We had been interested in installing photovoltaic roof panels to generate electricity for our household, but until this past year the economics were not favorable. Recently however, substantial decreases in material costs, generous rebates from CPS Energy, the San Antonio power company, and a federal tax credit have made it attractive enough that we did it. After discussions with representatives from Circular Energy (http://www.circularenergy.com), the Austin, TX company proposed a 16kW grid-tied design for our SE-facing roof surface. The roof layout of this system is shown here: Installation began during the last week of October 2011. First steps were to install supporting rails to the roof surface: These rails were fastened to the standing seams of the metal roof with heavy aluminum clamps, as can be seen here: Individual 240W photovoltaic panels were then fastened to the rails as can be seen here: An individual panel is shown here: A total of 66 panels were installed and connected to common cabling. Owing to the hillside location of the house, the array of panels can be seen only from the air. A conduit running down through the roof and along an exterior wall connected power from the solar panels to a pair of inverters shown here: The inverters have several functions. Output of the solar panels fluctuates depending on the amount of sunlight they receive. The inverters transform this output, converting direct current from the solar panels to the alternating current used by household electric wiring, and synchronizing the phase and frequency of their output to that of the CPS grid voltage. The inverters also monitor utility current and are programmed to shut down when grid voltages fall outside standard limits. This is particularly important in the case of grid power failure when contact with solar system voltage could be dangerous for electric utility crews. A more technical explanation of solar power inverter functions can be found at How solar inverters work. Cables running through our crawl space connect the inverters to a pair of CPS service panels where power comes to the house from the grid: The photo shows two electric meters – the one on the left registers current from the solar system; the meter supplied by CPS is on the right. The box on the far left houses a manual cutoff switch which disconnects the solar system when servicing is required. AC current from the solar system is kept at a slightly higher voltage than current from the grid so that solar power is used preferentially whenever it is generated. The grid supplies power to the house whenever household demand exceeds solar power production. This sharing of power supply is continuous and imperceptible within the house. The system includes a monitoring capability which produces an informative real-time display shown here: The graph displays most of three 24-hour periods beginning at 9:28AM CDT on January 24, 2012 and ending at about the same time on January 27. Light green areas indicate times when solar power exceeds household usage, and when the excess power is purchased by CPS. Pink indicates usage exceeding the power generated by the solar system, and when the deficit is made up by power purchased from the grid. The small panel on the far right gives a second-by-second indication of the current status of the system. At the moment this display was copied, household use was 1,501 watts while the system was generating 8,769 watts, leaving a surplus of 7,268 watts which was returned to the CPS grid. The blue and dark green upper panels show estimated measures of the relative contributions of solar and grid power. The row of purple boxes above the graph set the time period over which performance is graphed (note that the 3d box is highlighted here). The row of orange boxes controls the vertical scale of the graphs (set to auto scaling here). The continuous dark red line indicates our electrical power usage, which is highly variable over time. Most of the peaks occur when our heating system (heat pump) is running. Prominent peaks begin at around 6:00PM when passive solar heat gain in the house declines and the heating system comes on. Peaks are reduced at bedtime when thermostat settings are lowered, but recur at about 6:00AM when the heating system comes on again. The system generates power during daylight hours (between about 7:30AM and 6:00PM at this time of year). These times are represented by areas below the dark green line on the graph. The extent and shape of each of these areas reveals local weather conditions during each of the three days shown: Tuesday, January 24 was extremely cloudy, with light rain falling most of the afternoon. The small area under the green line on the graph reflects the absence of direct sunlight, although there was still enough light to generate some power. There was little passive solar gain in the house which resulted in frequent running of the heating system during the daytime. There was more rain during the early morning hours on Wednesday, but by dawn the skies had begun to clear. Early morning outside temperatures were mild (55° F at 7:40AM). The area under the green line was considerably larger than it was on the previous day, but the daylight hours were marked by intermittent sunshine and clouds which resulted in its irregular shape. The early morning hours of Thursday, January 26 were cool (42° F at 6:50AM) which led to more frequent running of the heating system. The daytime that followed was cloudless with brilliant sunshine. These conditions are reflected in the amplitude of power usage (red line) from about 6:00AM to 8:00AM, and the size and shape of the area under the green line for this day. Note also the usage peak at about 10:30AM when the electric clothes dryer was being used. We estimate that our solar power system has generated about 43% of our total household electrical use during the last three weeks of January.