I'm going to offer up something that I may regret and others may care less about :zz but I'm interested in learning something and those with more specialized knowledge can help me. I need a sanity check :o from those in the know.
After wandering around the web, here's what I think I know:
The watt is an electrical
unit of work done. Watts are watts. (No comments about how much work Charles does :wink .) 1500 watts DC is the same work as 1500 watts AC. Watts = amps x volts. A 12 volt DC device can do the same work as a 120 volt AC device only the 12 volt DC device needs more current to do the same work. If watts = amps x volts (it does) then amps = watts / volts. A 1500 watt heater needs 12.5 amps AC (1500 watts/120 volts AC=2.5 amps). The same heater will need 125 amps of 12 DC power (1500 watts/12 volts DC=125 amps).
It's probably no coincidence that the maximum power for most home electric heaters is 1500 watts. It draws 12.5 amps and most home wall sockets are on 15 amp circuit breakers/fuses. We like round numbers. A 2000 watt heater would require 16.7 amps, thus blowing a 15 amp fuse.
The smaller the wire, the greater the resistance (ohms) to flow (amps) it has. Also, the greater the flow, the greater the resistance. As an analogy, try blowing softly and then as hard as you can on a small bar straw. The higher the pressure the greater the resistance. Do the same on a normal straw. Less resistance at the higher pressures than the bar straw. No! Duh! as a young woman I know used to say. Anyhoo, it's similar for electrical
wiring. (For the purists it's not EXACTLY the same thing because with air through straws we're actually dealing with the Bernoulli effect, Reynolds numbers, laminar vs. turbulent flow etc.)
The voltage drop through a wire can be calculated. It's a function of the amps you're forcing through the wire, the wire's length and the wire's inherent resistance. I had some trouble getting consistent numbers for resistance in a wire, but the relative numbers still work. Using some ball park numbers and going back to the 1500 watt heater on a 50 foot extension cord I get a voltage drop of 2.0 volt AC drop for a 12 ga wire, 3.2 volt AC drop for a 14 ga wire and 5.1 volt AC drop for a 12 ga wire. Resistive loads like the heater element usually aren't fussy about the voltage, while inductive loads like the heater's fan motor are less forgiving.
So what? Well, while everything may still work, you're stressing the motor more with a 16 gauge extension than with a 14 ga or heavier extension. Further, the more current you try to pull through a wire, the hotter the wire gets (No! Duh!) Sometimes you can get wire failure as the insulation melts and the conductors touch. And don't forget start up loads (amps). Start up loads can double, perhaps triple the current requested. While a fuse may have a delay action to tolerate this, the wire will seem to 'bind up'; under this current draw, causing further stress on a motor. An air conditioner starting up can really spike the inrush current.
Hmmm. Watts are watts. This put me to thinking about generators. What if I added up all the watt demands I would see at one time and then chose a generator
with equal to or greater watt capacity. At that point in time I was introduced to something called the Power Factor. Something only a trained electrical
engineer can properly explain. It seems to boil down to the fact that whenever you convert power, especially between DC and AC, there are inefficiencies that get introduced. It turns out that the square wave AC voltage that most generators put out is considerably less efficient than true sine wave voltage (some Hondas and Yamahas). For converting between DC and AC and using a square wave generator
, you need to increase the requirement by about 20%. You can apparently disregard or minimize this if you use a pure sine wave generator
. As an example, I have a 45 amp DC converter in the Casita
. 45 amps times 14 volts (I believe it charges at 14 volts and to be conservative lets pretend the whole 45 amps is at 14 volts DC) equals 630 watts (45x14=630). However, converting from the generator's square wave 120 volt AC to 14 volt DC needs to be bumped up 20% (630x1.2=756 watts). So assuming a Coleman square wave generator, 756 of its watts can go to the converter/charger, while 630 watts of a Honda EU pure sine wave generator goes to the converter/charger.
To the 756 watts dedicated to the converter I'd have to add the pure AC loads, like a cube heater or an air conditioner. If my air conditioner used 15 amps at 120 volts AC, that would add 1800 watts (15 x 120=1800). So, if I had every DC load running simultaneously such that the converter/charger was generating 45 amps and I had the air conditioner running also, I'd need 2556 watts. Probably a 3000 watt square wave generator called for. Of course, if I'm only running a trickle charge on the batteries (13 volts DC at less than 1 amp) from the converter/charger, I could would probably need only 12 watts or so, and a 2000 watt square wave generator would suffice. Buy a pure sine wave generator and I'd have even more cushion with a 2000 watt generator. Lastly, if I can suffer through without the air conditioner, a 1000 watt generator would be sufficient.
So, how does this all sound? Am I on the right track? I apologize if this is intuitive to most others but I wanted to verbalize it to get some feedback.