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Old 06-25-2016, 09:56 PM   #1
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Power usage 120 volts versus 12 volts

I have heard that when you use a 120 volt appliance through the inverter, it uses 10 times as much electricity as does the same type appliance that is powered by a 12 volt cord, draining the batteries 10 times as fast. Do you know if that is true
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Old 06-25-2016, 10:06 PM   #2
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Originally Posted by RV Vagabond View Post
I have heard that when you use a 120 volt appliance through the inverter, it uses 10 times as much electricity as does the same type appliance that is powered by a 12 volt cord, draining the batteries 10 times as fast. Do you know if that is true
No, not true.

Power (Watts) = Volts x Amps

Assuming 2 appliances that do the same thing with the same efficiency, will be the same power but Volts and Amps differ but add up to the same Watts:

120 Watt Appliance using 12 Volts DC draws 10 Amps DC
120 Watt Appliance using 120 Volts AC RMS draws 1 Amp AC

The Inverter is not 100 % efficient so in theory the 120 Volt one will drain the battery SLIGHTLY faster but it's a SMALL difference.

Hope this helps.
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Old 06-25-2016, 10:14 PM   #3
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Wattage is power, amperage is flow and voltage is pressure. Electricity is analogous to water in that respect. When you reduce the voltage, you have to use a bigger pipe to get more amperage so you have the same power or total amount of electricity. That is where the 10 times comes from. If you are trying to achieve 120 watts, you can do it by one amp at 120 volts or 10 amps at 12 volts. Inverters are not so efficient. They max out at about 85% when at their max draw. As small power levels (wattage) their efficiency is less than 50%. A 3000 watt inverter putting 120 watts of will require about 15 amps input. Two amps to run the circuity and 3 amps of heat plus the 10 amps you are using.
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Old 06-26-2016, 06:47 PM   #4
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The inverter uses 10x as many amps cause it is working at 12v rather than 120, but the amount of electrical energy used is the same (except for heat loss in the inverter while converting). Others already explained the math.
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Old 06-26-2016, 06:54 PM   #5
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What he said. It's 10x the current. If you really dig into it there is a fudge factor because 12 VDC is more than 12. The difference is eaten up by the losses in the Inverter.
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Old 06-26-2016, 08:35 PM   #6
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in theory, the power (v * i) consumed by an appliance on 120v should be equal to that when on battery factored by inverter efficiency. in reality, there are more than that; the current draw is much greater than what appears on paper.

the example i am giving below is my real life experience.

appliance: sharp carousel convection microwave. 1600w input, 800w output
inverter: magnum ms2812. efficiency 88 - 90%
battery working voltage: 13.0-13.4v

when the microwave is working, on paper i should expect current draw

1600 13.2 0.89 = 136a (approx.)

but i have observed it drew 154 - 158a. why was that?

one of the reasons the extra energy was used was the loss occured during current traveling. i am using #2 awg cables between battery and the inverter. while it was working, i touched the cables i could feel the warmness. the heat, another form of energy, was converted from electricity and lost to the air. this heat issue occurs on all electrical transmissions, even on the main lines at 10000+v from a power plant to the transformer stations. if i were to change the cables to #4 awg, heat will still be there but in a lesser degree.

hope this clears some of the clouds.
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Old 06-26-2016, 09:19 PM   #7
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Just remember that the equation P=I*E is only valid for purely resistive loads. Power is measured in watts in this case. With less use of incandescent light bulbs, purely resistive loads are becoming less and less common.

Most modern loads are either inductive or capacitive. Inductive loads are things like motors and electronics with switching power supplies. Basically anything with an inrush current. Capacitive loads are things like fluorescent lighting.

As it relates to the RV world, when you see power represented by a VA (volt amps) rating don't just change the VA to watts. You have to know the power factor to be able to convert the two.

For example a UPS battery backup is usually sold in VA ratings (called apparent power) but the loads you are connecting (like a PC power supply) are listed in watts (called true power). You always need your VA rating to be higher than the watt rating in order to compensate for the power factor.
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Old 06-26-2016, 10:33 PM   #8
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but i have observed it drew 154 - 158a. why was that?

i am using #2 awg cables between battery and the inverter. while it was working, i touched the cables i could feel the warmness.
if i were to change the cables to #4 awg, heat will still be there but in a lesser degree.

hope this clears some of the clouds.

It looks like rain.

Do you mean 2/0 and 4/0? Because unless your inverter is super close to your battery, drawing a 150A load is too much for #2 and #4 would be untouchable! (#4 is smaller than #2)
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Old 06-26-2016, 11:15 PM   #9
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It looks like rain.

Do you mean 2/0 and 4/0? Because unless your inverter is super close to your battery, drawing a 150A load is too much for #2 and #4 would be untouchable! (#4 is smaller than #2)
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oops! yes, i meant 2/0 and 4/0 respectively. i was typing too fast while my alz' was kicking in my brain couldn't be catching up...
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Old 06-27-2016, 08:07 AM   #10
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FWIW If you are running off of battery the correct voltage to start at is 12.6 not 13.2. That is one of several items I would be checking as a way to discover where the errors are in the basic calculation. For instance I would want to measure the efficiency of the converter and the power draw of the microwave under the test conditions. There is also another drop in the battery voltage due to the internal discharge resistance of the battery. Then you are feeling lost heat in wiring to the Inverter. Another voltage drop. A good inverter will adjust for losses on the input side by raising the current to compensate for voltage loss.

All in all you have the kind of pretty puzzle it is good to give to students to teach them how many ways one can get unexpected results. They get to collect a lot of data by measuring all the circuit voltages so they can compute the various power losses. If you want to go that route it might be interesting. These are real world issues that are good to understand.
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Old 06-27-2016, 09:38 AM   #11
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FWIW If you are running off of battery the correct voltage to start at is 12.6 not 13.2. That is one of several items I would be checking as a way to discover where the errors are in the basic calculation. For instance I would want to measure the efficiency of the converter and the power draw of the microwave under the test conditions. There is also another drop in the battery voltage due to the internal discharge resistance of the battery. Then you are feeling lost heat in wiring to the Inverter. Another voltage drop. A good inverter will adjust for losses on the input side by raising the current to compensate for voltage loss.

All in all you have the kind of pretty puzzle it is good to give to students to teach them how many ways one can get unexpected results. They get to collect a lot of data by measuring all the circuit voltages so they can compute the various power losses. If you want to go that route it might be interesting. These are real world issues that are good to understand.
this is an interesting topic seeing so many uncharted attributes in the mix. we know or we don't know what we know but we don't what we don't know for sure.

the 12.6v to start may not be applicable to my case. my cells always settle at 3.35v, 4 in series is 13.4v. from 13.4v i started baking, for an instance of baking breads, after 20 minutes, i saw it ended at 13.0v. i have plenty of juice to use (560ah usable), so 13.2v for a calculation is dang close.
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Old 06-28-2016, 06:55 AM   #12
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I was going to say you should also check your meter because battery chemistry is pretty predictable. Then I re read your last post. With 4 cells at that voltage it sounds like you are running one of the Li chemistry's thus adding another unknown. The variables are all still there though, just some different expectations. ;-)
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Old 06-28-2016, 08:47 AM   #13
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I was going to say you should also check your meter because battery chemistry is pretty predictable. Then I re read your last post. With 4 cells at that voltage it sounds like you are running one of the Li chemistry's thus adding another unknown. The variables are all still there though, just some different expectations. ;-)
you are absolutely right, it is lifepo4 (guessing you might be a professor in engineering ). i am happy with the batts, not so much for the microwave. maybe because it's an older unit, its efficiency appears to be low; heating not as fast as panasonic or samsung, without convection.

back to the original subject, the reason i talked about my case was to say, there are way more attribites in real life than in theory. make use of it, enjoy rving .
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Old 06-28-2016, 11:09 PM   #14
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having fun yet?

cool,
and what about power factor correction, and why does the utility grid infrastructure use such high voltages. Do you think CGs will ever start peak demand charges..... J/K

Happy motoring.
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