Tag Archives: solar photovoltaic

Fifth Choice, How Big To Size The Solar System

Sizing the system will depend on your current usage, the amount of roof space you have, your budget, and how much time you want to spend optimizing the system.

Using the basic numbers from the PVWatts site and from our bill it looked like we would be good asking DTE to approve a 7,000 Watt DC rated installation. A couple of installers confirmed that range should be approved so I knew my baseline at that point.

However, I know the marginal cost of installing a system goes down as the system gets larger. This is a critical point to understand. A number of the quotes I received were derived and justified based on a certain cost per watt. But the actual cost per watt gets lower as the size of the system gets larger. (Think cheaper by the dozen.)

My desire was to utilize the maximum amount of the DTE renewable energy credit which paid $2.40 per DC watt, and also size my system so I wouldn’t need to come back later to add additional capacity. (Which would probably cost a lot more money per watt.)

I also knew that we currently use wood as our primary heat source and that as we get older we might want to use it less. And, we are planning on installing some super efficient mini-split heat pump systems to supply heat and air conditioning in the future.

I did some basic heat energy calculations and determined that our electric energy use could easily increase by 25 to 30% over the coming years as we used less wood and more electricity for heating.

So, I targeted a larger system for my proposal to DTE. I spoke to a couple installers who agreed it made sense to plan this way. The target number I ended up with was a 10,000 watt DC system.

I also have a 3,000 square foot roof so I know that about 1,500 square feet is facing South West. That gave me 1,500 square feet to play with which would turn out to be plenty to install our system.

Note that there are at least two different ways to describe the power capability of a solar system:

1. The maximum possible output of the solar panels themselves. (This is referred to as the DC Watts output or the “Nameplate Watts.”)

2. The maximum possible output of the system. (This is referred to as the AC Watts output.)

So I targeted a 10,000 DC Watt system, knowing that it would rarely if ever be possible to actually get a full 10,000 watts of output from that set of panels because it would take perfect conditions as far as the sun’s location in the sky, the temperature on the roof, shading, and the weather.

I ended up with what Detroit Edison called a “9.66 DC/ 8 AC kW Solar System” on my roof. And, today as I write this there have been a couple minutes where conditions have been good enough to hit the 8 kW level of system output, so I can confirm it is actually working as designed.

Since DTE is currently paying $2.40 per DC watt after installation, this means that when the installation was complete and had met all the approvals, DTE would write me a check for $23,184 to help pay for the system. (And after a few nervous weeks I actually did receive it!)

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The First Choice. Hot Water or Electricity?

Today’s post is about selecting between a hot water solar collector and a photoelectric solar collector, and the decision points involved in doing it.

When deciding on what sort of solar collector to deploy we considered cost, efficiency, return on investment, convenience once the installation was complete, and impact on the appearance of our home.

First we considered the cost. We could install a basic solar water heating system for a couple of thousand dollars by doing the work ourselves.  A solar electric power panel system would cost in the range of $2,000 for a small system self installed all the way up to $60,000 for a professionally installed system. But we found that in order to take advantage of the tax and utility company bonuses we would need to have installed by a licensed contractor.

Efficiency relates to how well the system turns the sun’s heat energy into usable energy for our home. In this area the solar water heater is much more useful, being able to convert about 90% of the sun’s rays into heat energy. Today’s solar electric systems range from 12 to 20% efficient.

The return on investment can depend a lot on how an installation is done. In our case we already have a quite effective water heater so even if we did install a great solar preheating system it still couldn’t save us that much money. (A water heater also needsa backup system in place anyway for over cast days.) On the other hand we can certainly use an inexpensive source of electricity and after the various incentives it looked like we would be at break even for our electric roof in about four or five years. As far as the convenience, both solar electric and solar hot water systems take a little continuous maintenance. The hot water system would take more, especially in the summertime when they can overheat.

Since the south side of our house is the front of the house the appearance of a solar collector became an important consideration for us. Neither type of system could be hidden, but we were more comfortable with flat panel collectors for this reason. If it had been on the back of our home we would have seriously considered vacuum tube hot water methods.

When all the ideas were evaluated we decided that a large solar electric collector was right for us. I hope this article was at least somewhat helpful.

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