How much sun does Austin Texas get? Annually, on average, about 3.1 KWh/m^2/day of direct sunlight, and an additional 1.7 KWh/m^2/day of indirect light. (Both components can be used to generate electricity, but only the direct component is useful for solar heating. Austin gets about 1000 BTU per square foot per day direct sunlight. 1 KWh=3,413 BTU. 1 BTU will raise 1 pound of water one degree Fahrenheit). See Sustainable Building Sourcebook, Appendix C (mirror) also: Renewable Energy in Texas and Texas Solar (mirror).
Photovoltaic modules cost about $5 per watt. See Photovoltaic Modules Comparisons (mirror) and Photovoltaic Module Manufacturers. Utility tie-in power inverters will cost another dollar per watt; see SolarElectric's Product page. Mounting brackets, wiring, panel modifications, etc. might run another $4 per watt, if you do-it-yourself: net-net, expect an installed cost of about $10/watt for a 2000 watt system. (Total cost: $20,000).
What sort of power output can we expect? Assuming a polycrystalline PV system with 11% efficiency, then Austin's 5KWh/m^2/day insolation equals 550Wh/m^2/day of usable electricity generated. But most panels generate 125 Watts peak per square meter: thus, for Austin, we have 550/125=4.4 hours a day of electrical generating capacity.
(This number may sound weird to you, so lets review its meaning. There are 24 hours a day, so if we could run our system 24 hours a day, we could produce 550/24 = 23 watts per sq. meter round the clock. But the sun doesn't shine 24 hours a day. If we assume it shines only for 12 hours, then we'd get 550/12 = 46 watts during the daytime. But direct sunlight is stronger mid-day than evening or morning, so we might get nearly 100 watts midday, and a few dozen morning/evening. It averages out to the same thing. All that the above division accomplishes is to point out that the panel, running at half-power, for half-the day, is the same as running full-power for 4.4 hours, and then being turned off. The average works out to be the same. Why is it that only the average counts? Imagine pushing that electricity into batteries, and then doling it out evenly during 24 hours ... )
These actual figures depend on the actual efficiency of the panel selected (monocrystalline are more efficient, amorphous are less so) and the power density (I don't know how that varies for monocrystalline and amorphous). Assuming that peak power scales as efficiency, then we have a crude rule of thumb: the usable hours/day is roughly equal to insolation/day, when insolation is measured in KWh/m^2. In other words, we can expect a single 100W-peak panel to generate 100*4.4 = 440 Wh/day. Thus, our hypothetical 2000W-peak system might generate 8.8KWh/day.
How should we amortize such a system? Electricity from the City of Austin costs about 7.6 cents per KWh: the system would save us about 7.6*8.8=67 cents a day. To amortize the $20,000 cost, we'd have to run it for 30,000 days, or about 81 years. Ouch! (Your mileage may vary. Wholesale spot prices in California linger near 18 cents per KWh, and, after retail markups, this might make solar electric far more appealing.)
Suppose we wanted to amortize the system over 15 years, which seems not unreasonable: one expects the solar cells to 'fade' in the hot sun, and have other problems in weathering the elements. The math, then: $20,000 dollars / (8.8 KWh/day * 15 years * 365 days/year) = 41.5 cents per KWh. This compares to the Wall Street Journal's estimates of 22 to 40 cents per KWh for solar, and 6.9 cents per KWh national utility average. (WSJ, 17 Sept 2001 issue supplement).
To make it worthwhile for common deployment, we'd have to shrink the amortization schedule from 80 years to 15 years. To do this, the installed cost would have to drop to under $2.5 per peak-watt. Note also, I failed to take into account interest rates: instead of installing a PV system, one might take that money and invest it in stocks and bonds, and reasonably expect to quadruple it or better in 20 years. Taking interest rates into account, the installed cost needs to be even less to make the system cost-effective. So it looks like we're not there yet, although maybe in another decade or two, we can hope that PV prices should be down to competitive levels.
See the LCRA's A Handbook for Assessing the Market Potential of Off-Grid PV Power Systems in Utility Service Territories) (mirror of google's mirror)