120Vac vs 12Vdc vs 24Vdc Consumption (Amps, Watts)

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Guest312
Guest312 Registered Users Posts: 22 ✭
in Solar Beginners Corner #1
Just wondering, when I do buy my Kill-A-Watt meter.

Ok, so.. If I'm going to buy solar panels for powering Laptops, HD TV's, Fans, Heaters, etc.

If my meter says that I'm using 750 watts for a heater on-grid (120Vac), and it says it's using 6.25Amps..

Does that mean that if I hook up a power inverter to a 12v Battery, that it will draw 10 times more than if I had it hooked up to a wall?

So if it's 120Vac @ 6.25 Amps (750 Watts), ... Will it be 62.50 Amps from a 12Vdc battery, with a power inverter hooked up to it?

So if I'm using a 100Ah AGM or GEL Battery battery, it will only last what..? 62.5A / 100Ah = 1 Hour 36 Minutes or (1.6 Hours) ?

The above is for a Electric heater (750 Watt Mode), that draws 6.25 Amps @ 120 Vac (750 Watts).

Just for fun, here's the high setting for the electric heater 1,500 Watts. (12.5 Amps x 120 Volts = 1,500 Watts)

So that's what.. 125 Amps x 12 Vdc = 1,500 Watts. Geesh.. Not getting very far with a 100Ah AGM Battery am I.. (0.8 Hours or 48 Minutes)

That's draining that battery to 0%. Not supposed to do that, even with a nice AGM Deep Cycle battery.. What is it again, I'm only supposed to drain it how far? 20%?

And here's a 24Vdc System.. Correct me if I'm wrong at any time with this post, I'm here to learn. Just trying to get everything straight before I go out and spend money on Solar Panels, etc. etc. Which I'm going to do, and start out small to get a feel for Solar Power Systems, etc.


24Vdc (750 Watts @ 6.25Amps @ 120Vac)

That's half..

So that's (6.25Amps x 5) = 31.25 Amps x 24Vdc = 750 Watts. So with a 100Ah Battery that gives me less than 3.2 Hours or 3 Hours 12 minutes.

So it really is better to go for the 24Vdc Systems then the 12Vdc systems. Right?

Do I understand how a 12Vdc system compared to a 24Vdc system will work?

If I know the Amps, Watts, Volts for a 120Vac system, it's very easy to figure out how much current will be drawn from either the solar panels or from the battery banks..


Please look over this, and let me know if I either got it or didn't get it. I'd really appreciate some feedback on this post. I'm sure newbies would really enjoy this post as well if I did or if I do understand how this whole thing works. :cool:


It doesn't really matter what the 120Vac end of the inverter is telling you does it... The real measurements will come from off the battery right? Seeing that it's pulling the juice from the battery (source). So it's 12Vdc or 24Vdc right?

So whatever my Kill-A-Watt 4460 is going to tell me, depending on what type of system I'm going to hook up, it's going to be 10x more current draw from a 12Vdc battery bank, vs 5x more current draw from a 24Vdc battery bank, vs ... Idk.. do they make a 36Vdc or even a 48Vdc Power inverters? I've heard of 24Vdc, but not 36 or even 48Vdc power inverter input voltage..

But it really doesn't matter if it's (2x 12Vdc in series 100Ah) 24Vdc system, or (2x 12Vdc in parallel (200Ah)) 12Vdc system. You can't buy 24Vdc batteries. Or can you? If you wanted less amp draw it's basically the same if you added 2x 12Vdc batteries in parallel, or in series. 2x 12Vdc 100Ah batteries in series will give you 3.2 Hours @ 31.25 Amps draw @ 3.2 Hour battery usage vs. 2x 12Vdc 100Ah batteries in parallel (200Ah) will give you 3.2 Hours @ 62.5 Amps Draw @ 3.2 Hours. It really makes no difference, except your drawing more current from the batteries. So since power inverters are more popular in 12Vdc input, it would probably be wise, and also to give a better selection of Power inverter as well as pure sine wave power inverters with a 12Vdc Battery bank system.. eh..? :confused::roll:

Ok, Let me here what you guys have to say. I'd really appreciate it.

The heater is this.. Just bought it last week, and that's what I was using as my example, just in case you were wondering. (Sunbeam Electric Tower Quartz Heater, Sun-Like Radiant Heat, SQH310-WM1-115 (WAL-MART))

It's listed on the box that at 120Vac it outputs 750 Watts @ 6.25A & 1,500 Watts @ 12.5A. According to Ohms law, that's spot on.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #2
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    Re: 120Vac vs 12Vdc vs 24Vdc Consumption (Amps, Watts)
    Guest312 wrote: »
    If my meter says that I'm using 750 watts for a heater on-grid (120Vac), and it says it's using 6.25Amps..

    Does that mean that if I hook up a power inverter to a 12v Battery, that it will draw 10 times more than if I had it hooked up to a wall?

    So if it's 120Vac @ 6.25 Amps (750 Watts), ... Will it be 62.50 Amps from a 12Vdc battery, with a power inverter hooked up to it?

    You got it...

    ...well, almost. Your inverter will also chew away 10-20% of the battery to convert 12 v DC to 120 v AC. Heating and cooling are hard to do 'off grid'. Heating is pretty much a nonstarter. My wood stove is heating away right now.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Guest312
    Guest312 Registered Users Posts: 22 ✭
    #3
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    Re: 120Vac vs 12Vdc vs 24Vdc Consumption (Amps, Watts)
    Photowhit wrote: »
    You got it...

    ...well, almost. Your inverter will also chew away 10-20% of the battery to convert 12 v DC to 120 v AC. Heating and cooling are hard to do 'off grid'. Heating is pretty much a nonstarter. My wood stove is heating away right now.


    Crap ! I almost forgot about the damn Inverters cost to run, i.e. to convert the 12Vdc to 120Vac. I think I remember seeing a video on YouTube where an inverter will use what.. Idk, I think something like 700mA (0.7A) or something.. How much does your inverter use without a load? I also heard that Kill-A-Watt is not very precise when telling you how many amps or watts a device is using.. Is that true? Seen a video on YouTube, didn't really watch it, but from the thumbnail it looked as if the person was using a side by side comparison with a nice FLUKE $500 meter, and testing the accuracy together..

    Thanks for checking my post, I really appreciate it. I think I have a very good understanding about Solar Systems work now. I haven't even bought a solar system or setup yet, and have "what I think" is a very good understanding how it all works. I will be buying some solar panels and getting into actually powering stuff besides an iPhone 4S or Samsung Galaxy S3, which I've done with three 1.5W, 4.5V, 0.334A in parallel. Turns out the output came to 5.5+ Vdc, 1.025A, 5.4W with some cheap RadioShack solar modules. I used to be very fuzzy on how solar systems work, and that they cost thousands and thousands of dollars (Large Systems are..) when you could put together a small to medium sized system for under $1000, if not less.

    One of the most important things I've learned..
    AGM Batteries or "Good Batteries" are expensive.. They are very important, if you want to power your system when it gets dark. ;)

    Nice work on the wood stove.. My father, and even we had a nice wood stove like last year.. Had to sell it because we moved to a new house. It was a Beaver oak type stove.. I really don't remember it's name.. but... yeah. I do realize how much power is needed to heat just 1 room, it's insane.
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  • rgk1
    rgk1 Solar Expert Posts: 134 ✭✭✭
    #4
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    Re: 120Vac vs 12Vdc vs 24Vdc Consumption (Amps, Watts)

    When you see the pros here on the forum talk about using solar for heating anything they are not kidding. Early on I kept thinking "it cant be that bad!". Granted I have a very small system but my tv, satellite receiver and clock draw 125 watts (watts/hr?) according to the kilowatt meter. Given that, I could almost run 5 tvs, 5 satellite receivers, and 5 clocks. Then the realization hit that I CAN NOT make a piece of toast with that same power.

    Sorta makes you look at anything with a cord attached in a whole other way!
    4-Risen 320 watt in series/parallel, 8-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/48 volt inverter, Tristar 45 MPPT controller.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: 120Vac vs 12Vdc vs 24Vdc Consumption (Amps, Watts)

    The actual formula for conversion is more like this:

    Watts AC / conversion efficiency (i.e. "85%") = Watts DC + inverter consumption (which could be as high as 40 Watts for a bad one) = Watts DC from batteries. We approximate Amp hours based on nominal system Voltage, as that is usually the lowest you want to go.

    So if you plug your 750 Watt heater into a typical 12 Volt inverter:

    750 / 0.85 = 882 Watts DC + 20 Watts for inverter = 902 Watts / 12 Volts = 75 Amps draw.

    So if you want to run that heater for, say, 8 hours you will use 600 Amp hours @ 12 VDC. That would under normal circumstances dictate a 2400 Amp hour battery bank just to run the heater.

    As the others said, it's a bad idea. Even on higher system Voltages.

    Oh and don't think of it as running things off panels: loads run off batteries, panels recharge batteries. The only time you run 'directly' off panels is if you have the batteries charged (Float) and the PV is still producing power. This is when opportunity loads are applied. But it's not something you should count on happening.
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