Solar Panel Selection

In the previous article in this series, we covered the design of an off-grid deep cycle battery. Be sure to read that article, or go back to the first article in this series for a complete overview of how to design an off-grid energy system. In this article, we cover some of the important aspects of solar panels to help select the right solar panel. This a broad subject, so we're going to cover the most important hightlights of this topic.

Practical Power Means Large Panels, or Many Smaller Panels

We often see people get excited about solar, buy a couple of 100 W panels from various retailers, and then get disillusioned when the output is disappointingly paltry. This is usually the result of deceptive marketing where a 100 W panel is sold with some cheap chargers and inverters, claiming it is a 100 watt "system", when it is, at best, a 20 watt system, more likely a 10 watt system in reality since the cheap chargers and inverters are likely throwing away a lot of power.

For the solar panels themselves, however, a minimum rule of thumb is to multiply the desired continuous power by five, and then add a little more. Want a 100 watt system? Then you'll need at least 500 watts of solar panels, and a little more would be better (for example, two 265 W panels). While you could build an array out of many 100 W panels, the cost per watt will usually be cheaper using large panels, assuming you can get them delivered without breakage. On the other hand, smaller 100 W panels represent less individual risk of breakage in use (a single broken panel only costs 100 W versus 265 watts), but cheaper (per watt) larger panels allow additional panels in reserve at the same overall cost.

Panel Configurations

There is no such thing as a standard solar panel, but there are a few common cell configurations that you will encounter. A solar panel module is composed of several individual solar cells. The size and wiring configuration of these cells determines the typical voltage, current and overall power produced by the module. After manufacturing, the cells and the panel will be tested to determine in what class that panel will be sold. In a given model series, a 260 W panel and 265 W panel will be physically similar; the only difference will be the amount of power the cells themselves produce. The table below lists some typical characteristics of various panel configurations.

Typical Solar Panel Configurations
Power 100 W 100 W 140 W 180 W 180 W 260 W 260 W 320 W
Nominal Voltage 12 V 12 V 12 V 35 V 36 V 24 V 36 V 36 V
Cells (WxH) 4 x 9 4 x 9 4 x 9 7 x 10 6 x 12 6 x 10 8 x 12 6 x 12
Open Circuit Voltage 22.4 V 21.4 V 22.1 V 44.9 V 43.6 V 37.5 V 60.0 V 44.9 V
Short Circuit Current 6.2 A 7.5 A 8.7 A 5.5 A 5.6 A 9.1 A 5.6 A 9.0 A
Max Power Voltage
and Current
22.4 V
5.6 A
17.1 V
5.8 A
17.7 V
7.9 A
35.2 V
5.1 A
35.8 V
5.0 A
30.4 V
8.6 A
49.3 V
5.3 A
37.7 V
8.5 A
Width (in.) x
Height (in.)
21.4 x
26.8 x
26.3 x
39.1 x
31.1 x
38.7 x
41.8 x
39.0 x
Weight (typ.) 20 lbs 24 lbs 28 lbs 31 lbs 34 lbs 38 lbs 44 lbs 53 lbs
Specifications shown are for comparison purposes only and are not definitive.

This chart shows just a sampling of the solar panel options found on the web. As you can see, not only is there no such thing as a standard solar panel, there isn't any such thing as a standard 100 W or 12 V solar panel, either! The 100 W, 180 W and 260 W versions show the dramatic difference in operating specs between panels. Note that the two 260 W panels are vastly different models from the same vendor (Canadian Solar). The two closest panel options, the 100 W, 12 V panels, would still result in degraded performance if used in the same array. More on the topic of panel arrays later.

Open-Circuit Voltage and Short-Circuit Current

All solar panels are characterized using the Open-Circuit Voltage (Voc) and Short-Circuit Current (Isc). These parameters are complements of each other, the former measured when the panel is disconnected from anything, and the latter when the terminals are shorted together. Both of these parameters are usually measured at a maximum light level of 1000 watts per meter. The open-circuit voltage, while not experience when under load, is helpful to determine how to size a solar panel array.

Maximum Power Voltage and Current

The maximum power voltage and current, when multiplied together, give the rated power output of the panel. Like the open-circuit voltage and short-circuit current, these are usually measured at a light level of 1000 watts per meter.

Realistic Power Output

The maximum power voltage and current values represent the available power under optimum conditions. However, most of the time your panels will not be operating in optimum conditions. Usually, some factor (sun not hitting the panels dead-on, dirt on the panels, clouds, fog, rain, snow, birds and bird extrudate, and so on) is keeping the panels from operating at their maximum power. To approximate all these effects, multiply the panel rating by 72% to get an effective operating power. For example, a 260 watt panel will really only produce 260 x .72 = 187 watts.

To avoid the limitations posed by incorrect sun angles, solar panel users once considered using tracking mechanisms to keep the panels pointed correctly regardless of the season or time of day. However, now that solar panels are so cheap, tracking mechanisms usually cost more per additional watt than simply buying more panels. Plus, tracking mechanisms can't handle all the other factors which reduce power, so it is best to overdesign the amount of panels instead.

Another limitation of solar panels is that they only produce their best power a few hours each day. Consider the drawing below.

This drawing shows an example of the output of a solar array during the day. Although some days the curve will be wider or thinner, or taller or shorter, this captures the essence of the available solar energy. In general, most of the available power is produced over a relatively narrow window of four or five hours, with some energy produced in the tails. Note that if we design our system to produce exactly the peak nameplate power, then most of the time the system will be under-performing. Also, if the 80% rule is applied, as shown by the dashed green line, then the system will always be under-performing, and the resulting power will be grossly disappointing.

Consider, however, the effect of overdesigning the capacity, as shown by the blue line. In this case, we have added an additional 25% capacity, so that the resulting degraded operation of that overdesigned system is shown by the original thick green line. During some near-optimal conditions, some power is being thrown away, as shown by the dashed blue area, but this loss is a small price to pay in exchange for more reliable power generation overall.

Note that in all cases, the tails produce very little power compared to the peak four or five hours during the day. Advanced practictioners may wish to optimize tail performance by staggering the azimuth angle of separate arrays, atmospheric degradation is a large contributor to tail losses. Accordingly, this design guide only considers arrays pointed due south, or slightly west of south.

Nominal Operating Cell Temperature (NOCT) Conditions

To account for a lot of these inefficiencies, most panel manufacturers will derate their panels using a term known as Nominal Operating Cell Temperature, or NOCT for short. Although the acronym NOCT really only means the temperature at which the panel was measured, the term NOCT has become a shorthand way of describing a derated panel. NOCT specifications generally apply for only 800 watts of light per square meter of panel, versus the 1000 watts per square meter value used for determining maximums. The NOCT derating usually results in a nominal voltage 80% that of the maximum voltage, and a nominal current 90% that of the maximum current, leading to an overall power output derated to 72 percent of the maximum full irradiance power.

We will use these derated NOCT values when sizing our array later. It is important to be able to extract the derated 800 watts per meter NOCT values from the datasheets, and if they can't be found directly, estimate these derated values from the information which is available.

If In Doubt, Add More Panels

As we will see throughout this guide, the investment in quality batteries, chargers, and inverters would be wasted if insufficient solar panels were provided on the front end. If in doubt, add a few more panels just to be sure. Given the relative costs of solar panels versus all the other components, it is better to make a mistake by adding too many panels than wasting expensive charger or battery capacity.

We'll return to the issue of arranging solar panels in an array, but first we need to talk about chargers. This is the topic of our next article: Solar Chargers ...


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