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"Real World" Operating Conditions for Solar Systems used on Boats, RVs or Off-Grid!

"Real World" conditions for a solar system on an RV are quite a bit different than they are for the same system used in an off-grid solar home. One giant difference is the placement of the solar panels.

An off-grid home can put the panels on a pole mounted, dual-axis tracking mechanism where the panels face the sun squarely and track it across the sky from dawn to dusk. Panels mounted on RVs are usually laid flat on the roof, don't face the sun squarely and don't track it across the sky throughout the day. The difference in the amount of power being delivered by the solar panels in each situation is substantial! Using solar panels on RVs has some challenges but they can be minimized.

Let's look at all the places where solar power is lost in a typical RV installation and see how we can address them. First, a quick review of the equation for power. Power is measured in watts and Watts = Volts x Amps. So, anyplace that the Voltage and/or Amps is reduced there will be a drop in the power (watts).

What causes Voltage Drop?
1) Anytime the temperature of the solar cell exceeds 25'C (77'F), there will be a drop in voltage from heat. As the heat increases, so does the amount of voltage dropped. When you consider that I've measured cell temperatures of solar panels on the roof of RVs above 65'C (150'F), this is significant. This is one reason why we have custom panels built with higher operating voltages. It is also why solar panels should be mounted with a few inches of clearance under them so the heat can escape instead of building up.

2) Every Amp traveling through every foot of copper wire has a known and measurable voltage drop. Smaller wire will have more voltage drop than a larger wire if they are carrying the same amperage. Voltage drop is also relative to the temperature of the wire. The higher the temperature of the wire, the greater the voltage drop. This is why we use larger wire sizes as the wattage of our systems increase and yet another reason we use higher voltage solar panels. Example: Our GO100 solar panel operates at 18.5 volts and 5.42 amps (18.5 x 5.42 = 100.27 watts). A competitor's 100 watt panel operates at 17.0 volts and 5.88 amps (17.2 x 5.88 = 99.96 watts). Assuming 15 feet of 10 gauge wire between the solar panel and the combiner box, our solar panel will deliver 99.39 watts to the combiner box. Their's will deliver 98.9 watts. That 0.49 watts may seem small but it is a significant difference in power delivered, especially as you add more solar panels to the equation. The difference in power also grows when you add in the rest of the wire harness and the path it takes to get to the batteries through the charge controller.

3) Every connection made in the wire harness has some resistance which causes some voltage drop including the connections to the solar panel, combiner box, charge controller, fuse or circuit breaker and batteries. Corrosion at these connections also causes voltage drop. This is why we use high quality copper lugs, crimps and ring terminals in our kits and protect them from corrosion by using glue-filled, heat-shrink tubing there.

4) Just passing the power through the electronics of a charge controller causes voltage drop. There is no charge controller out there that operates at 100% efficiency. They all have varying degrees of voltage drop depending on the quality of the electronic components used and how much power they are processing. This is why we choose high quality, high efficiency controllers to put in our systems.

5) Fuses and circuit breakers are designed to make the current (amps) pass through a small section of wire or metal so that if the current rating of the fuse or breaker is exceeded the wire will melt or the circuit breaker will open. There are varying degrees of voltage drop at these devices depending on how much power is passing through them.

What causes a reduction in Amperage?

1) The amperage (or current) a solar panel puts out is a straight line correlation with the intensity of sunlight shining on it. If the panel is receiving the equivalent of 1000 watts per square meter of sunlight, you will get the rated amperage of that panel. If it is receiving 500 watts per square meter, you will get half of the rating. If it is 100 watts per square meter, you will only get 10% of the rating.

2) Shade causes a rather dramatic drop in amperage. Even if only a portion of a few cells are shaded, it can cut the amperage by half or more! So pay attention to where the panels are placed. Keep them away from antennas, satellite dishes, and air-conditioners. Try to assure that they will at least see the 3 hours either side of solar noon without any shade because this is when you will get 90% to 95% of the day's solar power. Morning and evening light are less powerful and are basically "icing on the cake" so you needn't be too worried about those times of the day. However, zero shade from dawn to dusk is ideal.

3) The charge controller requires some power to operate its logic, LED lights and display. This is expressed as "Self Consumption" or "Quiescent Current Draw" in the technical specifications of the controller's data sheet. These little parasitic draws can add up. This is why we choose high efficiency controllers to use in our systems.

Summary of "Real World" Operating Conditions for RV Solar Systems:

1) Solar panels on RVs operate at higher temperatures than those used to define Standard Test Conditions (STC) and Normal Operating Cell Temperature (NOCT) ratings. This means the power ratings for solar panels used on the roofs of RVs will be less than the same panels used in other applications.

2) Solar panels on RVs are typically laid flat which means they are not squarely facing the sun. Even if you tilt them up to the south in winter, they only face the sun squarely for about an hour per day since you are not tracking the sun from dawn to dusk. This means the power output from solar panels used on the roofs of RVs will be less than the same panels used on a pole mounted rack facing the sun.

3) The wire used in RV solar systems is typically exposed to higher temperatures (because it is exposed to direct sunlight on the roof) and somewhat longer runs than in other applications (because it travels odd paths to get from the roof to the controller and from the controller to the batteries). This will cause some power to be lost in transmission between components. Our roof wire is round and light grey in color so it will operate at cooler temperatures than black wire that is flat like the commonly used "Tray Cable" in other systems.

4) Charge controllers used in RV systems are typically placed in areas where there is limited availability to move air over them to dissipate the heat generated when they are processing power from the solar panels. Higher temperatures mean less efficiency which means some power lost.

These conditions are different than almost every other application and require some thought when you are considering a system for use on an RV. The next topic will include these considerations when we go about designing RV solar systems for use in their unique "Real World" conditions.

 
 
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