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Solar Electric rooftop system basics A solar rooftop electric generation system consists of lightweight solar panels mounted on racks affixed via watertight bolts to roof joists, which generate DC current. Wires in conduit bring the DC to an inverter, which changes the current to familiar A/C house-current. There are two types of inverters: lower voltage battery-backup, and higher voltage grid-tie without backup. The inverter power is fed into the main electric service panel. If the system generates an excess of power, the meter runs backward as power is fed into the grid. Each such solar system must be authorized by SCE (or other utility) as a partner "co-generator" of electric power for the grid. The system panels are guaranteed for at least 20 years, and may last decades longer. System capacity is rated in KILO-WATTS (kW, 1000 Watts). A low-end residential system would have a rating of 3.0 kW "nominal" and 2.5 kW "actual" production (actual is lower due to angle of the sun, atmospheric haze, inverter efficiency, etc., also called the "Alternating Current" or "AC" rating). The lower actual number is the important one. In Southern California, the rule of thumb is that the actual number is multiplied by 5.5 equivalent hours of full sunlight, so this typical system will produce 2.5 kW times 5.5 hours or about 13 kWh of electric power per day, on average. Our grid's electric usage cycles very high in the daytime, peaking at about 60% of the grid's installed electric capacity. As you can see from one look at the usage curve, there never was an energy shortage in the state; actual usage peaks for two or three hours, and generator construction is planned for 10% over the greatest possible demand. Solar power, which peaks in performance during the times when our usage is greatest, automatically counterbalances peak demand, serving as clean "peaker" units. Solar system total price: About $9.00 per "Alternating Current rating" installed Watt of capacity, or about $22,500 for the 2.5 kW (2500 W) actual capacity system. Rebates from the Electric grid: Currently $2.60 per installed Watt, or about $6,800 for our sample system. California State Tax rebate: Currently Not Available, but there is a federal tax credit of 30% (capped for residential systems at $2000).. Final cost to homeowner: $22,500 minus $6800 rebate minus $2000 federal tax credit or about $13,700.
If you are billed at 12 cents per kWh, this system will shave at least $600 per
year from the electric bill, for a return on investment of about 5%. But you are saving after-tax dollars, equating
to a pre-tax saving typically about 7-10%. State law forbids re-assessment for this $22,500 home improvement, which the homeowner keeps and which may enhance property value in the event of selling the home. Electric bills may increase in the future, this investment protects against price rises and energy scams and (with optional battery backup) against possible blackouts too. Time-of-use pricing, where available, may increase the value of production during
peak hours. Each kWh of peak production can be leveraged to as much as 3 times the off-peak rate, effectively increasing
system performance. For example, a typical 4.4 kW system produced enough credits during last summer's daytime peak
periods to power two RAV4-EV plus all domestic electric, and donated $89
in excess electric to the grid. |