A few very important considerations go into purchasing and installing a solar electric system. There are siting issues, that means the location of the panels needs to have good isolation, and sizing of the array to produce the proper power output. These two go hand in hand. For example, if you have a less than optimal site, you can increase your power output by increasing the size of the solar panel array.

The solar panel array should be site free of shade from 9 am to 3 pm every day of the year. This can get a little tricky in the months of December/January (northern hemisphere) when the sun is low. Also, summer months can have shade problems due to deciduous trees, which can grow up into the sun’s path after the panels are installed.
The initial system size is based on your electrical usage and how much of that usage will be off set by the solar panels. The system types vary from Grid-tie, non-grid tie, and grid tie with battery backup.

1. Siting for solar panels. This is critical, because even a little bit of shade can cause a panel output to drop by seventy five percent. If the panel is a part of a string (a series connection of panels used to create higher voltage outputs than any one panel) then that string my not develop the voltage needed to drive the input of the connected inverter. This means that your array output will either be greatly reduced or be nothing at all. If your solar panels are not putting out any power when the sun is shining, then they were a bad investment.

In the northern hemisphere, solar panels should be installed so that they are facing due south (180 degrees true). You can use a compass to find true south, as long as you remember to add or subtract the magnetic declination for your location. This is because the magnetic North Pole (or south pole) is not the same as the true north pole. Magnetic declinations can be found at NOAA National Geophysical Data Center. It may not be possible to install an array facing due south. Deviations can be compensated for by increasing the size of an array, for example, a solar panel array installed facing 30 degrees away from due south will have an output of about 90 percent of a due south facing array. Some good technical information on site analysis is available at pvresources.com. There is also a tool called “Solar Path Finder” which appears to be pretty inexpensive compared to the cost of a solar system (and the cost of an incorrectly sited solar system) for those who are looking to do it them selves.

Also part of the siting consideration is the tilt angle. That is the angle at which the solar panels are tilted. Optimum performance is gained by tilting the panels to your latitude. For example, if you live in northern West Virginia, around 39 degrees North, then the best angle to tilt your solar panel is 39 degrees. Like the azimuth, this can be fudged a little bit without much degradation in performance. You can also optimize the array based on seasonal usage. More summer time use would mean reducing the angle by 15 degrees, and more winter time use would mean increasing the tilt angle by 15 degrees. This becomes more critical the further from the equator you are because of reduced isolation rates at higher latitudes.

Most home owner’s want a solar installation that will be as un-obtrusive as possible. That means mounting the panels to the roof, without much consideration for tilt or azimuth. Again, that can be compensated for by increasing the size of the array, so long as the no shade between 9am to 3 pm criteria is met.

2. Solar system sizing. If you are installing an off grid installation, you need to size the system for greater than 100 percent of your electrical usage, unless you plan to have a back up generator. The reason for this is to compensate for any cloudy days when you will be running off of battery power. The battery bank should have enough Amp hours (Ah) to run for at least two to three days of no sunlight. You can also augment an off grid solar system with a wind generator. These arrangements usually work well because when the sun is not shining, the wind is usually blowing. Off grid systems or grid tie systems with battery back up require more maintenance and batteries can be an explosion hazard.

For off grid sizing, you will have to add up all of your electrical loads, estimate how much each load is used every day, and what loads are going to running at the same time. When calculating load, use the heaviest load rating. Well pump motors are a good example of equipment that has a high starting current, but a normal running current. A 1 horsepower pump motor should draw around 820 watts when running (90 percent efficiency), but will draw almost 4,000 watts when starting. This will looks something like this:

Step 1:
Load X quantity X volts X amps = watts

Step 2:
Watts X hours per day X days per week /7 days = Average watt hours

Then, for a safety factor, I would multiply the total connected watts by 1.25. Your inverter (if you are using AC power instead of DC power) should be sized to handle at least 120 percent of your AC load. This will give you some room for expansion and for starting heavy loads.

Next, the battery bank needs to be sized. Battery bank sizing looks something like this:

Then, you have to size your solar panel array to the load.

On grid or grid tie systems can be sized to supply less than 100 percent of a building’s electrical usage. This can save money on the initial installation. Grid tie systems carry more regulatory headaches than non-grid tie systems. Most states now require local power company’s to have net metering agreements, but you will need to check and make sure that net metering is available where you live. A good web site is www.dsire.org (data base for state incentives for renewables and efficiency). Many states have good incentives that will off set up to fifty percent of the cost of a solar electric grid tie system. In these areas, the cost per installed watt can be as low as 4 dollars.

More in part III

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