Our previous post, Ground Solar, Part I, attracted a lot of visitors in only a few days. Before getting into more about our hurricane experience, it would be great to fill in the gaps with some general principles that are good to know when evaluating solar panels. Last time, we gave a homework assignment, and that was to visit the Off-Grid Solar Design Guide put together by one of our sponsors, SoftBaugh. We’ll refer to that guide often throughout our solar series, and fill in some of the gaps, so be sure to check that out.
There is a lot of misinformation out on the web about solar energy, especially when it comes to promised wattage, and how much energy can be extracted from a solar array on a sunny day. Plus, there is a bewildering array of options for solar panels, chargers, batteries, inverters and safety accessories. The principles outlined in this article will help cut through some of the noise for the solar panels. Future articles will address other components.
Cost per watt: for comparing apples to apples, this is the single most important metric you can apply. Simply divide the cost of a panel by the number of watts, and there is the cost per watt. For large panels, like the 265 watt units featured in the previous post, the price per watt will probably be the lowest of all, if purchased in pallet quantities (typically 24 to 26 panels per very large pallet). As of this writing, pallet pricing in the $0.70 to $0.80 range would be attractive (plus shipping, which can be hundreds of dollars). However, these panels are bulky and hard to handle. A smaller panel, in the 100 to 150 watt range, would be well over a dollar per watt, or about twice as much. In other words, each panel, 150 watt or 250 watt, will cost about the same number of dollars in either case. This is because a large portion of the cost of the panel is the cost to make a panel, regardless of how much wattage this panel produces. The silicon in modern panels costs very little compared to the framing, glass sheeting, internal wiring and inspection costs. Small panels, such as in the “solar recharging” range of 10W to 50W, will typically be the most expensive per-watt, often two to three dollars per-watt. Prices continue to drop on an almost monthly basis, but the relative pricing remains more or less constant.
Poly versus mono and efficiency: for comparing apples to apples, these are, surprisingly, relatively unimportant metrics. Poly (short for polycrystalline) panels (the bluish, crystalline kind) are usually less efficient, and less expensive per watt, than mono (short for monocrystalline) panels (the black, smooth kind). With each type, there can be a range of efficiencies. This efficiency number boils down to how large the panel is for a given wattage. Further, within a given panel series, the efficiency dictates the rated wattage: a series may contain 250W, 255W, 260W, 265W and 270W panels, each of which appears identical to the eye. The only difference between these ratings is how they tested at the factory. Sometimes, you can pick up oddball panels within a series cheaper per watt than a lower wattage panel. Unless you are constrained for space, like on the roof of an RV or a spacecraft, even a relatively low-efficiency poly is usually the correct choice.
Shipping costs: As panel costs drop, shipping costs become more important, making all but pallet delivery prohibitively expensive on a per-panel basis. Although fuel costs have plummeted in recent years, and some overseas shipping companies are going out of business, shipping costs appear to be as high as ever. Plus, residential pallet delivery will often incur an additional $50 to $100 lift gate fee. Want to borrow a friend’s truck and go pick them up at the shipping depot? Be prepared to pay a pickup fee in the range of, you guessed it, $50 to $100.
To avoid pallets, some companies will ship as few as one to four panels by regular ground delivery (even Amazon is into this) but it is important to realize that small cracks in the edge of the glass (hidden under the frame) can grow over time, much like a crack in a car’s windshield. Keep in mind that panels are necessarily subjected to daily and seasonal temperature extremes, exacerbating crack growth. A pallet will be less likely to suffer mishandling than a chain of people knocking around individual (very large and flat) cartons full of glass. We highly recommend getting pallets of panels at a time. Get some friends together and split the costs if you need to. Make several trips to bring yours home a few at a time, maybe even lying on some sofa cushions with blankets between them to cushion the road shocks. Try this a time or two and see if you trust UPS bringing them and tossing them out the back with just some cardboard around them. That cheap panel you got from Amazon with free shipping may be fine, but it may also be a spider-webbed mess in a couple of years, and good luck getting warranty service on that.
Cells per panel: You can easily determine the cells per panel by looking at the grid. The only real impact this has on your panel is how many volts you can expect the panel to generate. Estimate about one half of a volt per cell. A 60P panel (60P means 60 polycrystalline cells per panel, for example) will generate about 30 volts, with a minimum determined by the available sunlight, and the maximum peaking out around 38 to 40 volts at the optimum conditions. The number of cells drives other system parameters, but isn’t that important by itself. 60P is usually the most economical in a panel that is still fairly easy to handle.
Voltages and currents: Two voltages and two currents are commonly mentioned when describing solar panels, and each of these is usually defined at full sun. The open circuit voltage is the voltage you will see on the panel under no load. The maximum power point voltage is the voltage developed when the panel is creating the maximum amount of power. This voltage will always be less than the open circuit voltage value. The short circuit current is the maximum current the panel can create if the terminals were shorted, such as through an ammeter. The maximum power point current is the current developed when the panel is creating the maximum amount of power. This value will also be less than the short circuit current. The rating of the panel will be the product of the maximum power point voltage and current. All four of these values will be useful to know when designing other parts of the system, as we’ll see in future articles.
If you look at the datasheet for a typical solar panel, you will notice that the power rating is about 70% to 75% of the product of the open circuit voltage and short circuit current. This relationship is more or less fixed. So, if you want to estimate how much power a panel can create in its current circumstances (sun, clouds, time of day, angle, and so on), simply measure the actual open circuit voltage and short circuit current, multiply these, and take about 70% of that value. This is the effective wattage you can expect to get, with a high degree of reliability. In a future article, we’ll discuss a relatively safe way to do this using a combiner and an ammeter without risking damage to the connectors.
Connectors: Solar panels use several types of connectors, but one of the most popular is the MC4 connector. This one is our favorite, and we have lots of spare connectors and crimpers for this family on hand. Whatever connector type you pick, stick with it if you can.
Strings and arrays of panels: Solar panels rarely are used alone for significant power applications. Typically, these are connected in series to form a string, and then multiple strings used in parallel to form an array. Each array then feeds a single charger in an off-grid system. For many important technical reasons, each panel in an array should be of similar type. If not, the system performance will be limited to the weakest panel or string. Rather than use an oddball panel, it would be better to reconfigure the array to omit it. For example, if a panel in a 3×3 array were damaged, it would be better to reconfigure the remaining eight as 2×4 than to insert a weaker oddball panel in the mix.
Aiming technology: Back when panels were relatively expensive per-watt, many approaches were explored to get the most wattage out of each panel. Now that they are cheaper, all those optimizations are now more expensive than just buying more panels and being done with it. See the next topic also.
Peak versus tail power: SoftBaugh’s Off-Grid Solar Design Guide shows the following diagram for the amount of power a panel produces throughout the day:
As we can see, the amount of power varies widely as the sun tracks across the panel, with the maximum peak in the middle, and potentially useful amounts of power in the tails. It is this wide variability that tempts one to track the sun. However, as we discussed before, this approach is no longer cost-effective. During our hurricane experience, we threw so many panels at the problem that we were only using about a fourth or third of the available power midday, but still running the freezers and refrigerators even off tail energy. As the sun came up, we tapered in loads, and tapered them back out as evening came.
There are some optional physical arrangements that can be used to help do this tapering automatically. For example, by tilting one string to the east, that power curve is shifted to the left. Similarly tilting another string to the west, its power curve is shifted to the right. Combining these two, and leaving one straight up, the three power curves wind up flatter, increasing the overall tail power at the expense of peak power, which is likely to be thrown away anyway (this approach is better done with multiple arrays because of how chargers work, but that is the topic for another day). Do not be tempted to tilt individual panels in a string in different directions as each string will be dominated by the least favorably oriented panel.
String voltages and currents: In your system, power losses are determined by current. Higher current means higher power loss, larger required wiring to overcome this power loss, and thus more expensive wiring. Because power equals voltage times current, and high voltage means lower current for a given power level, it is important to design your strings, and thus your entire array, to use the highest voltage that your charger can tolerate. This value is based on the open circuit voltage, multiplied by the number of panels in the string, plus a fudge factor we’ll explain in a future article (hint: in some circumstances, clouds can actually increase the power a panel can produce). See the solar array calculator in the design guide for help with this decision. This principle of using high voltages and low currents repeats itself throughout the entire off-grid design.
Politics: Right now there is an anti-dumping tariff in place. That tariff is beyond the scope of this article, but if it ever goes away, panel prices could get chopped in half overnight. Alternatively, an increase in this tariff could double prices overnight. This, plus the steadily declining panel prices per-watt, means that solar panels are not a good speculative investment. Buy solar panels because you want to be able to produce power and you are OK with the price today.
Hopefully, this discussion has convinced you to solve some of your solar design problems by throwing more panels at the problem than might seem strictly necessary, tempered by the knowledge that declining prices and politics limit that strategy. Next time, we’ll drill into more about tapping power off those panels and doing something useful with that power.