inverter help please!

[pvcces]

Ruthi, heating and cooling loads are the most inefficient uses of electricity in our coaches. While household power is often referred to as KWH, most of the time RVers use amp hours. This means that a translation is needed to figure out what to do.

If you can get in the habit of thinking in terms of KWH consumption and cost, it will be easy to see what you need to do.

To start with, household power is often available for around $0.10 per KWH, while RV power can easily cost $1.00 per KWH. If you paid that much for household power, you would avoid using it for heating and cooling.

A gallon of diesel fuel might produce 15 horsepower hours, or about 12 KWH. This means that $4 per gallon fuel will cost about $0.33 per KWH. If you use gasoline, you are looking at around $0.50 per KWH and if you use propane, you could be looking at $0.65 per KWH. This is just the cost of the fuel; there is a lot of other cost to consider in running a generator.

For comparison, a golf cart battery can hold a charge of about 1 1/3 KWH, of which you should only use about 1/2 KWH before you recharge. All this boils down to the idea that it is expensive to store electrical power. We usually just store the fuel and produce the power as it is needed. In this comparison, 3 ounces of propane and one golf cart battery are nearly equal in stored energy.

All this may seem complicated, but it is really much easier once you use a common language for dealing with your power needs.

One other thing; running an AC unit and a big refrigerator is counterproductive. It is very easy for them to interfere with each other if you are pushing your generating capacity. A carefully picked low power consumption refrigerator will save you a lot of money. Many new low power refrigerators only use 450 KWH per year. That's around $45 at home, but more like $150 in your bus. A big refrigerator can easily use 3 times as much power.

Good luck working out your power needs.

Tom Caffrey

[artvonne]

While household power is often referred to as KWH, most of the time RVers use amp hours.


If you can get in the habit of thinking in terms of KWH consumption and cost, it will be easy to see what you need to do.

To start with, household power is often available for around $0.10 per KWH, while RV power can easily cost $1.00 per KWH. If you paid that much for household power, you would avoid using it for heating and cooling.

A gallon of diesel fuel might produce 15 horsepower hours, or about 12 KWH.

For comparison, a golf cart battery can hold a charge of about 1 1/3 KWH, of which you should only use about 1/2 KWH before you recharge. All this boils down to the idea that it is expensive to store electrical power, 3 ounces of propane and one golf cart battery are nearly equal in stored energy.


Good luck working out your power needs.

Tom Caffrey

  Tom, this is wonderful advice, thanks for sharing. I often try to calculate out things like this, its kind of fun comparing and understanding different energy sources and efficiencies. Its also interesting how closely the fuel companies have priced fuels to BTU energy.

  Some here may wonder why the Gas generator, or propane Gen, can cost more to operate with cheaper priced fuel. Its because the diesel is a much more efficient engine. It burns less fuel per horsepower than any other engine in reduced power applications. At full power they begin to match gas engines, but because they dont generally operate at full power 100% of the time, they will pay off.

  Another consideration to remember, is that anytime energy is converted into another form, you can generally figure a 10% (or greater) loss. So running an inverter, as is common in an RV, where you convert (invert) 12 or 24 volts DC, into 115/230 volt AC, you lose roughly 10% in the process. If your charging your batteries running your generator, your burning precious diesel to do the job. For every KWH you put into your batteries, 10% or more (0.1KWH) is lost through your battery charger everytime you use it. So at the end of the day, your burning roughly 20% more fuel everytime you use your inverter. IOW, it will be MORE efficient running the gen to directly run your AC, than charging batteries with your Gen and running the inverter to power the AC.

  The best approach then, would be to keep as many electrical loads using the most common readily available energy source. In a Bus this would be 12 or 24 volt lights and appliances. In heating or cooling, fans and pumps are high energy loads. Also, when moving fluids, air or water, its always more efficient to move large volumes slowly, than small volumes rapidly. Small high speed duct fans and small pumps, pushing water or air through small pipes and ducts, are by their very nature inefficient. You always give up energy efficiency for space efficiency. Think back to those monster pipes in the basements of old houses, that was gravity heat. No pumps, fans, or anything electrical needed to run them. For as inefficient as some of those gravity burners were, if you had to run your Gen to operate the electrical loads used today, the old gravity system would blow you out of the water. Cheap energy is whats given us inefficiency.

  And remember to keep fans, radiators, evaporators and condensors clean. Ive seen graphs that show a fan with just a fine film of dirt can lose as much as 20% airflow. Now your just burning KW's (diesel) doing nothing. And if the thing its blowing through is dirty, well, you can see it. Same goes for the radiator squirrel cages on MCI's. If those fans are dirty they arent going to move as much air.