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Electrical dummy back again with another question - DC vs AC system efficiency & design


noteven
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Is there a point in the design of an electrical system where it is sensible to invert the 12v DC electrical pressure to 110 AC from a pure efficiency standpoint? 

I am not including all the "convenience"  reasons for using inverted AC - I am assuming all needed appliances and devices are available and can be satisfactory for my needs on either 12v DC or 110v AC.

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If you're strictly concerned with EFFICIENCY when you Invert FROM 12 VDC TO 120 VAC there are wasted (even though they are fairly efficient) heat energy losses PERIOD.

 If you're talking about convenience  and necessity, sure if you need 120 VAC but aren't plugged to shore power the Inverter, despite its inefficiency, is the obvious way to go.

 Where possible and provided the appliance/device is capable of straight 12 VDC operation and you have adequate capacity ??? THATS THE WAY TO GO TO AVOID THE INVERTER INEFFICIENCY and wasted heat losses

 There are many devices out there (like my CPAP machine) which operate using 120 VAC, but that use in line brick converters (also with heat losses)  that change it to perhaps 12 or 24 VDC etc. If a person had the correct voltage and adequate amperage  to power it direct and toss the brick converter THAT IS LIKELY MORE EFFICIENT 

 There are some 3 way fridges (12 VDC, 120 VAC, LP Gas) you could run on what you have available. If its ONLY a 120 VAC or LP Gas of course you could power it with your batteries plus an Inverter provided you have adequate battery and recharging capacity. I have been known to run my 2 way fridge off battery/inverter power but I have plenty of battery and charging  

 You ask good questions 

John T

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There are a few cases where converting DC to AC and then back to DC makes sense. An example would be a 24v or 48v DC battery bank located in the front of a unit. Yes, they make DC to DC converters but they are not efficient and if your 12v loads are in the other end of the house, the wire size is huge to prevent an unacceptable voltage drop.

In that case, a lot of the time it makes more sense to run a 120v AC line to the rear of the unit, add a small converter to go from 120v AC to 12v DC. One major thing to remember is the higher the voltage on a DC circuit, the smaller the wire for a given wattage. The other main consideration on 12v DC circuits is that the entire run of wire (out from the load center, to the load and the return) has to be considered when determining the wire size.

Edited by GeorgiaHybrid
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1 hour ago, GeorgiaHybrid said:

The other main consideration on 12v DC circuits is that the entire run of wire (out from the load center, to the load and the return) has to be considered when determining the wire size.

Since watts is determined by multiplying amps times volts, that statement is true but it would also be true for AC circuits. It is true that alternating current is more efficient for transmission over long distances than direct current, but that relationship is the same for both and isn't the reason for that. Because of that relationship, it is far more efficient to transmit a very high voltage with very low current over long distances, no matter whether it is ac or dc current. But ac is far more easily stepped up to high voltage for transmission and back down to lower voltages for use, than is true for dc current. 

On the other hand, dc current is much more easily stored as batteries can only store dc. We could go into a lot of electrical theory to detail these relationships, but this involves much more than most RV folks would enjoy. 

Edited by Kirk W
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3 hours ago, GeorgiaHybrid said:

There are a few cases where converting DC to AC and then back to DC makes sense. An example would be a 24v or 48v DC battery bank located in the front of a unit. Yes, they make DC to DC converters but they are not efficient and if your 12v loads are in the other end of the house, the wire size is huge to prevent an unacceptable voltage drop.

In that case, a lot of the time it makes more sense to run a 120v AC line to the rear of the unit, add a small converter to go from 120v AC to 12v DC. One major thing to remember is the higher the voltage on a DC circuit, the smaller the wire for a given wattage. The other main consideration on 12v DC circuits is that the entire run of wire (out from the load center, to the load and the return) has to be considered when determining the wire size.

Now I got to ask. I am 48v and have a 110/12v standard rv converter unused. IOTA. Currently using a dc/dc unit for my 12v needs. Are you stating it more efficient to use the ac converter?

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Let me try to clear up any confusion:

Anytime you use use wire having resistance to conduct current (which is true for AC (Hot and Neutral return) or DC (+ and -) there are I Squared R wasted heat energy losses in BOTH conductors........ That's one reason to operate and transfer at HIGHER VOLTAGES (say 120 versus 12) so there's less current and less I Squared R  wasted heat SUBJECT OF COURSE TO WIRE SIZE. Its possible to have the exact same losses at 12 or 120 volt IF THE 12 VOLT SYSTEM USES MUCH BIGGER LOWER RESISTANCE WIRE

EACH TIME you make any conversion (12 to 120 or 120 to 12 or 48 to 12 etc) there are wasted heat energy losses .

THEREFORE if you want to make an accurate apples to apples comparison to decide at what voltage and how many conversions (12 to 120 or 120 to 12 or 48 to 12 etc) are necessary and which "system" is MOST ENERGY EFFICIENT, you need to know the currents and wire sizes and distances and efficiencies of each and every Inverter/Converter/Adapter etc.   Its pretty simple basic math but you would need all the data

That being said if there are long distances involved I prefer higher voltage so I can get by with much smaller wire !!!!!!! But sure I could also use lower voltage but WITH MUCH BIGGER WIRE REMEMBER.

As I already pointed out above if you can use an appliance that operates at 12 VDC (some may be available for 12 VDC or 120 VAC operation)  that saves any Inverter inefficiency losses even if you may need bigger wire.  Soooooooo in an RV if you have appliances that operate at ONLY 120 VAC and you're dry camped and have sufficient battery energy storage capacity you're stuck with an Inverter regardless of its inefficiency

NOTE I see more and more of 24 or even 48 volt RV battery banks nowadays so there's less current but of course, you would need a 24 or 48 down to 12 DC to DC to power up all they typical RV 12 VDC appliances. 

1 hour ago, GlennWest said:

Currently using a dc/dc unit for my 12v needs. Are you stating it more efficient to use the ac  converter?

Not quite sure I understand your question. You obviously need the DC to DC to get the necessary 12 VDC power. Are you asking about a 120 VAC powered 48 volt battery charger (efficiency of ?) or just what?? Again the overall system efficiency and which method is best depends on all the currents and wire sizes and distances and efficiencies of each piece of equipment, I cant answer that without a ton of data.........  

Hope this helps but as always NO warranty Im longgggggggg retired from power distribution design engineering and rusty plus codes change

John T 

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Kirk, watts are watts but in a DC circuit, volts times amps will give you watts. In an AC circuit you have volts times amps to give you volt amps (va) but the power (watts) will depend on the power factor.

A purely resistive load (think electric resistance heater) in a AC circuit has a power factor of 1. An electic motor might have a power factor of .8 and will will use the wattage of the va times the power factor with the remainder being mostly waste heat and other factors.

Look at some of the inverters out there with a va rating of 5000va and a wattage rating of 4000 watts. That is something that you need to take into account when looking at inverters.

You also need to carefully look at the wattage rating on inverters. If they say "maximum wattage", that is the rating for a purely resistive load only without a power factor. Others will be rated with an assumed power factor.

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6 hours ago, GlennWest said:

What I was asking if it better efficient  to use the Iota factory converter, no 12v battery, to power my 12v systems or continue with my 48/12 dc/dc converter. I know I would have no 12v that way without 110v. 

Hi Glenn, The Iota (120 VAC to 12 VDC) has a certain "X" efficiency with heat losses and will power your 12 volt loads....The 48 to 12 DC to DC converter has a certain "X" efficiency to power your 12 volt loads.......However use of the 48 volt battery bank and the 48 to 12 DC to DC would require not one but two conversions right?????? The DC to DC  PLUS the 120 VAC to 48 VDC Charger. 

However, one advantage of using a battery versus a converter only  is the battery can act as a buffer and is one huge electron bank and energy storage device.

I would have to have a lot of data and run a  lot of calculations find the most efficient systems

John T  

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44 minutes ago, GeorgiaHybrid said:

A purely resistive load (think electric resistance heater) in a AC circuit has a power factor of 1. An electic motor might have a power factor of .8 and will will use the wattage of the va times the power factor with the remainder being mostly waste heat and other factors.

Look at some of the inverters out there with a va rating of 5000va and a wattage rating of 4000 watts. That is something that you need to take into account when looking at inverters.

Georgia, good discussion of Power Factor. Of course it can get pretty technical. Here are a few very brief comments I gathered (without going deep in the weeds)  to help the others, looks like you have a good handle on it CONGRATULATIONS..

  In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. 

Mathematically it is the cosine of the phase difference between the source voltage and current.

Power Factor = True Power KW/Apparent Power KVA

Where I practiced the utility penalized us for a lousy power factor so we installed power factor correction capacitors where we had huge inductive loads such as motors

Pleasure sparky chatting with you, in my day I was tough on this but it been too long lol

 John T

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27 minutes ago, oldjohnt said:

Hi Glenn, The Iota (120 VAC to 12 VDC) has a certain "X" efficiency with heat losses and will power your 12 volt loads....The 48 to 12 DC to DC converter has a certain "X" efficiency to power your 12 volt loads.......However use of the 48 volt battery bank and the 48 to 12 DC to DC would require not one but two conversions right?????? The DC to DC  PLUS the 120 VAC to 48 VDC Charger. 

However, one advantage of using a battery versus a converter only  is the battery can act as a buffer and is one huge electron bank and energy storage device.

I would have to have a lot of data and run a  lot of calculations find the most efficient systems

John T  

Don't have my inverter yet. No work has put all my plans on hold. So reason asking. True once solar and inverter in, I would be converting twice.

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There are just too many appliances I use to worry about small inefficiencies in inverting.  Toaster, coffee maker, TV, DTV receiver, mattress heater...etc. 

Trying to find 12v versions of these things that actually work is fruitless.  You have to feed them a certain amount of watts to get the job done, regardless of the voltage.

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1 hour ago, hemsteadc said:

Trying to find 12v versions of these things that actually work is fruitless

Ive (49 year RV owner) seen plenty of 12 VDC versions if you get around RV supply houses, dealers, off grid vendors, etc. etc. Trouble is they may be more expensive, lesser known name brand, poor quality or require fairly high current, although I have used some that worked fine. I still wouldn't be without a good quality Pure Sine Wave Inverter, makes life so much simpler and as you indicate 120 VAC devices are so common and readily available. 

 Whatever works for a person is the way to go

John T

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