Solar Systen Output
A basic solar system for the collection of solar energy consists of the Sun, Solar Cells, Inverter and House Fuse Box. For long life each part of the system must be able to process all of the energy from the previous components. You need to read the Spec Sheet for the Solar Cells and the Inverter. Noon day Sun has a color temperature of about 6000 degrees Kelvin [k]. The power from noon day Summer Sun on a 1 square meter solar cell perpendicular to the Sun's rays is about 1 kW.
Solar Cells collect energy from the Sun by collecting electrons knocked off atoms exposed to the Sun's Rays. These atoms are on a conductive plate that is needed to return the missing electrons. Electrons knocked off the atoms are collected on a conductive plate beneath the photo voltaic plate. A solar panel is composed of many cells in series and in parallel. A given configuration and materials used for each cell determines the open circuit voltage of the panel. The maximum curant from a solar panel depends on the number of parallel cells, the number of cells and the many variations in the accuracy of construction. After solar panels are built, they are sorted into bins that represent their maximum power output lit with a calibrated light source. Often that light source matches noon day Summer Sun [1 kW/m2].
An example of an ideal distribution is shown above. Note that after measurement, the panels are sorted into "bins" based on their power output. Those in the 250 watt bin are guaranteed to output at least 250 watts with a "Summer Sun". If the panel generates more than 270 watts, it is moved to the 270 watt bin where it will be sold at a higher price. This process is repeated for the 290 and 310 watt bins. In other words, the maximum output of a "250 watt panel" is essentually 270 watts. This is very important in sizing the inverter.
The purpose of the inverter is to convert the direct current [DC] from the solar cells into alternating currant [AC] at the appropriate voltage and frequency to power your house and hopefully send excess energy to your power company. In the United States, most homes receive 240V power at 60 Hz. It is essential that all the inputs to the inverter be able to receive the maximum power of a solar array at its maximum voltage. Different inputs may be at different voltages. It is absolutly essential that full output of the inverter be passed on to your power panel. And it is also essential that your the breaker in your power panel be large enough and at the correct voltage to accept all of the power from the inverter.
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Experience
I bought my first Solar System from one of the early manufacturers of Solar Systems. They gave me a good price on their 250W panels. I said I would start off with 15 panels and later add more when my son buys a plug-in electric vehicle. I asked what size inverters they had and I selected their 4.4 kW inverter because that would be large enough to reliably convert the 4 kW I expected the 15 panels to generate. They prepaid a 20 year lease and they said they would provide the power I needed for the next 20 years. When they came to do the installtion I asked what size the inverter was and I was told that they found a 4.4 kW invertor for me and I said to install everything.After half a year I climbed a ladder to read the specs on the inverter and it said that it was a 3.6 invertor with a maximum intput of 3.6 kW. Damn. Electric equipment run beyond their specs have a tendency to fail. Surprizingly, the system did run for six years before it failed. They apparently based the size of the inverter on the minimum guaranteed output spec. Big mistake. It should be noted that the panels failed during a heavy rainstorm and rain water did enter the panels causing an intermittent short.
It took about a month for a service person to come out an verify nothing was working. After four months new panels were installed. These panels were 270 W panels. But the system still did not work. It needed a new inverter. The sizes available were one with a maximum output of 4800 W [20A at 240V] and one with maximum output of 3820 W [16A at 240V]. I asked for the 20A output but received the 16A. It apears that this inverter will protect itself by limiting the input power so the output will not exceed 16A. In other words the amount of power from solar has now dropped to 3840W which will provide me with about 3/4 of what I need. The rest I will have to buy from SDG&E. I do not feel they were listening to me. The 20A output inverter with all 15 solar cells connected would have given me all the power I needed for my house, power for my son to charge his car and a little extra energy to send to SDG&E.
By the way, an average power usage of:
1 kW is: 1 kW-year per year or about 8766 kWh/year.
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