Articles

Submitted by:  Brad Wade (bradwade@alum.pomona.edu)

Challenge

We knew that we would install a solar photovoltaic system “sometime,” but the need to replace the roof and gutters provided the motivation to actually go and do it.  Rightly or wrongly, we figured that the new roof would be more weatherproof if the supports for the solar panels were installed in conjunction with the new roof rather than afterward, so we did both at the same time.

Solution Details

Vendor & Proposal – We chose Regrid Power (now Real Goods Solar) to do the work. They proposed a 24-panel roof-mounted system that would produce a maximum of 4920 watts of DC power, which would turn into 4100 watts after being converted to AC. They also estimated that the system would displace 75% of our electricity usage and reduce our electric bill by 90%. Why the difference? They would sign us up for a “time of use” electric meter, and PG&E would charge us less than the standard rate for electricity consumed during periods of overall low demand and much more for electricity consumed during periods of overall high demand. Any time that we produced more electricity than we consumed, the excess would flow to PG&E and they would give us credit for it. Since “low demand” is night and morning, and “high demand” is afternoon (when the sun is shining), we would in effect be buying cheap electricity when we needed it and selling expensive electricity when our production exceeded our consumption.

Installation – The system was installed in early December of 2009 and activated in mid-January of 2010—you need a “permit to operate” from PG&E, and that took about 6 weeks to be issued.

Real-Time Monitoring – Regrid included a fascinating tool along with the basic solar installation: nearly real-time monitoring via our existing broadband Internet connection. At any time we can use a web browser to see how much power our system has produced that day, or any prior day. For example, our production for today (mid-July) looks like this:

You can see the effect of a few clouds passing by around 9am, the peak power of over 4000 watts generated around noon, and the fall-off in the afternoon as the sun’s rays make an increasingly shallow angle with respect to the panels. You can see a similar real-time display: Gaiam Inc. has a large solar installation in Colorado, and you can view its data by visiting http://www.pvwatch.com/?s=gu with your browser. In addition, there are vast quantities of data available in tabular form—30 data points taken every 5 minutes, more data than even this engineer knows what to do with.

Impact on Electricity Bill – So how did our solar installation work for us? One, our garage is cooler, since we put the panels on top of the garage. Two, the inverter (the unit that converts DC from the panels to AC for the house) reports a savings of 18,500 pounds of CO2. Third, we’ve saved a bunch of money. Over the first 365 days, our 24 panels generated 7058 kwhr of electricity (more than the Regrid/Real Goods estimate of 6368 kwhr), and we used 380 kwhr of additional PG&E electricity—so our panels actually displaced 95% of our electricity usage. We sold PG&E 552 kwhr of expensive “peak power” electricity and bought 863 kwhr of “off peak” electricity (plus 69 kwhr of “partial peak” electricity). The bottom line was that we paid PG&E about $10 over the 365 days for electricity. But the $10 is not the whole story. We also paid PG&E a “minimum daily meter charge” of about 40 cents per day, or $146 per year, which is in addition to the charge for the electricity we bought from them. I reason that this charge is for having PG&E serve as your “battery,” a place where you can store the excess electricity generated during the day and where you can get it back at night. It’s certainly a lot cheaper than having your own rechargeable battery farm to store your excess electricity. One side effect of this arrangement is that you do not get to generate your own electricity if there is a PG&E failure. It is a safety hazard to PG&E workers if solar panels are pouring energy into the grid at the same time that the linemen are trying to repair it, so the solar installation is disabled whenever the grid is disabled. When the power comes back on, the solar installation revives automatically; there is nothing that the homeowner needs to do.

Maintenance – There is pretty much nothing that the homeowner needs to do at any time. If you wish, you can hose the dust or pollen off the panels a couple of times during the summer (rain does a fine job in the winter), but the real-time graph did not show any detectable long-term change in our output after a summer washing. All we saw was a small increase in output for about 5 minutes because of the cooling effect of the water. How’s that?  You might think that on a hot day, you would get more output from the solar panels to help with the air conditioning, but it doesn’t work that way. Solar panels actually get less efficient with increasing temperature. The power generated by our 24-panel, 4920-watt system decreases by about 12 watts for every 1 degree rise in temperature.

Benefits & Payback

What about the cost? The system cost us about $19,000 out of pocket after Federal and state rebates. Real Goods did all the paperwork for the California rebate and reduced the amount we paid them by the amount of the rebate; they also provided all of the information that we needed to fill out the tax form to claim the Federal rebate. A model of our cash flow, including trying to predict what will happen to baseline and above-baseline electric rates and allowing for losing the interest we could have earned on the $19,000, showed a payback period of between 10 and 15 years for us—10 years if bank interests rates stay where they are now (i.e. negligible), 15 years if they go to 5%. Others have reported faster payback periods, but I suspect that those others use more Tier 3 and Tier 4 (i.e. really expensive) electricity than we do. That’s one of the big advantages of solar: it displaces your most expensive electricity first.

Informational Links

Real Goods Solar – http://www.realgoodssolar.com/residential/getting-started/