Energy Pyramid Says Solar Tops All

By Green Home Consulting

Once you’ve tackled the first two layers of the energy pyramid (the building envelope and mechanical systems), it may make sense to consider renewable energy sources such as solar photovoltaics (PV) for electricity or possibly solar thermal for hot water.

In past newsletters we have discussed ways to make your home do the work of saving energy by improving its tightness and adding insulation to the building envelope.  This is the all important first layer of the energy pyramid and results in the greatest savings for dollars expended.  Next you should maximize efficiency in the mechanical systems such as hot water heating  and heating and air conditioning since these are the greatest uses of energy in your home.  But once those pyramid levels are optimized, what about addressing long term electrical needs?

Even with more efficient light bulbs and Energy Star appliances, our demand for electricity is growing exponentially with technological advances and even electric cars.  Plus, geothermal and other heat pump systems run on highly efficient electric pumps and thus are a perfect pairing with solar PV.
The Federal government is offering a 30% of cost tax credit through the end of 2016 and New York and Connecticut both have substantial rebates to reduce the cost of solar installations.

So what are the considerations? First, a big enough area for installation.  Solar PV panels must be grouped in arrays of 10-12 panels in order for a single group to generate enough voltage to turn on the inverter (which converts DC power to the AC power that is used in the home). Too few panels and the system will not operate.

Next, shading and orientation.  PV panels are extremely sensitive to shading so it is not only necessary to have a SSW to SSE orientation but also that no shade is being cast by trees, chimneys, dormers or other roofs, vents, etc. on any part of the array.  The entire array will degrade to the amount of electricity being produced by the lowest performing panel so if one panel is shaded, the entire array will not operate properly.  This is not only an electricity generation issue but also affects cost because the array must achieve at least 80% efficiency on average to be eligible for financial incentives.

Pitch. The pitch of an existing roof, if the array is to be installed on a roof rather than a ground mount, is less important because the mounting structure can be adjusted to optimize the pitch during installation. Alternatively, during new construction the roof pitch can be optimized.

Other considerations are roofing material (for instance, if you have a slate or tile roof, solar is not an option) and your current electricity use.  You are not permitted by the utilities to become a net power generator and thus in most cases can install a system that generates no more power than your past 12 months of electricity use.

Finally, it is worth checking the rules governing solar in your town.  Although the initial resistance has abated in most towns, architectural review boards may have special requirements such as arrays appearing on the front of the home must be laid out in perfect rectangles, which may limit the size of the system.

Some common questions:

  1. What is net metering?  Net metering occurs when you generate more electricity at a given time than you are using in your home.  Typically this occurs during the summer months when the system is producing the most, possibly more than you need.  Then the system is designed to send your excess electricity back into the grid and your meter actually runs backwards.  If your system is not grid-tied, most excess electricity that is generated will be lost.  But if it is grid-tied, your system will not function during a power outage.
  2. Will technological advances make my system obsolete?  Although there have been increases in panel efficiency in recent years, there have not been any dramatic changes nor are major changes anticipated.  In addition, while there are new products such as solar film, the relationship between cost and electricity generation still points to traditional solar panels as the most economical choice.When considering different proposals, it is worth evaluating the cost of the most efficient panels relative to their electrical output and comparing that cost to the next most efficient panel.  You may be able to get most of the power of the former for a much lower cost.  The highest efficiency panels make the most sense when it is important to maximize power generation in a limited space.  Finally, it is important to keep in mind that whatever panels you choose will generate most of their rated electrical output for 25 years or more, regardless of technological advances.

Solar thermal systems are used for heating water.  One common use is for pool heating but they can also be used for domestic hot water generation.  The roof top systems are smaller (usually only 2 or 3 panels) and much less expensive than PV systems and may be a wise choice if there is limited roof space.  They are also less sensitive to small amounts of shading. The heated water is stored in a separate storage tank near your existing hot water heater and is used to pre-heat hot water for use in the home.  Tax credits and financial incentives are available for domestic hot water systems though not pool heating systems.

In sum, solar systems should have a good rate of return and are worth considering when other energy savings steps have been completed.  Summer is a great time to tackle all of your home’s inefficiencies.

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