LETTERS. Most readers find it annoying since that implies that you’re shouting at everyone, which I know you really don’t mean to do.

  4. Wolfe

    Most inverters, if they show anything, report OUTPUT voltage, watts, sometimes amps. I’ve not seen one that reports the hundreds of 12V amps going in. So, yes, OP was seeing 6A@120, 720W. Simple answer.

    Mine only report being on/low input voltage/overload output, so i put meters on both sides (because I’m weirdly addicted to data?). And yes, math works… under full load, my 4KW PSW runs 80-90% efficient and snorts up a brutal 400A@12V…

  5. Michael St Denis

    And, it could also be that he looked at the current draw when the pump was off, since a fridge pump cycles on and off. With the pump off, the current draw would be small (just running a fan inside the fridge).

    1. Mike Sokol

      Very true. At some point I’ll write an article about refrigerator battery usage correlating inverter efficiency, compressor starting amps, running amps with and without the compressor running, and duty cycle. Then I’ll hook up a real refrigerator with a battery and inverter to gather some empirical data to check my formula. Too much fun.

  6. Irv

    Mike, you didn’t answer his question. Your diagram of battery-inverter-microwave shows that the DC amps should be 10x the AC amps. Thus his refer drawing 6A at 115v should be drawing 60A at 12v as he asked.

    Maybe his refer spec (6A at 115v) reflects startup surge current…

    1. Mike Sokol

      My interpretation of this setup is that the GoPower controller must be monitoring the 120v-volt side of the inverter. If the refrigerator pulls 6 amps on the 120-volt side, then the inverter MUST be pulling 10 times that (60 amps) on the 12-volt side. Perhaps Dave has an AC-DC clamp meter he can use the measure the actual currents involved and report back.

      1. Al & Sharon Florida

        The residential refrigerator is labeled as using 6amps at 115V, but that doesn’t mean it pulls a constant 6 amps continuously. Certainly it may pull 6amps when the compressor starts, probably quite a bit less once the compressor is running. Then there is automatic defrost cycle with has some kind of electric heater element to dry the air.
        I have seen many reports from boondockers or dry campers with residential fridges that report the fridge pulls between 125 to 200 amp hours (AH) in 24 hours. Even if this number is not exact, a fridge which pulled 60amps of 12V DC continuously would consume 1440 AH’s a day. Far more than almost any RV battery system could keep up with on a daily basis.
        We would need more info from the person asking the question as to if he was reading the 12V amps or the 120V amps. Also it would help to know what the fridge was doing at the time, compressor running etc.

        1. Mike Sokol

          Yes, total battery usage will be dependent on duty-cycle of the compressor, which is dependent on ambient temperature and how many times you open the refrigerator door, etc…

          To get a handle on this I’ll have to measure my own home residential refrigerator for starting amps, running amps, and duty cycle. From there I’ll be able to come up with a reasonable guess as to how long the house battery bank should be able to power it. But the OP (Original Poster) was asking about amperes of draw, not amp-hours, which are two totally different (but related) things. More to study and write about…

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