A LITTLE LIGHT ON THE SUBJECT
Solar electric systems generate electricity when the solar panels are exposed to light. Full and direct sunlight exposure will produce the most power from the systems, however, the systems will generate electricity anytime they are exposed to light. The electricity produced by the sun can be used to power direct current (DC) devices such a battery or it can be converted into normal household type power that will run any device currently powered by utility power. Solar systems that produce electricity are sometimes referred to as "photovoltaic" (PV) systems and contain NO WATER whatsoever. The systems are simple, reliable, quiet, emissions free, and long lasting. There are no moving parts in a typical system, and recent advances in manufacturing processes have boosted performance while also decreasing the cost. A typical solar system is an integration of a few basic components: a group of solar panels to generate electricity, a series of safety disconnects to protect the system, and an "inverter" to convert the electricity produced from the solar panels into usable household type electricity. Inverters are also designed to work in sync with utility power if the outside power meets minimum standards of quality. Inverters that supplement utility power in this way are called "utility interactive" inverters or "grid tied" inverters. If back-up power is also desired for use when the utility power suffers an outage, or if there is no utility power available in a remote area, then a different type of inverter is used to convert the solar power into usable household power independently of any utility power source. "Off grid" systems such as this require a means to store excess power for use at night, when the solar system is not generating power. Traditionally, a set of specialized deep-reserve type batteries are used to power the inverter at night until the sun comes up again and powers the loads and also recharges the batteries. Finally, there are hybrid solar systems that utilize batteries only when utility power fails. These sorts of systems send excess power to the utility in sync with the utility when it is normally available but will also provide power to your loads via the solar array and batteries should the utility power suffer an outage.
Below are brief descriptions of the individual components that make up a solar system and how they are generally integrated into either a power system that interact in sync with utility power or used independently from any utility power system ("off grid").
SOLAR CELLS, MODULES, & PANELS
The building block of the solar panel is the solar cell (shown above). It is what generates electricity when it is exposed to light. Made of treated silicon (a type of sand), the photovoltaic cell generates electricity when bits of energy (photons) from the sun are absorbed into the atomic structure of the cell. As photons enter a solar cell, they "knock loose," or displace part of the silicon atom known as the "electron" and a small "direct current" (DC) voltage is generated. The DC voltage is produced with varying amounts of current (amperes) depending upon the size of the cell and the amount of sunlight the cell is exposed to. Since this process takes place at the atomic level, solar cells produce power without any moving parts, without making any noise, without emitting any radiation or pollution, and without any periodic maintenance or re-fueling. So long as the cell is exposed to sunlight, electricity will flow. Solar cells are highly reliable, and produce power for many decades. Most manufacturers carry a 25-year warranty. This is not because they "wear out" in 25 years, but rather because a warranty for longer than several decades is not a viable business insurance model.
Solar cells produces a voltage similar to that found in batteries, and the electricity produced from a solar cell will similarly power any DC operated appliances if the voltage were high enough. A single solar cell generates about ½ volt with varying amounts of current (called "amperes"). This is true regardless of the size of the individual cell. Since this relatively small amount of voltage is generally not enough power to operate most appliances, manufacturers boost the voltage by connecting several cells together in series to boost the voltage. Historically, 36 individual cells were wired together into a framed weather resistant enclosure called a "module" to achieve about 17 or 18 volts. This was a popular size because the solar industry grew up around and connected to batteries . . . which are most commonly 12 volts . . . and they were connected to a solar module. This design has changed in the last several years however, as most solar systems have completely eliminated the battery component to work in conjunction with the utility power. Modules now more commonly include more cells in wired in series to achieve higher voltages per module to more efficiently connect with other modules to boost the voltage even higher. The most efficient inverter on the market today for example requires an input dc voltage from the solar array of 200-450 volts dc to accomplish its task.
A solar "panel" is defined as such when two or more solar "modules" are wired together in series. If several solar panels are connected in parallel then you suddenly have a solar "array." Solar cells, modules, panels and arrays all share the same power generating characteristics, so the difference is more a matter of how the photo-voltaic technology is packaged. Regardless of packaging, the amount of square footage exposed to light will hold a few basic power generation potentials. As a rule of thumb, the power generation capability for a photo-voltaic system is that it will generate approximately 1,000 watts (1 kilowatt or KW) for every 100-square feet that is exposed to direct sunlight over a period of one hour. Thus, if you see a generator rated at 2.8 KWh you can estimate that it will produce 2.8 kilowatts (KWh) every hour it operates. A "watt" is a handy unit to help us compare overall power when there is a wide difference between voltage and the amount of current that the voltage carries. Watts are simply volts multiplied by the current (or amperes).
INVERTERS
Once a source of electricity is generated from some source, it needs to be regulated and converted into a stable and usable form of power.
Inverters are devices that accomplish just that. Inverters designed for use with solar pv systems may also be able to convert the dc power from the solar array into a pure sine wave alternating current (AC) that can match the phase and voltage of another AC source they are connected to. Usually the other source of AC power the inverters match is your local utility power. In this way inverters are said to interact with the utility grid and supplement its use. There is no power "storage" ability for these sorts of inverters, they simply and very efficiently convert the dc power from the solar panels directly into power that is immediately available for use by whatever circuits are connected in your service panel. Since the "fridge" or the fax machine don't care where they get their power from, they are powered by whatever power comes down the line. This also means that power from the utility grid may not enter your home or business at all when the inverter is generating power sufficient to power all the loads at any given time. In fact, if the solar system/inverter conversion produces more power than your loads at any given time, the excess power literally flows back into the utility for them to sell to your neighbor!
When this happens your power meter will stop rotating forward and will actually rotate backwards. You will receive a credit for this power in the form of power you never purchased... and in this way you will "store" your credit with the public grid for use when the sun goes back down. It is an ingenious way of "banking" your power with the public grid system for use later when the sun goes back down. In sizing a solar system for your use, the very best size is the one that will "spin" your power meter backwards during the daytime the same number of revolutions you typically spin it forward during the day and night! Grid interactive inverters operate completely automatically and require no additional re-wiring of your home or business to operate, other than the installation of a circuit breaker in your service panel.
Utility interactive inverter solar systems are especially well suited for use where your utility power rarely fails. However, since they derive their ability to operate from matching the power characteristics of the utility power source they are attached to, they will immediately shut down if the utility suffers an outage. This is done by design to protect the utility lineman who spring into action to repair the power lines during an outage. The lineman don't expect power to come from a house and if the systems did not shut down there would not be a way to shut off power in the lines they are repairing. All grid interactive inverters are therefore UL listed to immediately cease generating when they lose their AC hookup source.
Inverters that are non-utility interactive have a means to generate the timing and phase characteristics to power conventional AC loads independent of any additional power source. Inverters of this sort are called "off grid" or standby type inverters and they require some means of storage of the excess power that is available from the generation source (such as solar). In these sorts of solar power systems a specialized set of deep reserve type batteries are incorporated. In concept, the inverters are actually powered from a battery, and the batteries are charged by the solar panels. In this way, the solar system must be made to "govern" itself around the battery bank by means of various charge regulators or controllers. The solar industry got its start with these sorts of systems and they are well suited for locations that have no utility power available.
SAFETY DISCONNECTS, WIRE, & SUCH...
In order to satisfy the National Electric code a few circuit breakers and disconnects must be designed into your solar system. The first and most obvious is the installation of a circuit breaker located with your service panel. This circuit breaker both provides a means of attaching your solar system to the rest of your electrical system but also provides another layer of protection for your inverter in case of extreme power surges. AC circuit breakers are sized according to the amount of amperage they are allowed to pass before they "trip" or disconnect. Your electrician is well versed in the proper sizing of AC circuit breakers. In addition, the specific circuit breaker size will be called out in your inverter owner’s manual.
Another necessary safety disconnect should be placed between the solar panels and the inverter to allow servicing (if ever needed). In addition, an individual fuse is required for every solar "string" or series of solar panels that you attach to your inverter. The fuse size should be sized according to the rules stipulated in article 690 of the NEC.
If your solar array is to be placed upon the rooftop of a dwelling then it should have a ground fault protection (GFP) circuit built into the system. Most inverters will have this circuit already built into their system or will offer it as an option.
Wire sizing is another important aspect of your solar power system. Generally, a wire needs to be large enough in diameter to prevent it from heating up when current is passed through it. You need to protect the wire from heat buildup caused by resistance and its insulation should also be rated to withstand external and ambient temperatures from degradation. This rating is usually a "THHN-2" type of copper wire that can be purchased at any electrical supply store. As a rule of thumb, a high voltage power system will require a smaller wire size as it will pass a smaller amount of current to pass the same number of watts; and a lower voltage power system will require a larger wire size diameter to pass the higher amount of current. When sizing wire, the length of distance the power travels is also a consideration as a longer distance will dictate more wire and therefore a higher resistance to power transmission.
Finally, since solar pv is still in its infancy as a mainstream technology, some local utilities still require an additional ac disconnect to be placed somewhere near the power meter that serves to disconnect the power between the ac output of the inverter and the circuit breaker in your service panel. This is a holdover requirement from the old "co-generation" era of non UL listed automatic cutoff capabilities that all utility interactive inverters incorporate. Check with your local utility company to see if they still require this additional and redundant disconnect.