Category Archives: General

3 Year Summary of Life With a Solar Roof

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 Solar Array On Our Roof
The Solar Array On Our Roof

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.

Our Solar Inverter
Our Solar Inverter

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:

Solar Output Monitor From A Web Browser
Solar Output Monitor From A Web Browser

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.

Our conclusions:

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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What Difference Panel & Roof Angles Make

The question of angle of solar panels comes up in about every serious discussion of PV installations.

We recently had a chance to photograph two different arrays on the same day at the same time. It gives a great comparison:

Snow Covered Solar Array At 40 Degree Angle
Snow Covered Solar Array At 40 Degree Angle
Snow Covered Array At 23 Degree Angle
Snow Covered Array At 23 Degree Angle

The top array is basically at full power, the bottom is at zero.

But as with any design question, there is more to consider:

1. What additional cost might be involved to increase a tilt angle from 23 degrees, (a typical 5/12 pitch roof) to 40 degrees?

2. How often does this condition actually exist in use?

3. What is the optimum angle for yearly power generation?

When I designed our system I looked at the cost of fancy racking to lift the panels to 40 degrees, It was quite significant. Additionally there are maintenance costs that come with a more sophisticated arrangement.

Our Michigan weather conditions don’t produce this condition very often so in terms of actual power lost I probably estimated less than $100 worth of energy a year.

Finally the 23 degree angle of our roof is somewhat short of the optimum yearly location, so I would be generating less power by about 10%.

However, with the above in mind I determined I would be better off using the money I would have spent for a fancier racking system to buy and install more solar panels at the 23 degrees. That way I would receive benefit from that investment on the vast majority of the days the system is in use, even if i sacrificed generation when the system is snow covered.

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Three Years With a Zero Net Utility Bill

This month marks three years since we’ve had our solar array turned on.

And even though we spend five to eight hundred dollars a year on our firewood we are still coming out with a net of zero on our utility bills.

Some background:

With the contract we have with DTE they pay us about ten cents a killowatt hour for generating power, then they allow us to use what we generated. The net result is a negative electric bill for the last three years.

However, we do use some natural gas, primarily for the dryer, but also for the cooktop and for some space heating and some hot water heating. So we have a gas bill, it is just far less than the electric credit.

The balance comes to us every year or so in the form of a check. This money has basically paid our bill for hardwood delivery. (We really enjoy wood heat, I’m not convinced that it actually saves us money.)

Our total power generation for 3 years is 31 Mega Watt Hours, or roughly $3,100 of electricity at our current rates.

Not bad!

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Green Energy and Unintended Consequences

Unbelievably bad decisions justified by a desire to lower CO2. In the UK they are importing wood from the US to replace coal in their power plants.

Here is the original shocking article from the BBC:

As someone who uses wood as a major source of heat, it is clear that even though it is a renewable resource, it takes time, money, and energy to get it in a burnable form. In our case I’ve been cutting, moving, and splitting wood from our own yard the last two years to help heat our home, but to do that on a commercial basis, importing wood from across the ocean to replace coal that is a few miles away seems completely idiotic.

Here is a more worldly view that points out how ridiculous it is to make political decisions based on junk science: Here

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Zero Juice Today!

There are plenty of days in winter when the solar panel array is completely covered in snow. (At least there have been this winter.)

Any day when you can’t see the black of the panel you can be sure the panel can’t see the sun. All the more reason to enjoy the sun when it is available!

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Two Years With No Net Utility Bill!

Last month we marked the two year point with our utility supplier.

The array has been functioning for 25 months but the bills lag the generation. So last month we’ve now gone two years with an average heating and electric bill of zero.

How does that work? Well, it starts with the Solar Currents plan that was in place in 2010. It gave us .10 per KWh for generating the electrical energy and then also let us use what we generated without cost. Almost like getting paid twice.

And we have gotten paid. For the first two years we generated more income from the solar array than we consumed with combined electric and gas bill. So we have actually gotten checks from DTE when they balance out the year.

Then we do ~90% of our heating with wood. (Because we like to, not because we need to.) The wood costs us about what we have gotten paid by DTE.

The net result is no utility bills. (Or a net zero utility bill.)

More importantly, it has been a fun two years, and, we’ve hit break-even on our Return On Investment (ROI)

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We Hit Our Generation Target after Only 351 Days!

Well we did it! We generated as much electricity in 2011 as we consumed in 2010, with five days to spare!

We need to make long term comparisons like this because actual generation depends so much on weather, but a one year mark makes sense to me.

Now e also plan on consuming more electricity in 2012 than in 2011 because we will have a full year with our mini-split heat pumps. But don’t worry. For every BTU from the heat pumps we will use a BTU less of natural gas or wood.

The mini-split systems also provide the benefit of mini-zones. That is, temperature control by room.

For example right now my office is being heated up to be in the seventies. Yesterday morning my wife was cooling our family room down to the sixties. Most of the home isn’t using any energy now and stays in the mid sixties to mid seventies range. In a conventional home heating/cooling system all rooms would typically end up being heated or cooled to the same point.

This video shows a similar system in use in another solar home:

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