Potential energy

WilliamA

I've been kicking this around a bit as a topic for discussion as it comes up often. Perhaps it deserves a thread of its own so here it is.
Hauling energy around is something that just needs doing. We all do it as part of the job of camping. The how and why come in many different shapes and sizes. Basically, it comes down to a combination of three choices: battery, solar or generator. I don't include plugging in to shore power as that's standard equipment. At its core, we are just hauling different kinds of potential energy. Batteries store a finite amount of energy, generators have, by virtue of fuel capacity, a fixed amount of potential energy. You could make the argument that solar power is unlimited, but that has drawbacks as well. It really comes down to how much you need and how you use it.
Inductive versus resistive energy:
It's often bantied about but not so well understood that, for our purposes, energy demands fall into just two categories. At the end of the day, the total energy used for either is less important than how it's made. Resistive energy can be thought of as a constant. I'll use lighting as my example. Turn on a light and energy "starts and runs" at a fixed rate. If a light takes 5 watts to run, it will not take more than 5 watts to "start up". This resistance won't ever change (for all practical purposes anyway) during the life of the light. Anything with a motor, however, uses "inductive energy". Motors take much more energy to start than they do to run. Why? When you first apply energy to a motor it is in a stalled state. Motors have very low resistance, almost zero, when they are not turning. They are by design, a short circuit. As they spin up to speed, the resistance gets higher. That's because the armature is spinning and the brushes only contact the armature as a short circuit for a fraction of a second. Additionally, the load that the motor is turning (think: A/C compressor) is also stalled and therefore needs a lot of power to coax into moving. That's inductive load. Examples of this are the A/C unit, Microwave, ceiling fan and cooling fans for the converter and A/C housing. The small 12 volt fans take very little power, nevertheless they take exponentially more for that fraction of a second to get going. So if all you need is resistive power for lights and clocks and radios, then batteries will do just fine.
Heaters:
There are a few kinds of heaters but most have some sort of fan motor to push air around. The electric furnace I have in my trailer usus, oddly enough, mostly resistive energy. It uses 800 watts for the elements but never takes more than that to "start up". What that means in the real world is that I can calculate my power consumption as 800 watts. There is no "start up" (surge) wattage like in an A/C unit or refrigerator. (To be fair, my furnace has a small fan motor which draws surge wattage, but it's tiny in comparison to the size of the heating elements.) So that means I can calculate my start up and run wattage ad essentially the same. For an A/C unit's inductive load, I need to have a power source that has twice the running energy to start it up. If the A/C unit is rated at, say, 750 watts, I need to have a power source capable of producing twice that, or 1500 watts.
If we didn't have heaters or air conditioning, it would be easy to go a week on the stock battery. There's not much in the way of inductive load that needs to be considered. Few of us, however, do without those things for long.
So what remains is how to carry the energy we need to run the things we want. Back to the basic kinds of stored energy we go.
The best batteries and solar cells in the world will still provide low voltage direct current. Low voltage d.c. Is a fine way to power modest lights, radios and entertainment systems. Heating (including crock pots, toasters and the like) simply draw more power than it's handy to haul around. I won't stump a primer here on Ohms law but suffice to say, if you need high energy (let's just say, more than 200 watts) you need a/c voltage. You simply can't haul anough batteries to get more than that for more than a couple hours. Yes, you can use an inverter to convert d/c to a/c, but the same rules still apply, plus, converters use power and are not particularly efficient either. You end up adding problems as fast as you solve them.
Batteries have half of their rated power available to use. You wouldn't draw your battery to zero any more than you'd drive your TV until you run out. Any energy calculation must be made upon half the capacity of the battery. A generator can produce 100% of its carried capacity. If your generator is rated at X watts and the gad tank holds Y gallons, you'll get X watts for as long as the generator has fuel. It's easy to calculate how much energy you can haul. It's X watts for Y hours. If the gennie puts out 2000 watts and can run for 6 hours on a tank, you have 12000 watts available. Easy. By comparison, if a battery is rated at X amp hours, by converting to watts, then dividing by 2 (half of its capacity) you have its total available energy.
Solar cells are a variation on a theme. Watts available based upon their size and output. The gigantic variable there is the weather. It's reasonable to assume that most of us use much more electricity at night. So any solar option takes us back to batteries.
I get most of my power from my Jeeps generator. It charges my trailer battery as I travel. When I can, I plug into shore power for convenience. When I want to use my heater (most of you can say A/C) off grid, I use my generator. Whichever way works best for you, bear in mind that the best, most efficient way to haul energy around is with a generator. For those who don't like gennies (even the best generator is, or can be, a hassle) then it remains to choose between the remaining options.
Generators are heavy and cumbersome, but so are a couple of high amperage batteries. By any standard other than convenience, generators, being a poor, distant cousin to shore power, are still the most efficient way to carry lots of power. Nothing else comes close. The math requires it.
So for the vast majority of us, the answer is to plug in (one way or another) or do without...
WilliamA

TomD

Awsome information. I just finished 3 nights and 4 days of Boondock camping at around 57 degrees of temp per day and night. The 6V battery system I installed was still at 60% after 4 days. Norcold, Fantastic Fan, Lights, I-Pad Movies, and Water Pump.

GulfCoast

"If the gennie puts out 2000 watts and can run for 6 hours on a tank, you have 12000 watts available."

A small typo here. But so no one gets confused, that should be 12,000 Watt-hours or 12 kWh.