Simple 12 volt energy use figuring?

softdown

Recently dawned on me that for practical purposes I think we can may figure 12 volt energy use this way: A 10 amp 12 volt appliance using 120 volt energy should be consuming about 1 amp of energy. Or 10% of the 12 volt figure. Of course that figure may vary significantly but I suspect it might be close enough to give one a usable idea.

Then we might want to convert wattage to horsepower at times. My 3 cylinder diesel genset is rated at 6500 watts. Tractors rated at 6.5 kW deliver roughly 10 hp - as I recall. But I'm pretty sure that genset motor would be rated at well over 10 hp. After all it is used in some skid steer and tractors.

So maybe there are big losses in the conversion to 120/240 volt energy?
Or perhaps the genset could deliver a lot more wattage at higher rpms. The issue with that is it is not a low speed genset. Going from old memories here - normal gensets operate around ~3000 rpms while true industrial gensets lope around  ~1600 rpm. Those figures will be off but you get the idea.

mike95490

Step-up & down transformers are pretty efficient, as are modern inverters.

Conventional generators use the RPM's to set the frequency,  for 60 hz, most small gensets are run at 3600rpm
Long life diesels run at 1800 rpm.   Voltage is set by a voltage regulator.  Changing rpm's will reduce the available HP, and the drive train pulleys re-sized to maintain the proper frequency - often not worth the effort.

Saggys

I was taught 2 horsepower engine per 1000 watts of generator power. Not sure if this is correct or not.

mike95490

Saggys said:

I was taught 2 horsepower engine per 1000 watts of generator power. Not sure if this is correct or not.

Roughly, thats about right

BB.

Be really careful tossing numbers/equations/rules of thumbs around...

Volts * Amps = Watts
12 volts * 10 amps = 120 Watts
120 volts * 1 amp = 120 Watts

Yes, that is true... However when sending power around there are other limits to worry about.

• Voltage drop: 12 volts @ 0.5 volts max drop to loads; 120 VAC * 3% = 3.6 volt drop

So the higher voltage means less current means less voltage drop and higher voltage allows higher voltage drop which lets you send power at the higher voltage much farther...

Just for the heck of it... How far can you send 120 Watts at 12 and 120 volts using voltage drop limit. Using a simple voltage drop calculator:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=0&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=8&distanceunit=feet&amperes=10&x=0&y=0&ctype=nec

12 volts
0.5 volt drop max
10 amps
120 Watts
14 AWG
8 feet run way run:

Result

Voltage drop: 0.50
Voltage drop percentage: 4.15%
Voltage at the end: 11.5

What about 120 VAC @ 120 Watts on 14 awg cable @ 3% drop?

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=0&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=120&phase=ac&noofconductor=1&distance=575&distanceunit=feet&amperes=1&x=0&y=0&ctype=nec

120 VAC
3.6 volt / 3% drop
1 amps
120 Watts
14 AWG
575 foot one way run

Result

Voltage drop: 3.58
Voltage drop percentage: 2.98%
Voltage at the end: 116.42

So you can send 120 Watts on the same 14 AWG cable over 70x farther@ 120 vac vs 12 vdc....

Now--HP vs kWatts (and Watts)... There is the question of converting the output of tractor motor between US Horsepower and Eur Watt/kWatt numbers.

A US Brake Horse Power is 735.49875 Watts or 0.73549875 kWatts:

• 10 HP * 735.49875 Watts per HP = 7,355 Watts
• 6.5 kWatt = 6,500 Watts
• 6,500 Watts  / 0.73549875 Watts per HP = 8.84 HP

Note that US HP numbers are a bit of a lie (at times) by the marketing department (US marking wants more HP. Cheaper than redesigning a car motor).

And then there is the rule of thumb for sizing a gasoline engine to electrical output (via DC Generator or AC Alternator)... In theory, 1 HP should equal 736 Watts electrical. But all of the conversion losses (you mean the gas motor needs to power the cooling fan, oil pump, water pump, electrical system of car, fan belt losses, etc...) the basic rule is for every:

• 1 kWatt of electrical output, you need a (at least) 2 HP rated gasoline motor. (as Saggys and Mike said above)

Another handy rule of thumb is 1 HP hour (from gasoline engine) is 0.5 lbs of gasoline (gasoline weights 6.1 lbs per gallon).

• 3,500 Watts from genset * 3 hours = 10,500 Watt*Hours
• 10.5 kWH * 2 HP per kWH = 21 HP*Hour
• 21 HP*Hour * 0.5 lbs of gasoline per HP hour = 10.5 lbs of gasoline
• 10.5 lbs of gasoline / 6.1 lbs per gallon = 1.72 gallons of fuel
• 10,500 WH / 1.72 gallon = 6,105 WH per gallon

A mid size genset should burn provide 5,000 to 6,000 Watt*hours per ballon... So not a bad estimate.

How efficient are AC inverters (and similar for DC to DC converters) is around 85% to 95% conversion efficiency. The wider the voltage range, especially for "step up" or "boost" voltage conversions, typically a bit higher losses. I use 85% inverter AC inverter efficiency for planning (I am a bit of a pesimist).

If you are deciding between 12 VDC and 120 VAC circuits... the 120 VAC base system will need the solar array and battery bank increased by:

• 1/0.85 DC to AC conversion efficiency = 1.17 or roughly 20% larger panels + battery bank for AC system

And that is not bad to make an AC standard home/cabin wiring system vs DC.

You have other issues with 12 VDC switches/breakers/fuses costing more than the 120 VAC versions as DC current tends to "sustain" an arc much more easily than AC power (technically, 12 VDC and above tend to sustain arcs, and below ~12 VDC does not arc).

For alternator based gensets... 3,600 or 1,800 RPM for 60 Hz systems (number of poles in alternator--2 poles = 3,600 RPM, 4 poles = 1,800 RPM, etc. for single phase gensets).

For 50 Hz power, 3,000 and 1,500 RPM (2/4 pole).

Of course that is for direct drive alternators... Belt drive, gear drive, higher pole count, etc. can be different.

And the number of pole? Don't have to stick with 2/4 (single phase) or 3/6 (3 phase)--Howabout 100 poles for your hydroelectric dam alternator set?

https://www.jeumontelectric.com/wp-content/uploads/2017/06/JE-Hydro-Alternators-4-to-100-Poles-.pdf

And Inverter-Generators are "wild AC input" from genset (wild meaning uncontrolled frequency/RPM from motor). Converted to high voltage DC then back to 120/240 VAC @ 50 / 60 Hz... Because the gasoline motor now only needs to turn the wild (typically 3 phase) alternator to give enough energy to run the AC inverter actual AC loads--The "ECO" throttle can slow the engine down and save fuel/noise (Otto Cycle gasoline engines are lossier because of throttle plate, and other windage/cooling are/oiling system losses, etc.).

The standard genset--RPM = AC frequency... So genset has to hold 3,600 or 1,800 Hz (60 Hz +/- 1% to 5% or so).

Anyway... I like to keep the rules of thumbs "simple" (like inverter efficiency, off grid power efficiency, etc.). For things that involve multiple variables--Such as AWG wire size vs current vs voltage drop vs distance--I suggest going through the numbers and using a voltage drop calculator (in this case) to make the actual determinations--Vs trying to create and remember the assumptions for "another rule of thumb".

-Bill

softdown

^^^^ I see the point.

I was specifically thinking about 12 volt battery chargers that I use all the time. They have about a 6' run after transforming 120 volts AC to ~ 12 volts DC. Just trying to determine their power consumption since I charge about 20 misc. batteries every month.

RCinFLA

softdown said:

^^^^ I see the point.

I was specifically thinking about 12 volt battery chargers that I use all the time.



For chargers, power factor of charger is another important factor.  Generators, inverters, and transformers are actually rated for V*A, not watts.  Only get the rated watts if power factor is 1.0.

Many chargers have poor power factor for AC input power.  Typically they will be between 0.6 and 0.7 power factor.

BB.

On Amazon, EBay, etc., there are so many cheap energy monitors out there--If you are are interested in how much 120/240 VAC power your misc. battery charging consumes... You can just buy one (or several) Kill-a-Watt type meters to measure those charger(s).

https://www.amazon.com/s?k=kill+a+watt+meter (120 VAC plug in meter like "kill-a-watt")
https://www.amazon.com/s?k=AC+power+meter (more AC power meters)
https://www.amazon.com/s?k=DC+power+meter (DC power meters)
https://www.amazon.com/s?k=battery+monitor ("cheap" battery monitors)

-Bill