System Sizing:  By Actual Use

There are many ways to calculate what size system will meet your needs. Some are quite involved mathematically, some utilize an accumulating amp-hour meter, and some are basically "rules of thumb". Here are two methods that have proven to work best for us, based on actual use by you, the customer.

We use cardboard rectangles to plan our solar panel layouts.

We use cardboard rectangles to plan our solar panel layouts.

Method #1
Our recommendation is for you to go out and "boondock" in your RV (without running your generator or plugging into shore power) for as long as it takes to run your batteries down. Use electricity like you want to, and don't change your habits while you're doing it. This will indicate how much power you consume on the average day. 

For Example:
Let's say you were able to "boondock" for two days before you noticed your batteries getting weak.

First, we have to determine what your batteries’ storage capacity is. Let's say you have two (2) relatively new Group 27 deep cycle batteries that are rated at 100 amp hours of storage each. This means you theoretically have 200 amp hours of energy to draw on (2 x 100 = 200). However, only about 50% of that is safely usable so you really only have 100 amp hours of energy to draw on (0.5 x 200 = 100).  NOTE:  Drawing 80% of the charge from lead acid batteries is possible, but could harm the batteries.  We recommend only drawing 50% during daily usage.

Once we have established what your battery bank storage capacity is, we divide it by the number of days you "boondocked" (in this example it was 2 days). So, 160 amp-hours of storage divided by 2 days = 80 amp hours of energy consumed on the average day.

Now we need to determine how many solar panels you will need in order to replace that 80 amp hours of energy you consume per day. We will assume that you use your RV during the sunnier half of the year and/or you follow the sun south during the darker half of the year. This will give you an average of five (5) "peak sun hours" per day.

A 100 watt panel produces an average of about 6 amps per peak sun hour, or about 30 amp-hours per day.

Given the above example, you would need three 100 watt solar panels to fully recharge on the average day (80 / 30 ≈ 3).

For additional accuracy, we highly recommended installing a Battery Monitor before you go boondocking. These devices record your consumption, and give you a reading that tells you how many amp hours were removed from your batteries. This eliminates guesswork and mental math. You will know what you used, and can then make an informed decision on what size system you will need to meet your lifestyle.

Method #2
This approach works best if you have very modest energy consumption.  For example, if all you want to power is a blender and a TV, you may be able to get away with a relatively small solar charging system.  The first thing you will want to determine is the wattage of each of the discretionary devices you plan to power.  This is usually printed on the actual device or in the owner’s manual.  If you can’t find this information, purchase a Kill-A-Watt Meter at a local hardware store.  The Kill-A-Watt meter will tell you the exact wattage of your device.  Multiply that by the average run-time of each device and sum the total.

If your blender consumes 1500 watts and you plan to use it for 2 minutes each day, your blender consumption would be 1500W x (2/60)h/day = 50Wh/day.
If your TV consumes 150 watts and you plan to use it for 3 hours each day, your TV consumption would be 150W x 3h/day = 450Wh/day
Now, sum the daily watt hour consumption for each device. 50Wh/day + 450Wh/day = 500Wh/day.  
To convert this into Ah/day, divide it by 12V.  500Wh/day / 12V = 42Ah/day.
Add to this your base load (refrigerator in LPG mode, gas detectors) = 34Ah/day
Based on the example above in Method #1, you would need about three 100 watt solar panels to produce 90 amp hours of charge per day to cover the 76Ah of daily consumption.
It is our experience that most RVers consume between 75 and 150 amp hours of power per day depending on their lifestyle and degree of frugality. This means that some people will only get about one day of boondocking out of the battery bank used in the above example. These folks would need three to five 100 watt panels in order to break even on a daily basis.  We regularly install systems that produce more than 300 amp hours of power per day!