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July 11, 2011 | Leave A Comment

In Oregon, in order to receive the rebate from the Energy Trust a system must have a 75% solar resource. This is known as the TSRF or Total Solar Resource Fraction. Solar arrays lose efficiency due to losses from shading, and when tilt and orientation are not ideal. For the Portland metro the ideal tilt and orientation is 190 degrees (slightly west of South) and a 32 degree roof pitch.

Of course, it is extremely rare for a home to have perfect pitch, orientation, and no shade. More often than not, there are shading obstacles like trees, and roofs are facing East to West. Have no fear, solar arrays can still work on East-West facing roofs but if there is some shade as well your roof may not have a 75% TSRF.

One possible solution, although not ideal from an aesthetic perspective, is to pitch the solar modules to the South off of an East or West facing roof. Recently, we had just such a scenario where a SE roof was heavily shaded but the NW roof was far enough away from the trees to receive only minimal shade. Of course we can’t install the solar modules flush to a NW roof but what if we pitch them up off the roof towards the SW?

And furthermore, what is the new roof pitch and orientation? It might seem like a simple question but it is actually quite complex. A new compound angle is being created with a distinct pitch and orientation that depends on how steeply you pitch the modules off the East-West roof. I scratched my head for a long time on this one and attempted to conjure up some clever trigonometry. I googled away and couldn’t find an answer.

Finally, I stumbled on Solmetric’s website where they have created a compound angle tool. Great job Solmetric!

Here is how it worked out. The unshaded roof is facing NW at 315 degrees. The roof is sloped at 13 degrees. If the module is pitched at 25 degrees towards the SW than the compound angle creates a new array that has an azimuth of 251 degrees and a tilt of 28 degrees. This new azimuth and tilt was enough to achieve that 75% TSRF that is required by Energy Trust.


Posted in FAQ's, Pacific Northwest Solar, Residential Solar | Tags: , , ,

May 3, 2011 | 2 comments so far

How much energy will my solar array produce and what will that do to my electric bill?

This is one of the most common questions that we hear and often times there are misconceptions about just how much energy your residential solar system will produce. A good place to start for any home or business owner is with your electric bill from your utility provider. In most cases there is a bar graph that shows your yearly electric usage in units of  kilowatt-hrs(kwh)/day.

In January, for example, this home used on average 144 kwh/day. Multiplying this number by 31 days and we can calculate that 4464 kwh were used in the month of January. Going month by month and adding up all of the kwh usage, you can calculate the usage for the entire year. For this particular graph the yearly usage is about 36,840 kwh.

What exactly is a kwh you may ask?

Think of a standard 60W incandescent light bulb. 60W is the power requirement for this bulb. If you leave this light bulb on for one hour this bulb has used 60 watt-hours of energy. If the light is on for 10 hours the bulb has used 600 watt-hours of energy. Dividing watt-hours by one thousand gives you kilowatt-hours. So, leaving a 60W light bulb on for ten hours uses 0.6 kwh of electricity.

How many kwh will my solar energy system produce?

Now that we know how to determine our yearly kwh usage from one electric bill statement, the next step is to understand how many kwh a solar electric system will produce. Solar energy system production is based on many factors including system size, shading, roof pitch, and roof orientation. The ideal roof pitch and orientation for a system in Portland, OR is 32 degrees at 190 degrees.

As a general rule of thumb, you can assume that for every 1kW of solar energy your system in Portland will produce 1000 kwh/year of electricity.

For example, if you install a 3kW solar electric system it will produce approximately 3000 kwh/year of electricity. If you calculate from your electric bill that your yearly usage is 10000kwh then you know your system will offset 30% of your yearly bill.

 

 

 

 


Posted in FAQ's, Residential Solar, Solar 101 | Tags: , ,