This last month marked three years since we’ve had our solar roof. I receive questions on the viability of the system so I wanted to provide a summary of things so far.
About the system:
Our system has 42 230 watt polycrystalline solar panels from ET solar. There are 3 strings of 14 panels providing a total power rating of 9,660 watts mounted on the roof of our home. The panels would only put out their full rating under perfect conditions which will almost never happen because of the orientation of the roof and the location of the installation. (Ann Arbor Michigan)
The device that converts the power from the panels to usable power for our home is called an inverter and is rated at 8,000 watts. It sends excess power back to the energy grid so it is referred to as a grid-tied system. This device has no ability to store power and in fact must shut down when there is a power outage because putting power back on the grid during a power outage would be very dangerous to anyone working on the lines.
About the power:
Our average daily consumption was about 27 Kilowatt hours in 2010 and the system was designed to provide 100% of our home’s electric power demand.
The actual output is dramatically different at different times of the day and with different weather conditions. If there is two inches of snow on the roof and the panels are covered there might be 100 watts even with bright sun. In the summer the power output might be 8,000 watts and then when a cloud goes in front of the sun it drops down to about 2,500 watts.
Our daily generation varies from a high of about 62 kilowatt hours to a low of zero. Over three years the system has produced 31,000 kilowatt hours of energy which is roughly equal to our household electric usage and meets the original design goal of 27 kilowatt hours a day average.
The system has its own local website so we can watch the output on a browser on our WIFI network:
About the economics:
The system was installed by a licensed electrical contractor. The cost for the electrical installation and mounting was about $4.20 per watt, so around $42,000.
If you include the tax incentives and the utility company incentives in force when we installed our system then it has been a great investment, already paying for itself. However the utility company incentives were much higher in 2010-2011 than they are now.
Our agreement with DTE called for them to pay us 10 cents for every kilowatt hour we produce for 20 years. And, they allow us to use that power without paying them for it. The current incentives are significantly different.
About back-up power in case of a grid failure:
The basic system as installed three years ago has no ability to store energy. And no ability to generate power when there is a blackout. Systems that are tied to the grid must be designed that way to protect other systems and people who service the grid. So this system monitors the grid constantly and if there is a power drop out the system shuts down. When it senses the grid is back up it turns back on after a few minutes.
As a result the basic system provides no benefit during a power outage.
An off grid solar power system sized to provide our normal daily usage would need a large bank of batteries, would not have provided the tax and utility company incentives, and would have almost double the cost over a 20 year period. (The batteries have a much shorter life than the rest of the system components and would probably need to be replaced at least three times during those 20 years.)
A back-up generator is still the most logical and cost effective system during a power outage.
About the aesthetics:
Our home faces West South West and as a result we had to put the panels in the front to optimize the power generation. As a result they are very noticeable from the street. We wouldn’t have installed them at all except that we plan on living in this home for a very long time. Some people seem to find them very interesting or even attractive but we are sure some people would not.
About the reliability:
The major expense in a system like ours is the panel array and the installation. Solar panels normally have a very long life. (Some systems are still working 40 years after they were installed.) The mounting hardware is almost all aluminum and it should easily last that long. The roof under that panels is a 50 year shingle so it should last a long time also.
The inverter will probably not last nearly as long as the panels. However the cost to replace the inverter will probably be fairly reasonable when the time comes because there isn’t as much labor involved and inverter pricing will probably continue to come down as the technology develops.
We have had two problems with the system in the three years of use:
1. Some of the cables at the array on the roof were bundled with plastic tie wraps. A few of those tie wraps failed, probably because they were not UV resistant. We will need to fix those in the spring with proper UV resistant tie wraps.
2. The fuses on two of the three strings blew open last summer. I got on the roof and replaced those myself but it is still not clear why they went open. (Note: This is not a job for an amateur, remember I spent the first ten years of my professional life as an electrical engineer.) I’m currently investigating this with the panel manufacturer.
Overall we are quite happy with the solar power system. We have enjoyed tracking the power output during different weather conditions and we’ve been very pleased with it from an economic standpoint. However, I think systems like ours are still a ways away from being simple enough for the average homeowner. It will still be a while before these become normal in a neighborhood like ours.
If you haven’t already seen it here is a time-lapse video of the original installation back three years ago:
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