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When it comes to buying a solar system, many people wisely turn to a local PV dealer / installer who can select the components and ensure that all of them work well together. Although this option may cost a bit more than those I’ll explain shortly, it’s a good approach. A competent local installer can answer all of your questions and take care of problems that may arise. (Be sure they really know what they’re doing.)
Unless you’re mechanically inclined and pretty knowledgeable, embarking on this process yourself can put you on a steep and treacherous uphill climb! “Even licensed electricians often need help from experienced PV installers” notes SEI. You can find a list of local installers here. Also, be sure to check out the local business directory your phone company provides.
Another approach is to buy a system from an Internet supplier. This approach can save you a substantial amount of money, as deep discounts are available through the Internet. Once the system arrives, you will have to install it yourself, or try to hire a local installer to do it for you. Bear in mind, local installers may not be happy that you cut them out of the first part of the deal!
If you do buy through an Internet sup plier, you’ll need to know quite a bit more about PV modules, racks, inverters, charge controllers, disconnects, and batteries than when purchasing a system from a local sup plier/installer. If you decide to take this route, you should read one of the books on solar electricity in the Resource Guide to deepen your knowledge. I’ve provided a lot of information in this section, but there’s much more to know. As SEI points out, “PV systems are not plug and play”.
For years, our favorite book on solar electric systems has been The New Solar Electric Home. It presents a lot of technical information in a way that is amazingly comprehensible and was recently updated. Unfortunately, most of the rest of the books on solar electricity are penned by engineers or tech-heads for whom writing is not their strength. There is at least one exception, though, a book from Johnny Weiss and his colleagues at the nonprofit organization Solar Energy International (SEI) that might be useful to you: Photovoltaics: Design and Installation Manual. This book is a manual for individuals who want to size, design, and install solar electric systems. It is very well-written and full of good information, It should bring you up to speed on the subject so that you can size and design your own system and purchase its components with confidence. Who knows, after reading it — and taking a workshop on the subject from Solar Energy International in Colorado, the Solar Living Institute in California, or the Midwest Renewable Energy Association in Wisconsin — you might want to install the system yourself!
Be very careful when shopping for and purchasing the components of a solar electric system. This requires a lot of knowledge and attention to detail, be certain that you are working with a very knowledgeable dealer who really knows what he or she is talking about and offers solid technical support. Look for a supplier who’s been around for a long time. and look for one who will sell you what you need, not what they have in surplus.
In recent years, there been an onslaught of Internet renewable energy suppliers. Some of them operate from remote sites; they have no inventory, so everything must be shipped from the manufacturers. They typically offer little, if any, technical support, lack expertise, and may have lousy return policies. They may even charge you to replace items that they shouldn’t have sold you in the first place.
As a starter, I recommend that you log on to Solatron Technologies and read their piece titled “Six Important Questions to Ask before Choosing an Alternative Energy Dealer.” (As a side note, we’ve purchased PV modules, Water Miser battery caps, batteries, and an inverter through this company — and even sold a used inverter through their website — and have found their service and products to be exceptional. Moreover, their website is one of the most secure sites from which you can order. End of free advertisement.)
Another highly reputable supplier is Real Goods. They’ve got excellent, highly knowledgeable people who can help you size and select components.
Other online suppliers provide top-notch service and products, too, but you need to be careful when shopping for an online supplier. Look through Home Power and Solar Today magazines. Check out each company’s web- site, its return policies, its expertise and level of technical support, and other aspects of their business, Does the company actually have a location or are they operating from someone home? Ask friends who may have dealt with online dealers for recommendations,
Buying Solar Panels
When buying PV modules, you’ll find that you have quite a lot of options; over a half- dozen manufacturers produce several different sizes of modules. Fortunately, they’re all pretty good, so it is hard to go wrong.
Given this, what criteria do you use to select a PV module?
If you are working with a local installer, he or she may have a favorite company he or she has had good luck with. In fact, most local installers are distributors for one or two of the major PV manufacturers. Call several local suppliers to see what each one offers.
If you want more options and want to save some money, you can contact a reputable online source, Experienced and knowledge able sales people may be able to give you sound advice on which panels will work best for you. Be sure to ask for sale items, too, You can often obtain some amazing deals this way. Also, be sure to ask if their companies offer any solar kits. Solar kits are complete pack ages that include the modules, racks, inverters, controllers, and everything else you’ll need, Kits usually include whatever type of module the supplier can order in bulk at a deep discount, which they pass on to you.
When it comes to studying your options, we found ETA Engineering’s website very useful, They provide information on modules from six different manufacturers (although there are a few important manufacturers like BP Solar missing from the list). ETAs descriptions of the various PV modules that they sell include tables that list key information you need to know when purchasing one, including power output, maximum power voltage, maximum power current, cell efficiency, and physical dimensions. You can use this information to compare modules from different manufacturers. (I’ll fill you in on this shortly.)
One criterion many solar buyers use when comparing modules from different manufacturers is the cost per watt. To deter mine this, simply divide the cost of a module by its maximum power output. The maxi mum power output is the maximum wattage under standard test conditions (1,000 watts per square meter of irradiance at 25°C or 77°F cell temperature). Sharp’s 165-watt panel, for instance, produces 165 watts under standard test conditions and costs $639 through ETA at this writing. The cost per watt is $3.87. Kyocera’s 125-watt PV module sells for $489 or about $3.91 per watt. These are good prices, by the way, although you can sometimes purchase PV modules on the Internet for as low as $2.08 per peak watt — so shop around.
While you shopping around, be sure to check out the manufacturers’ warranties. The ETA site lists this for the modules they sell, Sharp and Kyocera offer a 25-year warranty on their modules; Astropower offers a 20- year warranty. Most manufacturer warranties fall within this range,
Finally, we recommend that you also check out peak voltage very carefully, especially if you live in a hot, sunny area. PV modules that operate below 16.5 peak operating volts tend not to perform as well in hot climates as higher volt age panels. “Be wary of suppliers who do not post a solar module’s peak operating voltage” “Before buying any panel” they add, “be sure to ask what the peak or maximum power voltage (not the open circuit voltage) is for each panel”. Always look for panels that operate at 16.5 or higher (for 12 volt panels) or 33 volts or higher (for 24 volt panels) for best performance in hot weather.
Types of PV Modules
To help you in your selection of PV modules, let me tell you a bit about the types of silicon cells that are on the market today and the pros and cons of each. Solar electric cells come in three basic varieties, all made from silicon.
The first commercial solar cell produced was the single crystal cell. These are made from a purcingot of silicon — a long solid cylinder that was then sliced into thin wafers.
Unfortunately, producing pure silicon ingots required lots of energy and cutting the ingot into thin wafers produced lots of waste. The process was not only expensive; but wasteful; however, single crystal solar cells boast the highest efficiency of all three solar cells on the market today. They are about 15 percent efficient — that is, they convert about 15 percent of the sunlight energy striking them into electricity.
Single crystal cells are still manufactured today, but they’re not as commonly used in commercial PV production as their newer, less pure cousin, the polycrystalline cell (image below). Polycrystalline cells are so named because they consist of numerous silicon crystals of varying size. They’re beautiful to behold, but slightly less efficient than single crystal models, about 12 percent, compared to 15 percent. However, because they require less energy to manufacture, they are cheaper. Lower production costs easily outweigh their lower efficiency. By and large, they’re the main type of solar cell in use today.
Silicon solar cells can also be produced by depositing thin layers of silicon on a thin metal backing, producing long ribbons. This technology is known as amorphous silicon. It uses less energy less silicon, and is less expensive than either the single crystal or polycrystalline cells.
Amorphous or thin film silicon technology was first used to create tiny solar cells for calculators and watches. Although this technique is less expensive than crystalline cell production techniques, the very first thin film materials were damaged by direct sunlight and had very low conversion efficiencies — only about five percent.
In the years that followed the introduction of thin film solar materials, however, manufacturers have found ways to layer thin film materials to boost efficiency to about eight percent and to make this material resistant to photodegradation. Today, thanks to this research, amorphous silicon is being used to produce solar cells and solar modules by a company called UniSolar. UniSolar produces amorphous silicon in long rolls with sticky backing that can be applied to metal roofing (___ a). UniSolar is also using its thin-film process to create solar cells incorporated in roof shingles, as in b, although this product is not ready for prime time, according to SEI. When perfected, this amazing product will protect homes from the weather while generating electrical energy (___ b).
In recent years, thin coats of amorphous silicon have even been sprayed on glass, creating solar electric window glass that produces electricity from sunlight. This application is ideal for skylights and glass canopies, for example, over gas pumps at gas stations. It is being used in large commercial buildings, too, BP, the third largest oil company in the world, which h made a major commitment to renew able energy and currently manufactures BP solar modules, has installed solar electricity canopies in numerous gas stations through out the world.
Thin film glass and solar roofing materials are called building-integrated photovoltaics. Although you may want to consider solar roof materials, if you are thinking about installing solar electricity and reroofing your home, a coated glass skylight is probably not going to be an affordable option just yet.
Buying an Inverter
Inverters are a key component of virtually all residential solar electric systems. They come in many shapes, sizes, and prices. When purchasing an inverter from a local supplier who’s also going to install the system, your choices may be relatively limited. They typically have inverters they like and therefore will probably make a recommendation that fits your needs, Even though a local supplier! installer will do the thinking for you, you should still understand something about inverters, so they don supply you with a model that doesn’t work for you.
First and foremost, you need to deter mine whether you need an inverter for a grid- connected or stand-alone system.
Second, when selecting an inverter, be sure that its voltage corresponds to the voltage of your system. As noted earlier, solar electric systems are either 12-, 24-, or 48-volts (the most common are 24- and 48-volt systems; 12-volt systems are common in small applications such as cabins or summer cottages). This means that the panels produce 12-, 24-, or 48-volt electricity; the inverter boosts the voltage to 120 standard household current, So remember, the inverter you buy must fit your system voltage. A 12-volt inverter won’t work in a 24-volt system.
The next selection criterion is the wave output form. Here, you will find two basic options: modified sine wave or pure sine wave model. Basically, output wave form tells you how pure the electricity is. Sine wave is purer than modified sine wave. It is equivalent to the electricity you buy from the electrical grid, without the hassles of blackouts, brownouts, and dangerous power surges!
Sine wave is also more expensive.
Unless money is a problem, we strongly recommend that you purchase a sine wave inverter, not a modified sine wave inverter. Sine wave inverters produce “cleaner” AC electricity, so they tend to work much better with modern electronic equipment. In fact, some of the newest electronic equipment — like most of the energy- and water-efficient front- loading washing machines — won’t operate on modified sine wave electricity. The sensitive computers that run these washing machine just plain won work! Some laser printers apparently also have a problem with modified sine wave electricity as do some cheaper battery tool chargers. Furthermore, and here’s an important thing, I found that electronic equipment like TVs and stereos give off a rather annoying high-pitched buzz when they’re operating on modified sine wave electricity.
When selecting an inverter, you will also need to check out three additional factors: out put power, surge rating, and efficiency. Let’s start with output power.
Output power is a measure that tells you how many watts the unit can produce on a continuous basis. The Xantrex R53000 inverter, for instance, produces 3,000 warts of continuous power, which means it will be able to power a microwave using 1,200 watts, an electric hair dryer using 1,200 watts, and many other smaller loads simultaneously without problem. The Sunny Boy 2500U, made in Germany but available in the US, produces 2,500 watts of continuous power, still quite a lot for most households (that’s the size inverter I have in my house and we rarely, if ever, reach this level).
Surge rating is the wattage an inverter can put out over a short period, usually around five seconds. The Xantrex RS3000 inverter has a 7,500-watt surge power rating (60 amps). That means it can produce a surge of power up to 7,500 watts. Why is this important?
Many appliances like washing machines, refrigerators, and power tools like table saws require a surge of power when first turned on. It’s required to get the ball rolling, so to speak — to start parts moving, overcoming inertia. Typically, these devices only need this surge power for a tiny fraction of a second, but without it, the tool won’t start! (Kind of like many people who need their morning coffee to get going.)
Next is efficiency.
Inverters consume energy to change DC to AC and to boost voltage. Look for models with the highest efficiency possible. The Xantrex RS3000 has a peak efficiency of 90 percent. The Sunny Boy 2500U is 93 to 94.4 percent efficient.
If you are going to have a battery bank you also need to see whether the unit you are interested in contains a battery charger. Not all sine wave and modified sine wave inverters have the additional circuitry needed to charge batteries from an external AC source (a generator or utility power, for example). Even if you are not planning on installing a battery bank, you may want to purchase an inverter with a battery charger just in case you decide to add battery backup or go off-grid.
You should also check into noise, especially if the inverter is going to be installed inside your home. Be sure to ask about this feature upfront, and , if possible, ask to see a model you are considering in operation to be sure it’s quiet. My Trace inverter, now manufactured by Xantrex, is located inside my home, The manufacturer describes it as quiet, but the unit emits a loud and annoying buzz. (If that’s their notion of quiet, they must be deaf.) The first six months after I moved in, it drove me nuts, though now I’m used to it. The Sunny Boy inverters define the word quiet.
There are other things to look for as well, for example, ease of programming, the type of cooling system in the inverter, and search mode power consumption. My first Trace inverter (DR2424, modified sine wave) was a dream when it came to programming: all of the controls were manual, I simply adjusted a dial to change the settings and I was done. My new Trace PS2524, a sine wave inverter that works wonderfully in all respects except for programming, is a nightmare. I have found the digital programming to be virtually impenetrable; the instructions don’t help a whole lot, either. You pretty much have to be a genius to figure them out! So find out in advance, how easy it is to change settings, and don rely on the biased view of a salesperson or a knowledgeable tech person. Ask friends or dealers/installers for their opinions or, better yet, have them show you.
Inverters produce heat and need to be cooled. Ideally, you want an inverter that cools itself passively rather than an inverter that requires a fan, which consumes energy
Last, but certainly not least, is the power consumption under search mode, Search mode is an operation that lets your inverter turn off completely when there are no active loads in your home — no devices or appliances drawing power. To stay on the alert, though, inverters send out tiny pulses of electricity that search for a load (an open circuit). If you switch on an appliance, the system immediately kicks into gear and starts sup plying electricity to meet the demand.
The search mode saves energy because it allows the inverter to shut down and go to sleep. During the day that means that all of the electricity you are producing either goes on the grid, as in a grid-connected system, or goes into your batteries, in a stand-alone sys tem. Search mode consumption of under 0.5 amps is good.
Be sure when buying an inverter that the supplier, be it an Internet supplier or a local vendor, takes the time to determine which inverter is the correct choice for you. Ask lots of questions. According to the folks at solaronsale.com, “Too many inverters are sold to customers by dealers that fail to ask the customer what the inverter is being used for, which is no different than selling someone a pair of shoes without asking what size he wears”. They add, “Four of ten inverters sold in the United States are returned to the vendor due to poor manufacturing quality or due to a lack of technical support before or after the sale!” (Solaronsale.com tests each inverter before it is shipped to avoid this problem.) Furthermore, “a full 20 percent of inverters that are purchased on the Internet without pre-sale technical support will actually cause irreversible damage to the appliance that is plugged into it’
Supplying 240-Volt Electricity
Many homes require 240-volt electricity, for example, to operate appliances such as electric clothes dryers and electric stoves. As a general rule, you should try to avoid such appliances, not because a solar system can't be designed to take care of them, but because they use lots of electricity and you’ll need many PV modules to supply them.
If you must have 240-volt AC electricity don’t despair. All you need to do is purchase and install a step-up transformer such as the Xantrex/Trace T-240. This unit takes 120- volt AC electricity in and steps it up to 240-volt AC electricity. Whatever you do, don wire two 120-volt inverters together in series to double your voltage. You burn out both units instantaneously!
If you are going to install a grid-connected solar electric system with a battery bank or a stand-alone system you’ll need batteries. The more energy your home consumes and the longer the cloudy spells, the more batteries you’ll need, (As a friendly reminder: because batteries are expensive and require periodic maintenance, be sure to cut your electrical demand through efficiency and other measures first. Efficiency will save you a fortune on PV modules and batteries!)
Most batteries used in solar electric systems are 6-volt, deep-cycle lead acid batteries. Trojan L-16s have been the mainstay of the solar electric industry for years, but their dominance in the battery market has been challenged in recent years by Surrette Batteries, among others (see image below).
Batteries are wired in a combination of series and parallel circuits to produce 12-, 24-, and 48- volt systems. As you will soon see, batteries are rated by their capacity to store electricity The common measure for battery storage is amp-hours. An amp-hour is one amp of current flowing for one hour, Just to give you an idea of what you will be looking for, a battery in a solar electric system should probably store over 350 amp-hours of electricity to be useful.
When shopping for batteries, it’s hard to go wrong. Most deep-cycle lead acid batteries manufactured for solar systems are pretty good. But be sure to check out the storage capacity and manufacturer warranties first.
Surrette S460s come with a seven-year warranty. The manufacturer will replace the battery free of charge for the first two years if it fails during that period. After that time period, the manufacturer will replace it at a pro rated value,
Although lead acid batteries are less efficient than some of the newer battery technologies on the market today, old batteries are recycled. In fact, nearly 100 percent of the lead from used batteries makes its way back into the production cycle.
Solar electric systems can run on ordinary car batteries, but not for long. Car batteries are not designed for deep discharging — drawing off lots of power. They’re designed to crank out tons of amps to start a car, but they’re the rabbits of the battery world. What you need is a tortoise, a battery that can give you all it has for long periods of time. No sprinters need apply.
So be sure not to make the mistake of running a solar electric system on car batteries. It is a waste of your time and money.
Also, be sure not to purchase marine deep- cycle batteries, They are only slightly better than car batteries, You be lucky to get more than a year or two of service out of this type of battery in a stand-alone solar electric system. Both car and marine batteries are manufactured to optimize cranking power, that is, they are manufactured with thin plates to provide a surge of power to start engines. Thin plates, however, are damaged by the deep discharges that typically occur in solar electric systems.
Golf cart and forklift batteries make a better choice, as they contain many thick lead plates capable of undergoing deep discharges day after day. (Remember: they need to be recharged at night!) A properly maintained golf cart battery could last three to five years in a stand-alone solar electric system. If you can get them and get them cheaply, this might be a viable option. Bear in mind, however, that the typical golf cart battery only stores about two- thirds of the electricity of a standard deep-cycle battery designed for solar electric systems.
Far better are the batteries manufactured specifically for renewable energy (RE) systems by companies like Surrette and Trojan. These batteries have thicker plates that can withstand deeper and more frequent deep discharges than the standard golf cart battery. As a result, they have longer life expectancies — they’ll last 7 to 15 years.
How long an RE system battery can last depends on how often it is deep discharged and how well you take care of it. Generally, the more often a battery is deep discharged, the shorter its lifespan. (They’ll typically with stand 750 to 1,000 deep discharges before needing replacement.) If battery acid levels are not maintained by periodically adding distilled water or if the batteries are not periodically equalized, expect a much shorter lifespan. Also, if batteries are kept in a cold place, they’ll not function optimally. They lose capacity when cold, according to Weiss.
Battery terminals need to be cleaned at least once a year, too, If they’re not kept free of corrosion, they’ll not perform well. While you are at it, you should also be certain to keep the batteries clean. Wipe them off regularly for optimal performance. I like to spray the terminals with a corrosion-resisting product like Permatex’s Battery Protector and Sealer, sold in the automotive department of hardware stores.
Lead acid batteries designed for solar systems are your best bet. Lead acid batteries contain sulfuric acid (a 30 percent solution). Although they work well if well taken care of, lead acid batteries produce hydrogen gas when they’re being charged, either by a solar array, a back-up generator, or grid power. They also produce a corrosive acid mist. They need to be installed in a well-ventilated area away from people and sources of combustion.
Whatever you do, don buy used batteries. According Home Power magazine, used batteries are probably abused. Most of the people who have installed used batteries ended up being sorry they did. To learn more about battery maintenance, we recommend the video An Introduction to Storage Batteries for Renewable Energy Systems.
Sizing Your Battery Bank
Most professional installers design stand alone solar electric systems with about three to five days backup. That is to say, they size the battery bank so that it can supply your needs for three to five cloudy days. Generally, the sunnier climate, the fewer batteries you will need. That said, I think a three-to-five cloudy-day storage capacity is best for most climates. While creating a more substantial storage capacity may give homeowners greater peace of mind, few beginners realize that PV modules produce electricity in cloudy weather, although at a reduced rate. The system doesn’t go dead the minute a few clouds block the sun.
To determine how many batteries are needed to provide sufficient backup, you’ll need to run through some fairly complicated calculations that require computing your household’s average daily consumption of electricity in amp-hours.
Suppliers should be able to help you deter mine your needs, especially if they have worked with you to size your solar electric system. They also help you determine how many amp-hours of electricity you’ll need to store in your battery system to ensure that you can achieve the desired number of days of autonomy.
Once you determine the number of amp- hours, all you have to do is to select a battery you like. Next, divide its storage capacity in amp-hours (minus its discharge limit — how deeply it can be discharged) into the total amp- hours of electricity you need to get by for three days. This tells you the number of batteries you’ll need in your system. For example, let’s say you need 3,500 amp-hours of electricity to provide three days of back-up power. The Surrette S460 batteries you are thinking about buying store 350 amp-hours of electricity each (at 6-volt DC). If they can be discharged by 80 percent, they effectively supply about 280 amp-hours each. To deter mine how many Surrette S460s you will need, simply divide 3,500 amp-hours by 280 amp-hours, which gives you 12.5 batteries.
Other Types of Batteries
Most batteries used in solar electric systems are 6-volt, deep-cycle lead acid batteries. However, they’re not the only batteries in use today. Nickel cadmium and nickel iron batteries can also be used. These batteries can be deep discharged many more times than lead acid batteries and therefore last a lot longer. Unfortunately, they don’t store as much electricity as the standard lead acid battery and cost a beck of lot more. They’re also not widely available,
Another type of battery that is useful in certain applications is the sealed battery. (They’re also called captive electrolyte batteries for reasons that will be clear shortly.)
Two types of “sealed” batteries are currently available: absorbent glass mat (AGM) batteries and gel cell batteries.
In absorbent glass mat batteries, thin absorbent fiberglass mats are placed between the lead plates to immobilize the acid. Furthermore, the mat is a microporous mesh work that creates tiny pockets that capture hydrogen and oxygen gases given off by the battery during charging. The gases recombine in these pockets, forming water. (As a side note, AGM batteries tend to tolerate over charging a bit more than gel cell batteries.)
In gel batteries, the lead plates are separated by cavities, as they are in a standard lead acid battery However, the electrolyte (sulfuric acid) is in a gel state, not a liquid. The electrolyte is gelled by the addition of a small amount of silica gel, which turns the electrolyte into a material much like hardened Jell-O.
Sealed batteries are maintenance-free, which means they don’t need to be filled with water or equalized. This saves lots of time and hassle and makes them a good choice for very remote locations where routine maintenance is unlikely, according to Photovoltaics: Design and Installation Manual by Solar Energy International.
Sealed batteries are also spill-proof. The gel cell batteries won’t even leak if the battery casing is broken (a rare occurrence).
Because of their design, sealed batteries charge faster than standard lead acid batteries. Sealed batteries also release no explosive or toxic gases like conventional lead acid batteries, In addition, sealed batteries are much more tolerant of low temperatures than lead acid batteries, They can even tolerate occasional freezing, although this is not recommended,
Sealed batteries are commonly used for storing electricity in solar electric and wind generating systems on sailboats and RVs where the rocking motion would spill the sulfuric acid of flooded lead acid batteries, and where space is limited and batteries are frequently crammed into out-of-the way locations, They also have a lower rate of self- discharge, which means they discharge more slowly than conventional lead acid batteries,
Sealed batteries can be used for grid-tied systems with battery backup. In such instances, the batteries are typically kept at a full state of charge (they’re regularly recharged by the solar array and the electrical grid), Unfortunately, sealed gel batteries are much more expensive than flooded lead acid batteries. They also typically store less electricity and have a shorter lifespan than the more commonly used lead acid battery. Steven Strong, solar electricity expert and author of The Solar Electric House, says “They should be considered for all photovoltaic applications, especially those where site access for regular periodic maintenance is impractical”. Frankly, we’re no experts in this area, but we wouldn’t recommend them for most home systems.
Buying a Solar Electric Kit
If all of this seems like too much work, you might consider buying a system package. As noted earlier, many suppliers offer packages that include all of the components you will need for grid-connected and stand-alone systems. Solatron Technologies, mentioned earlier, for instance, offers seven different grid-connected systems that provide different amounts of energy. The kits include PV modules, an inverter, disconnect switches, and rack and mounting hardware, Their 1.9 kilowatt system purchased separately would cost nearly $15,000. The kit runs for a little over $10,000!