Case Study: How does the productivity of a PV solar panel in your region compare to those of others in the United States?

Note: This chapter was retired in July 2015 as the tools and data are no longer available. The pages are available here for reference.

Factors Affecting PV Productivity

A number of factors can affect photovoltaic (PV) energy productivity of a solar panel, including the time of day, the season of the year, the panel's position and orientation to the sun, the location of the panel, and the type of weather occurring on any given day.

You might expect that on sunny days and longer summer days there would be more output from a solar panel installation.

  1. Below is a graph of daily power output data from a series of solar panels at a site in Texas over a year's time. Describe the overall shape of the graph.
  2. The shape is a sinusoidal curve with two peaks.

  3. What does the shape of the graph indicate about this site?
  4. The x-axis is the day of the year and displays 365 days across the graph, beginning with January 1st as day 1. There are days when very little energy is generated. Plus, there seems to be a seasonal profile to the graph. There is higher power output in the spring and the fall than in the summer or winter. The low power days could be stormy, cloudy weather. The maximum output from this system is between 425 and 624 kilowatt-hours (kWh) per month, calculated from 30 days of power output.

  5. This graph displays a single day of power output, in watts per 15 minute samples. What does the graph show?
  6. There is no solar power generated at night. The daylight extends from about 5 AM until about 8 PM.

What are units of power and energy?

The unit of power in the International System of Units (SI) is the watt. One thousand watts is a kilowatt.

For electrical power, work is done at a rate of one watt when one ampere flows through a potential difference of one volt.
P (power in watts) = I (current in amps) x V (voltage in volts)

The light bulbs typically found in homes used to be 60 watts or 100 watts. However, today many people are using CFLs, or compact florescent lights which have much less wattage. The equivalent light produced from a 60 watt incandescent bulb is a CFL 13 watt bulb.

Multiplying power by time produces units of energy. If you leave a light on with a 60 watt bulb for one hour, then you will use 60 watt-hours (Wh) of energy. If you leave a light on with a 13 watt CFL for one hour, then you will use 13 watt-hours (Wh) of energy for the same illumination. Over the course of a month, your units of electrical use for your home will be in kilowatt-hours (kWh). 1000 watt-hours (Wh) equal a kilowatt-hour (kWh). Solar panel power output is also reported in kilowatt-hours (kWh).

A monthly electric bill for a home will typically be 400 - 600 kWh unless the house uses electric heating or cooling or some other very high energy use appliances. The above numbers are approximate for the sum of monthly use of most American home appliances such as a refrigerator, washing machine, electric lights, stereo, etc.

What are the solar output possibilities near where I live?

In the solar energy monitoring websites in Part 1, you can find data for an area near you that has a record of solar energy generation. You may be able to compare this with your own electrical use at your home by looking at the kilowatt-hours (kWh) of energy you use in a month or a year as recorded on your electric bills.

In this chapter, you will see how much difference there is between solar power installations. Keep in mind that these solar power installations were chosen because they have a good solar exposure. Not every home or business will have that option. First, you will explore hourly, daily, and monthly data from one location. Then you will compare a year's worth of data from two sites. Last, you'll compare long term data at two schools and consider solar panel productivity in light of a home electrical bill.

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