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Why solar electric rooftop Photo-Voltaic ("PV") system? PV is non-polluting, once the panels are made, and give a good feeling when you watch your meter at times spinning backward, pumping electric into the grid, visible proof that you are part of the solution of our energy-independence problem. What does PV cost, and why buy it? PV is a prudent investment. The bulk of the cost is the PV solar panels, which are guaranteed for 25 years and may last much longer, an enduring, visible asset. For each $20,000 in system cost, PV returns about $1000 in electric per year at an "average" utility rate, a 5% Return on Investment ("ROI"). But with rebates and tax credits, the price is reduced to about $14,000 and the ROI increased to about 7%. It's even better than that, though, because: 1. You're saving after-tax dollars; frees up more pre-tax income. What does PV consist of? Lightweight (3 lbs. per square ft.) 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 AC house-current. Each system must be permitted and approved under building codes and by the utility, which must allow it to feed excess electric into the local grid. Each grid-connected PV is a "cogenerator" with the utility. Some electric facts Volts and Amps: Electrician-talk. Suffice it to say that "Watts=Volts times Amps", and wires have to be thicker with low voltages (like your car's battery cable) and high-voltage systems are more efficient than lower-voltage systems. Watt: Measure of electric consumption. 30 Watt light bulb, 300 Watt computer, and 2000 Watt hair dryer. kilo-Watt: 1000 Watts, like ten 100 Watt light bulbs turned on at the same time. Also, about what a refrigerator uses when its motor goes on, and less than a hair dryer when it's turned on. Abbreviated "kW". kilo-Watt-hour: 1000 Watts of appliances running one hour. This is the normal unit of work that the electric company charges from 8 cents to 50 cents to deliver to your electric meter. Abbreviated "kWh". PV size: Measured in Watts. A 1000 Watt system needed, as a rule of thumb, for each resident. So the "average" home (2.5 persons) needs, as a rule of thumb, 2500 Watts, or 2.5 kW. So if every home had a 2500 Watt solar system, we'd have a chance of producing almost as much energy as we used, overall. Such a system would, on the average, produce about 12 kWh per day (360 kWh per month) assuming about 5.5 hours of full sun per day. Energy efficiency conservation can lower usage closer to this number. Battery backup optional PV requires hookup to the grid to get the rebate, pumping excess electric into the grid in the day, but drawing down at night. But these "grid-tied" systems stop producing electric if the grid goes down. Battery backup systems are also grid-tied, but in addition can stay on if the grid fails. These systems require a place to put the battery, a special inverter, and rewiring of the main panel so that the inverter can detect grid failure. If this happens, the inverter temporarily breaks the connection to the grid and draws on the stored power of the battery. Service is uninterrupted on the backed-up circuits as long as the batteries last. Hooking such an inverter up to a solar PV system means that you only have to last on the battery pack until the next sunny day, when the batteries can be recharged by the PV. If the grid were to fail for a long time, you could still have power and would be, in effect, "off-grid". When the grid comes back up, the inverter detects it, locks into
it, and restores your full power to all circuits. Battery backup systems use an inverter that operates at a much
lower voltage and lower efficiency. Battery backup systems sacrifice some efficiency in exchange for the benefit
of backing up the home. |