Photovoltaic Power Systems FAQ

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ETA Engineering, Inc.

4049 E. Presidio St., Suite 117

Mesa, AZ 85215

Phone: 480-966-1380

Toll Free: 1-877-964-4188

Fax: 1-480-966-1516

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Note: Prices are subject to change without notice due to changes in other vendors' prices to us. These changes may arise from normal product development plus fluctuations in foreign exchange rates and commodity prices.

Frequently asked questions (FAQ).

FAQ Regarding Photovoltaic Power Systems

How does a solar electric (photovoltaic) system work? Photovoltaic modules (a.k.a. solar panels) convert sunlight into direct current (DC) electricity. This electricity is either used immediately or stored in batteries. It may be used either directly (as direct current) or converted to alternating current (AC) by a device called an inverter.

How much power can a photovoltaic (PV) module (solar panel) produce?

PV modules vary in size and output. One of the most common size module is about 4' 5" x 2' 2" and produces 120 Watts peak or about 7 Amps for use in a 12 volt DC system.

How much energy can a photovoltaic module produce?

Electrical energy is generally measured in kilowatt-hours (kWh). Thus, if a module produces 100 Watts for 1 hours, it has produced 100 Watt-hours or 0.1 kWh. The amount of energy produced on a given day will depend on location, shading, and module orientation (direction and tilt).

In a good area for solar power (such as Phoenix, Arizona), a properly oriented module which produces 100 Watts at noon on a clear day will produce an average of about 0.5 kWh/day in January and 0.8 kWh/day in May and June. (Fluctuations result from the amount of variation in direct sunlight on a typical day).

In a relatively "poor" area for solar power (such as Albany, NY), the same module will still produce about 0.25 kWh/day in January and 0.6 kWh/day in July.

What is the difference between solar panels that produce electricity and those that produce hot water?

There are two types of "solar panels" -- electrical and thermal. The electrical type is general referred to as a "photovoltaic module" or a "solar module." It is a solid state device or assembly of solid state devices and produces electricity only. This is the type we carry. The thermal type of panel (which we don't carry) generally consists of water piping, glass and insulation and is generally much larger in size (often 4' x 10').

I want to have AC power available from my battery bank. How do I know which inverter to buy?

1. First, you should see if you can operate as much as you can from DC power. DC appliances are available for many applications including fluorescent lighting, TVs, computers, refrigerators, water pumping, and air handling (coolers and fans). DC appliances are generally more efficient than AC appliances and avoid the loss of energy due to inverter inefficiencies.

2. Once you have determined what AC appliances you need to operate and how many will be in use at any one time, you can add up the loads to find out the minimum size inverter you will need. Example: Microwave (1000 W), computer (300 W), TV (175 W), miscellaneous (200 W) = 1675 W. Choose, at least, the next larger size.

3. Important note about AC motors: AC motors generally require a lot more power to start up than they do to run and may require an inverter much larger than their simple wattage rating would indicate. See questions on types of inverters for more information.

4. Determine the type of inverter you will need -- sine wave or modified sine wave. (See the question on types of inverters.) You also need to determine what type of AC power you will need. In the Americas it is generally 120 V 60 Hz or a combination of 120 and 240 V, 60 Hz. In much of Europe, Africa, and Asia the most common electrical standard is 230 VAC, 50 Hz.

5. Once you know the size (output capacity) of the inverter you need, you will need to determine the input voltage. If you have only 12 VDC available, you already have your answer but you will be restricted to a maximum of about 3000 W AC output.

If you require an inverter in the 2000 W to 4000 W (2 to 4 kW) range, you should consider a 24 VDC input. (For over 3 kW, you will generally be forced to go with either 24 or 48 VDC). Above 4 kW you will generally need 48 VDC or higher input voltage.

6. A further consideration is whether you will want to be able to charge your batteries with AC power (e.g., from a generator, the utility grid or shore-power). If so, you will probably want to buy an inverter/charger combination. When the inverter detects AC power available on its input side, it automatically switches to its charger mode. In this mode it both charges the batteries and provides AC power (through a transfer relay) to any AC loads.

What is the difference among the different types of inverters?

There are three basic types of inverters: square wave, modified sine wave, and sine wave.

Square wave inverters simply reverse the polarity of the DC voltage 120 times per second. (Each cycle consists of two polarity reversals.) Their output is basically either +120V or -120V. Square wave inverters tend to produce a lot of "hum" in equipment connected to them and cause motors to run hotter than normal. They are no longer in common use in solar electric systems.

Modified sine wave inverters can also be considered "modified square wave inverters." Their output consists of 4 voltage changes per cycle -- 0, +peak, 0, -peak, and back to 0. Most of these vary the percentage of the cycle that voltage is either +peak or -peak depending on the load (this is called pulse width modification or PWM). The peak voltage (+ or -) is usually set so that the average value of the voltage is approximately 120V under normal conditions.

Modified sine wave inverters are relatively inexpensive and will run most appliances. However, they may produce some hum or static and certain AC motors will tend to run a little hotter than they would with normal utility power or on a sine wave inverter. Also, certain battery chargers for cordless tools, photocopy machines, and laser printers have been reported to be incompatible with this type of inverter.

Sine wave inverters produce power very similar to that produced by the utility companies -- in some situations, even better quality power. The output of the most common type of sine wave inverter (such as the Trace SW series) has up to 256 voltage changes per cycle and very closely follows a true sine wave. Somewhat more expensive units such as the XANTREX PRO sine series produce even better output (basically a completely smooth sine wave).

Sine wave inverters are more expensive than "modified sine wave" inverters but will run virtually any AC equipment just as well as utility power will (as long as the equipment's power requirements do not exceed the continuous and surge ratings of the inverter).

How much does it cost to:

[Asking the questions in this section is sort of like asking "How much does a car cost?" Well, is it a compact or an SUV, a stripped-down model or fully equipped, new or used, etc.? This being the case we will give ranges and a couple of examples.]

...run my 400 ft2 cabin with solar?

A simple, 12VDC/120VAC system can cost under $1000. This could include a 120W PV module and module mount, 2 deep-cycle batteries, a charge controller, a 400W inverter, fuses, and wiring (installation not included). This could run lights (AC, or preferably DC), radio, small TV, etc. but not a typical refrigerator. The PV module would produce about 0.5 kWh/day (under good solar conditions) and the batteries would store a useful 2 kWh. Thus, on a 2-day visit you could typically use up to 3 kWh of electricity (1 from the sun and 2 from the batteries) and let the batteries recover while the cabin is not in use.

But what about a fridge to keep the drinks and sandwiches cold? Even the portable 12VDC refrigerators typically use about 4 Amps (or 48W). When used all day (24 hours), that adds up to 1.15 kWh/per day which is most of what you'd have available for a 2-day visit. Thus, this is just barely doable as long as you keep your other electrical use to a bare minimum. To make this more practical, you could double the capacity of your battery bank (at a cost of about $150) or add both extra batteries and a second PV module (at a cost of about $800).

For fulltime use of such a cabin, you would be better off with a more efficient refrigerator (see the appliances section of our website), more batteries, more PV modules (typically 2 for the fridge alone) and a bigger and better inverter. Figure an equipment cost of between $3000 and $4000 excluding appliances.

...run my 2000 ft2 house with solar?

If you are planning on changing nothing else, look at your utility bill and find out how many kWh you use each month, especially the peak use (either summer or winter depending on your climate). Let's say you use 2000 kWh/month in the summer in the Phoenix area. That implies a fixed PV array of about 13,000 to 16,000 Watts! A system like that would cover about 1400 ft2 and typically cost between $70,000 and $90,000 (roughly the cost of two nicely equipped SUVs which, of course, don't produce any energy let alone clean energy). Your net cost would normally be lower due to tax credits and, possibly, utility rebates.

Now back to reality...

If your are planning a remote home (off the utility grid) either we, or one of our dealers, can help you make it more energy-efficient. This is best done during the planning stage. Most remote homes use propane for cooking and heating, propane or solar thermal for hot water, and evaporative cooling for cooling. Taking these measures, designing the house well, and using energy-efficient lights and appliances, one can often bring the cost of a solar electric system down to $25,000 to $30,000. In areas with good winds, a hybrid solar-wind system may be even more cost effective, plus have an extra element of reliability.

If your house is on the electrical utility grid, you may want to consider a grid-tie or line-tie system. These have been a popular way for people to produce part -- or even all -- of their electrical needs but without the costs involved with a large battery bank. When your system produces more than your house is using, you sell the power to the utility company. And when you need more power than your PV system is producing (especially at night!), you simply use utility power like normal. This is the type of system where your electrical meter can "spin backwards" during the day. A typical line-tie system size produces about 2 kW of AC power under full sun and can cost approximately $15,000-$16,000 installed (note: installation costs vary greatly). In the Phoenix area, such a system can produce an annual average of about 400 kWh/month. Many utility companies (especially in California but also APS, SRP and TEP in Arizona) offer substantial subsidies to their customers who install this type of system.

...put solar modules on my boat or RV?

We offer appropriate kits ranging from $399 to $889 for this purpose. They include one or two PV modules, module mounting feet, wire and a charge controller that connects to your existing deep-cycle batteries. Larger systems (and extra or replacement batteries as well as inverters) are also available.

What is the difference between a watt and a kilowatt-hour (kWh)? (Or, how many watts does a refrigerator use in an hour?)

Many people are confused by these terms -- and for good reason.

A watt (or a kilowatt) is a measure of power (the rate at which energy is used) while the kWh is a measure of energy.

Corresponding, non-metric terms for the watt include "horsepower," and "calories/hour." Corresponding, non-metric terms for the kWh include "horsepower-hours," and "calories." (Trivia of the day: a kWh happens to be approximately 860,000 calories or 860 kilocalories, the type of calories used to measure the energy content of food.) Kilowatt-hours -- just like calories -- can be stored. Power cannot.

Thus, we can say that a typical deep-cycle battery stores approximately 1 kWh of useful energy. And if we use that energy at a rate of 100 watts (power) we will drain the battery in 10 hours. In fact, the typical classroom illustration of a kilowatt is the amount of electrical power used by a ten 100W light bulbs. And a kWh is the amount of electrical energy consumed by those 10 light bulbs in one hour's time.

As for the refrigerator, what the questioner really should ask is: how many watts (power) does the refrigerator use when on? And, how many kWh does it use in a 24-hour day? (A more appropriate time period, as the compressor will usually run more during the day than at night). A typical refrigerator may use 500W with the compressor running and 2.5 kWh/day (indicating that it is running the compressor about 21% of the time).

KFE, ETA Engineering, Inc., Sept 2004

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Last updated: December 19, 2008