Friday, September 8, 2006

Solar Panels on the Roof

Working in the back yard one day, I looked at the roof of our house . There was no shade on it. The sun was just beating down on it. And I thought, this would be a perfect spot for a solar system. We have a 3000 square foot one-story house, facing North, with the longest axis going East/West. So we have a large roof facing South in the back of the house, between a chimney on one end and a skylight on the other.

 The City of Austin provides a lot of help to home owners with their utility bills, both water and electricity. One insert in our utility bill mentioned "Solar Energy for Your Home", a meeting at an elementary school. It was a short presentation followed by a question/answer session. 

Austin Energy is the electrical department for the City of Austin. It has a rebate program to encourage the installation and use of solar systems. The rebate program will pay up to 75% of a system, up to $12,000 (at $4 per watt with a 3KW system). 

A solar system produces electricity from the sun. But at what cost? And how much would we need? The system promoted by the City is a "grid-tie" system. We are still on the normal power grid, using power provided by the City. When the PV system works, it produces power. If we are using more than the system provides, we get the difference from the City. If it produces more than we need, the extra (above what we can use) goes back to the City. 

With the City's program, we have "net-metering". We pay for the net amount of electricity we use from the City. That means that the City will buy the electricity we provide, at the same rate that they would charge us, at least as long as we don't produce a total amount which is greater than we use. 

This is about the best arrangement we could hope for. We don't need to store the power we generate and don't need. We just give it to the City, and later, at night, we can have it back. The meter runs forward, backwards, forward, and so on, and we just pay for the net amount that we use: the amount the City delivers to us minus the amount we give back to them. 

To look at the economics of the purchase, I looked back at the last 12 months of electrical usage.

Month KWatts Used Cost
May 2005 715 $61.55
June 2005 1014 $93.77
July 2005 1447 $140.44
Aug 2005 1249 $119.10
Sept 2005 1305 $125.13
Oct 2005 903 $81.81
Nov 2005 573 $44.91
Dec 2005 634 $50.39
Jan 2005 750 $60.78
Feb 2005 605 $47.78
Mar 2005 586 $46.09
Apr 2005 674 $53.96

12 months 10,455 $925.71
Average month 871 $77.14

A 3KW system will produce 4050 KWh per year, or 337 KWh a month -- about half our minimum monthly usage. This would be a savings of $30 a month. If our system costs $21K, and the City pays $13K, and we get a $2K tax credit, we end up paying $6K. At $30 a month, this is 200 months, or 16 years to get it back. Without interest. Assuming power costs stay the same.

But we expect power costs to go up. And installing and using the system should help create demand and a market for these systems. And more demand should mean more supply, which will help bring costs down. It's a good thing to do and the economics aren't too bad (as long as someone else is paying 2/3 the cost!)

The first step was getting the City out for an inspection and approval for the rebate program. They want to make sure that the system will work. You need to own your house. There has to be a place to put the solar panels, facing South, that is not shaded. They filled in a form and then approved us with no problem.

The rebate program requires that the system be installed by an installer registered with Austin Energy. There were 8 companies on the list at the time that we started to work on it. I sent e-mails to each company, saying I had been approved by the City for the rebate program, attaching a photo of our house and asking for a proposed 3kW system. 

Two companies did not respond. Two companies said they were so busy they didn't want any more work ("Unfortunately our installation crew is booked out for the next several months."). That left 4 contractors. Each of these came out and looked at the house, took measurements, and sent me a proposal. This process took about a month.

Panel Manufacturer Configuration Inverter Total Watts Cost
1. Kyocera 24 panels x 130 W Xantrex GT3.0 3120 Watts $19,500
2. BP 170 18 panels x 170 W Xantrex GT3.0 3060 Watts $19,887
3. Isofoton 21 panels x 150 W PV Powered PVP2800-XV 3150 Watts $23,356
4. Sharp 16 panels x 208 W Fronius 3328 Watts $21,983

As you can see, each bid had a different panel manufacturer and configuration. The sizes of the panels is quite different. The costs also vary.

So, we went with Proposal 1, which was Armadillo Solar. Armadillo computed that the City of Austin rebate program would pay $11,793.60, leaving a balance for us of $7,706.40. We accepted this contract on 26 June. By 6 July, the City had approved the rebate application, and notified Armadillo. On 14 July, we paid Armadillo our part, as a deposit. At that point, they ordered the panels and inverter. 

The materials arrived in late August, and the installation was done 23 to 25 August. Everything seemed to go smoothly. An inspection by the City was scheduled for 10:00 AM on 5 Sept, and by 8 Sept we had our new meters and were operational.

The solar panels went on the roof, and the controls on the side of the house.

The power from the solar panels is in Direct Current (DC). It comes into the panels from the left, in the metal pipe. The first box is a cut-off that allows the panel to be turned off. Next, the black box, is the inverter, that converts the DC power (about 180 volts) to 240 volts AC to match what the house uses. Then there is another cut-off switch. After the cut-off switch is a meter which measures how much solar power is produced. From that meter, it goes into my electrical box, so it could be used by the house, or if there is an excess over what we need, it goes out thru our standard electric meter, but running the meter backwards.

Despite what was quoted in the estimate, we ended up with a slightly different inverter.  The installed unit is an Aurora Magnetek.  A tag on the bottom says Model PVI-3000-I-OUTD-US.