An Overview of Solar Power

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Burbank Water and Power's Solar Support Rebate Program

An Overview of Solar Photovoltaic (PV) Power

We appreciate your interest in solar PV power generation and hope the overview presented here helps further your understanding of solar PV technology.

What is a Solar PV cell and how does it work?

You've probably seen calculators that have solar cells -- calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You may have seen larger solar panels -- on emergency road signs or call boxes, on buoys, even in parking lots to power lights. Although these larger panels aren't as common as solar powered calculators, they're out there, and not that hard to spot if you know where to look. The first solar cell arrays were on satellites, where they were used to power the electrical systems.

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They can even power your house!

How do these devices work? PV cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a direct current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

How all the parts of a solar PV system work together

Now that we have our PV module, how do you power your house with solar energy? Although it's not as simple as just slapping some modules on your roof, it's not extremely difficult to do.

First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy. Non-tracking (most typical) PV systems in the Northern Hemisphere should point toward true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year-round. In Burbank that's about 34 degrees. A different orientation and/or inclination could be used if you want to maximize energy production for the morning or afternoon, and/or the summer or winter. Of course, for optimal operation, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In most PV modules, if just one of its many cells is shaded, power production will be reduced by more than half.

Several things go into the decision of what size system you need. This is complicated by the fact that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary.

With that data, and knowing your average household usage there are simple methods your solar installer can use to determine just how many PV modules you'll need. Your BWP utility bill has this information or you can call BWP at 818-238-3700 to obtain billing history. You should design for slightly below your average needs since BWP does NOT pay nor allow you to bank excess electric generation over more than the course of a year. Your installer will also need to decide on a system voltage, which will be determined by the type of inverter used (see below) and this will determine how many modules are wired in series.

Some problems solar PV faces and possible solutions

You may have already guessed a couple of problems that you'll have to solve. First, what do you do when the sun isn't shining? Certainly, no one would accept only having electricity during the day, and then only on clear days, if they have a choice. You need someplace to store the extra energy you generate. In remote locations with no electric utility service, this is done with batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system. Currently, however, it's a necessity if you want to be completely independent from BWP.

Fortunately there is an alternative that solves this problem since BWP allows you to connect your solar PV system to the utility grid under certain circumstances. You buy power from BWP when you need it and bank a credit by turning the meter backward when you produce more than you need. This way, the BWP grid acts as a convenient storage system and the meter becomes your "bank." This method of crediting you for excess power is required by law in California and is known as "net metering." (See Net-Metering - How does it work? below.)

If you decide to use batteries as backup in case of an outage, keep in mind that they will have to be maintained, and then replaced after a certain number of years. The PV modules should last 20-25 years or more, but today’s batteries don't have that kind of life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them.

The other problem is that the electricity generated by your PV modules, and extracted from your batteries if you choose to use them, is direct current (DC), while the electricity supplied by your utility (and the kind that every appliance in your house uses) is alternating current (AC). So you will need an inverter, a device that converts from DC to AC. Most of today's sophisticated inverters will also automatically control how your system works.

The inverter needs to have special electronic circuitry to make sure that the power you sell to BWP is synchronous with our generated power -- that it shares the same sinusoidal waveform and frequency. Safety is a big issue here! BWP also has to make sure that if there's a power outage in your neighborhood; your PV system won't try to feed electricity into lines that our lineman may think is shut off. This is called islanding. Inverters approved for use in Burbank are designed to not allow islanding to occur.

Throw in the mounting hardware, wiring, junction boxes, grounding equipment, over current protection, DC and AC disconnects and other accessories and you have yourself a system. Electrical codes must be followed (there's a section in the National Electrical Code just for PV), and it's highly recommended that the installation be done by a licensed electrician who has experience with PV systems. Once installed, a PV system requires little maintenance (especially if no batteries are used), and should provide electricity cleanly and quietly for 20 years or more.

There's only one drawback, the cost!

If photovoltaics are such a wonderful source of free energy, then why doesn't the whole world run on solar power? Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. As you can see from our discussion of a household PV system, there’s quite a bit of hardware needed. Currently, an installed PV system will cost somewhere between $7 and $10 per Watt.

For example, A 2,000 Watt (2 kW) PV system would be able to supply the average Burbank house of 1,500-2,000 square feet with 20%-80% of its annual power needs. This varies depending upon the actual power consumption rate, if the home has a roof space that's ideal for solar (one that faces generally south) how much shading there is, and so on. Installation prices also vary, but generally a 2 kW system costs approximately $20,000 before the rebate payment and any available tax incentives. Please check with a tax professional regarding the availability of tax incentives. Even after the rebate and tax incentives, a residential system is likely to take over twenty years to pay for itself at today's electricity rates.

That's why PV is often used in remote areas, far from a conventional source of electricity. Right now, it simply can't compete with BWP's electric rates. However, costs are coming down as research is being done. Researchers say that PV will one day be cost effective in urban areas as well as remote ones. With programs like Burbank's helping to support the solar industry, demand and module efficiencies are rising, we hope to see prices falling, and the world is becoming increasingly aware of environmental concerns associated with conventional power sources. Part of the problem with solar PV is that manufacturing needs to be done on a large scale to reduce costs as much as possible. That kind of demand for PV, however, won't exist until prices fall to competitive levels. So it's a Catch-22 situation.

That's where Burbank's Solar Support Rebate (SSR) Program comes into the picture.
See How BWP’s Solar Support Rebate (SSR) Program Works

Net-Metering - How does it work?

California law requires that PV systems in the state be allowed "net-metering" by their utility. Net metering means that when the PV system is supplying all of the power that the residence or business needs, the electricity meter stops spinning. If there is more generation than current needs in the building, the meter will turn backwards and the kWh read decreases.

When the PV system isn't operating, such as at night, the BWP utility grid provides power for your electric needs, the meter spins forward, and the read increases again. In effect, the meter becomes a kWh "bank" for your excess generation. It's likely that during most of the daylight hours, both the PV system and BWP will be providing power and the meter will spin forward more slowly than usual (at the rate of actual consumption from the utility grid). If you do generate excess power from your PV system, it is fed into the BWP grid and offsets utility generation that would have otherwise been required to serve your neighbors.

In other parts of the country the utility might only buy excess power from you at a much lower price than their own selling price. California has been leading the way by requiring net metering for solar PV for several years.

Recommended Solar Websites:

Tax Incentives Assistance Project
http://www.energytaxincentives.org/

California Solar Center
http://www.californiasolarcenter.org/

U.S. Dept. of Energy - Energy Efficiency and Renewable Energy
http://www.eere.energy.gov/solar/

National Center for Photovoltaics
www.nrel.gov/ncpv

National Renewable Energy Laboratory
http://www.nrel.gov/solar/

Solar Electric Power Association
http://www.solarelectricpower.org/

Thank you for your interest in Solar Power!