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November 25, 2010 by · 7 Comments 


I watched with amusement, as my camping neighbor religiously placed a small solar panel no larger than an envelope  toward the morning sun.  He did this every day – removing the panel at night and repeating the process the next day.  While I never had the opportunity to see what the rated output of the panel might be, I would estimate it to be no more than two watts.

(Photo Caption:  Typical 2-watt solar panel)

Two watts?  Some of you may be wondering what is wrong with that.  In fact, you may not even know what a watt is.  A lack of understanding should not be an embarrassment – not everyone knows the principles of electrical power.  Shucks, if they did, I would have been out of work years ago!

Teaching the basic principles of Ohm’s Law and electrical power is not the most interesting concept.  There were many times that I looked out into the faces of my students only to see blank stares or minds miles away – totally engrossed in other thoughts.

I found that one of the more successful ways to teach the concept was to use analogies that related to what students already knew.  All they had to do was compare the concept in the analogy to the electrical theory and bingo – they grasped what was all about.

Let’s start here:

It takes a lot of POWER to pull a camper.

I believe every one understand what power is.  Your truck may have a 100-horse power engine or a 400-horse power engine.  The bigger the engine, the easier it is to pull your trailer.  You know the more power something has, the more work it can accomplish. You see this everyday with construction machinery, garbage trucks, vacuum cleaners, mixers, and tools.

You also currently understand that the more power a device has the more fuel it uses. The fuel may be gasoline, diesel, electricity, or even man-power.

This leads to the simple, absolute, and factual statement:  You can neither create nor destroy energy, but you can convert it to another form of energy. Gasoline is chemical energy.  It is converted to heat energy.  The heat energy is converted to mechanical energy.

Fuel for a car or truck to develop power to do work.

Here is another analogy:  Your car or truck needs gasoline.  As long as it has gas it will do work.  When it runs out of gas the work stops.  So, you continually refill the gas tank – usually before it is completely empty.

In a camper the 12 volt appliances such as the water pump and lights are like your car.  They must have energy to operate.  The energy comes from a storage battery that is full of chemical energy.  It converts chemical energy to electrical energy that does the work you want.  The battery is analogous to the gas tank in your car.  How long the fuel in the battery will last depends on how much power is needed and how quickly you use it up.  If you use up a battery, you can possibly use a second battery.  Or, you can refill the current battery just like the tank in your car.

12 Volt RV Storage Battery (Typical)

The question is how many watts you want to refill the battery with where is  it going to come from.  This is analogous to how much gas you want to put in your car and where you get it.

The most common method of refilling a battery is from the alternator in your truck or the converter in your camper.  But, as you know to do this, the truck’s engine must be running or the converter connected to regular 120 volt AC electrical power.  When we are camping where there are no electrical connections we must find another way to refill the battery.

Our hot, bright SUN - 93,000,000 miles away.

This is when we start looking for an alternative source of power.  The most convenient and accessible power source is the sun.

The sun is capable of supplying all the power your camper will ever need.  The problem is how to capture the power from the sun and convert it to the type of energy you specifically need.  This means finding a way to put the sun’s power in the battery much like how we refine crude oil into gasoline.

Currently the best technology for capturing and converting the sun’s power to refill a battery are solar cells or panels.

Would you fill your fuel tank one spoonfull at a time?

But what size of solar panel do we need?  Let’s try another analogy:  What if you try to fill your gas tank a teaspoon full of gas at a time?  Compared to a fueling station gas pump, it will take a long, long time.

For right now, let’s think of one watt as one spoon of gasoline.  If we have nowhere to drive a car and plenty of time to wait, putting gas in the tank one spoon at a time may be OK.  Of course, it could take hours – even days – to fill the tank one spoon at a time.

Solar panels are rated in watts (and volts).  The two-watt panel I observed works like the teaspoon, filling the battery slowly so it is full enough to use later.  But if we are going to do a lot of work, we need to fill the battery much faster, so we select a solar panel that has more watts.

Have you got it now?  It is exactly the same principle we operate with everyday – we just have different terms and tools to do the job.

You’ve got to forgive me for the next part.  I know it is electrical theory stuff and requires some simple mathematics – but it is no harder than learning how to bake a cake.

You hear the term volts all the time.  But what is a volt?  Well, rather than describe a volt with pages and pages, let us just say a volt is analogous to how fast a car is going – or pressure.  Imagine we run into a brick wall at less than one mile per hour.  The wall does not get knocked down and (hopefully) nothing is hurt.  Try it again at 80 mph.  The wall is knocked down and we have a lot of damage. Why?  More pressure from a higher momentum.

Then someone comes along and throws out the word AMP or AMPERAGE.  “Hey, your camper uses a 30 amp power cord and his uses a 50 amp cord.”  OK – just what is an amp?  Again using an analogy, one amp is like one truck.  30 amps is like 30 trucks.  Thirty trucks working together can move a big rock much easier than one truck can.  So, the more amps we have the more work we can do.

WATTS is a measurement of POWER, and power is how much work electricity can do. If we have low voltage, we need more amps to make up the difference between a higher voltage with fewer amps to do the same amount of work.  In the end, it all comes back to power – which is measured in watts..

REMEMBER - The formula is as simple as PIE

I always told my students it was as easy as PIE to determine how much power we have as long as we know the  voltage and amperage.  I gave them this formula where P=Power in Watts, I=Amperage and E=Electrical Voltage (PIE).  Multiply volts x amps and you get watts – or how much power you have.  If needed, you can divide amps into power and you get voltage.  Again – it is easy as PIE!

Quick Quiz:  Which has more power?

(a) 12 volts and 10 amps

(b) 120 volts and 1 amp

(c) both are the same power.

I hope you picked (c).  Both have 120 watts of power and can do the same amount of work!

OK – Electrical Theory class is dismissed.  Hopefully you now have a better grasp of what volts, amps and watts are and how they are interrelated.

Perhaps you now understand why attempting to recharge a 12 volt, 1,200 watt-hour RV battery with a two-watt solar panel was so amusing to me.  The owner didn’t have a clue about what power really was, but he knew it was available at some level or he would not have been using even a small solar panel.  Gosh, it would take at least 600 hours of sunlight to completely recharge an empty or dead RV battery with a two-watt solar panel.  That translates to twenty-five sunny days!!

Time to put our new found knowledge to work!

All of your 12 volt camper appliances have a label somewhere that shows  either amps or watts.  In any event, we want to determine the wattage of each appliance, then we want to estimate how much time it will be used over a 24 hour period.  Use the owner’s manual or the label on the device to get the numbers and make a list of power and then how much time it will be turned ON.

Typical RV refrigerator electronic control board

If the control board for your refrigerator is rated at 3 watts it will consume 3 watts per hour for 24 hours.  This is 68 72 watt-hours.

To keep your battery from running out you will need to replace that 72 watt- hours every day.

Your have a solar panel that is rated for 15 watts output.  We get good sunlight maybe 8 hours a day.  8 hours x 15 watts is 120 watt-hours.

So, will a single 15-watt solar panel provide enough energy to refill the battery with what it needs to run the refrigerator until the sun comes up again?

YES.  120 watts is more than the needed 72.  We actually have some extra energy we can use with other appliances or save for a cloudy day.

Typical 12 volt RV fresh water pump

Of course, we will be using other 12-volt direct current appliances that take power from a battery.  Examples are a water pump, radio, possibly camper jacks and slide-out motor, maybe a fan and a light or two.  Each of these must be added to the power computation.

Have you got it?  See, it really is not hard to sort out watts, volts, and amps along with batteries and solar panels.

In Lesson 3 we will look at the size, cost and availability of larger solar panels for campers.

Until next week – Happy Camping Trails to everyone!


7 Responses to “Campsite Power – Chapter 2 FREE ELECTRICITY FOR EVERYONE FOR LIFE?”
  1. butterbean carpenter says:

    HOWDY RANDY(prof 95),

  2. jimjan says:

    Thanks Randy,

    I’m slowly beginning to see the light. Although you need never fear being out of a job if I’m working with something electrical.

  3. Stacy and Mike says:


    NO ONE ever tried to explain something to me like that. I understand! Man, where were you when I was in science class? Now I dont hane to wonder WATT you are talking about. Thanks for the lesson. 5***** all the way!

  4. Jim says:

    Prof. 3 watts for 24 hr is 68 wh ?????

  5. Professor95 says:

    Jim Wrote: “Prof. 3 watts for 24 hr is 68 wh ?????”

    Jim, if I had a prize to award, you would be the winner. While the 68 was an admitted goof (72 is correct) – I would intentionally insert small numerical errors into handouts and often during labs or lectures just to see if students were paying close enough attention to catch the error or were just floating along nodding their heads. It was always an indicator for me to tell if I was really getting through to them or not.

    Apparently, at least in your case , I was. You just made my day – I now know of one person out there in cyber-land is not only reading – but he is paying attention!

    P.S. – I have now edited out the error in the original text.



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