The Argosy Solar Project

Dave & Ann
Lowrey

WBCCI: 5074

So, to put it in a nut shell, the solar panels charge your batteries and your appliances drain your batteries.

In order to keep things running, in the long term you need to be putting in more than you are taking out. To do this, you need a combination of the following:

• Proper sized battery bank
• Proper sized solar panel array
• Controlled usage of appliances

When sizing a solar system, the first and most important thing you must know is: How much power will I consume?

As you may (or, if you are like me, may not) remember from your High School Physics class, power is measured in Watts. Current draw is measured in Amps. You need to determine how much current each appliance uses, and how long you plan to use that appliance per day.

Some of this is simple. If I have a water pump that draws 5 Amps, that I will run 1/2 hour per day. All I need to do is multiply the number of amps by the amount of time I use the appliance to come up with the number of Amp-Hours the appliance uses. In the case of my water pump, it uses 2.5 amp-hours (5 Amps x .5 Hours) per day.

Other appliances may not tell you amps but, instead, will tell you it how many Watts it draws. You can easily convert watts into amps by dividing the watts by the voltage you are using (which is typically 12 in a RV). So if I have a 12 volt light that draws 24 watts, and I plan to run it 4 hours a day, I divide my watts by 12 then multiply the result by the time I plan to use it. So my light will consume 8 amp-hours (24 Watts / 12 Volts x 4 Hours) per day.

What if I don't know how many amps or watts an appliance uses? I would first try to find out from the manufacturer, via their web site or calling their technical support department. If that doesn't work, you can use a table (one is included in the Resources section of this site) that gives "typical" power consumption numbers for common RV appliances. However, the best way to find out is to measure it.

To measure an appliance's current draw you need a Digital Volt Meter (DVM). You can buy inexpensive ones at Radio Shack or home center stores. You simply disconnect one of the power wires to the appliance and insert the meter, set to read current, in between the power source and the appliance. Then simply read the meter. Note that some appliance's current draw is not a constant number, so you may need to read the meter for a period of time and take an average.

CAUTION! Make sure you are dealing with a 12 volt appliance when using the above procedure. Messing with a 110 volt AC appliance in the above mentioned procedure could lead to pain, suffering or even death.

Using the above methods, I obtained the following numbers for my trailer:

I now know that I plan to use approximately 50 Amp-Hours per day.

Now it's on to: Sizing