Picking a Power Plant
by George Myers Dave Galey called his power plant the largest, heaviest, most expensive, and least used appliance in his coach. This seems to hold true for most people, as I have been told that there are very few power plants in RVs today that have more than 200 hours on them. That makes the cost per hour very high. On the other hand, when we were dry camping with outside temperatures more than 100 degrees, it was worth every penny to have it running the air conditioner. To borrow a computer term, the "killer application" for most power plants (the one that, by itself, justifies the cost) is to have air conditioning when there is no place to plug in.
We will be using the power plant and roof air conditioners in our new coach for on-the-road air conditioning. This will put many more hours on the power plant than it would get if the coach had a separate engine driven air conditioner. In our case, this is the killer application.
My purpose here is to try to give you some ideas to help make a decision as to the size and type of power plant you should buy. I would hope these ideas will help you find the power plant, if any, that is best for you and that it will not be a heavy expensive extra that you later decide was wasted money.
The first factor to consider is how you will use the RV. If you will be staying in full hookup parks almost all the time, there is little use for a power plant. The same is true if you will use the unit primarily as a snow bird in the winter. However, you must realize that you will have to do without air conditioning when you are dry camping on hot summer days. Once you decide you need a power plant, you must pick the size and the fuel you want it to use.
Size The power plant should be large enough to power everything that you need, or want, to run at the same time. There are two concepts in that last sentence that you must resolve. They are "need" and "want." If you want a luxury unit where you can have everything on at the same time, and are willing to pay the cost, add up the rated loads and give a little margin to be sure you will be happy. On the other hand, if you are worried about cost, go for the basic unit that will give them what you need, but be sure you realize you can't turn everything on at the same time.
For most people, a 4000-watt unit is probably the best choice to balance cost and needed capability. This will handle one air conditioner plus a few other things. Except for large RVs, or use in extreme summer heat, this should be adequate. If you want two air conditioners, a 6500-watt power plant is recommended, even though the two air conditioners together are rated at less than 4000 watts. This extra capacity is needed because the motors in the air conditioners take more than their rated current when they first start up. This is called a starting load. Even with units this size, you may need to turn the air conditioners on first and then turn on any other items you want on at the same time.
In the large motor home and bus conversion area, larger power plants in the 8 to 15 KW range are common. If you want to run everything without having to worry about power management, units in this range are necessary. With a power plant in the 4-6.5 KW range, you will have to limit what is on at any one time.
My coach is all electric. Even with all the electrical gadgets I have, I could not reasonably come up with a list of things I would want on at the same time that exceeded 10 KW. I truly do not see a need to run both the heaters and air conditioning at the same time, and I can turn off the hot water heater while the microwave is on, if necessary. In fact, the 8 KW unit I have will handle our needs very nicely.
There is also a problem of having a power plant that is too big. Most manufacturers recommend that the power plant be operated near its capacity. The engine may not warm up properly if it does not have enough load. Many people believe operation at a small fraction of capacity will significantly reduce the life of the engine. Where one air conditioner would be enough for a 4-kW unit, a 15-kW power plant would hardly know it was there. Operation at low power output can cause carbon build up, as well as many other problems, in gas power plants. It can also cause diesels to slobber. As I have not had this problem, I don't know what slobbering really is, but I have been assured by people with experience that it is not good.
Fuel There are three common fuels used in power plants. They are gasoline, LP, and diesel. In a motor home, it is much more convenient if the power plant uses the same fuel, and fuel tank, as the main engine. The gasoline fueled units are the cheapest, and there are several brands and models available in the 4-6.6 KW range. They do require a separate fuel tank if the RV does not have a gasoline tank already. They seem to suffer the most from disuse. I have had the choke on a two-year-old gasoline unit stick, making it unusable after sitting over the winter.
The LP units are next in price. While a little more complicated in the fuel system, they have a reputation for not getting their carburetors gummed up from sitting unused all winter. An LP power plant is usually fed off the same supply as the stove, refrigerator, etc. However, a special LP tank is needed because the power plant needs liquid LP off the bottom while the appliances use gaseous LP from the top of the tank. The tank has to be larger than the typical removable tanks used on most RVs because the power plant uses a lot more fuel than typical appliances. For design purposes, figure about 5 pounds of LP per hour.
The diesel units are the top of the line, and priced accordingly. They also weigh more, a consideration even in a converted coach. The diesel power plants are heavier both in their weight and in their design. Add the fact that a diesel engine is a simpler system, and the result is a much more durable and reliable unit. For a customer who will use the unit a lot, diesel is the fuel of choice.
I received some feedback from friends who went to the convention in Aims, IO (FMCA 1999 Summer Convention). It was hot and most people ran their power plants during the day to keep the air conditioner going. The folks who had power plants running off the main fuel tank made it through the convention without refueling. However, because most LP tanks have much less capacity than the motor home's main fuel tank, several of the folks who had LP power plants had to make at least one trip to refuel, and some had to make two.
Conclusion Almost all bus conversions have a power plant; however, I think many folks pay far too much by buying one that is much larger and fancier than they need. I remember talking to one power plant salesman who implied that any coach was trash if the owner could not turn on whatever he wants whenever he wants. This is nonsense. Everyone has other uses for their money, and the added weight does matter in the life of the coach. If you want an all electric coach, or if you often camp where there are no electrical hookups, a high quality diesel power plant is the way to go, as you will work lessor units to death and find yourself stranded. On the other hand, if you are only without a hookup for a day or two a few times a year, a smaller gasoline or LP unit should do nicely.
There is a more detailed discussion on power plants in "Designing Electrical Layouts for Coach Conversions." The material is in the text (starting on page 10) and in an appendix in the latest version.
George Myers is a retired 32 year veteran Electrical Engineer with over twenty years of experience working on bus conversions. George wrote a monthly feature article for Bus Conversions Magazine entitled "Electrical Shorts" and is now producing "The Coach-Builder's Bulletin", www.coach-builder.com, along with his wife Sue, for their company Epic Conversion Support.
