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The Design

When planning a solar panel system for your home, the first consideration for any solar designer is the tilt and orientation of your roof areas.  We need to know which roof(s) will ensure the most optimum solar output – which translates to the best Return on Investment.   For us here in Maryland, the most optimum solar roof orientation is Due South at 180 degrees.  Of course, not everyone has this perfectly oriented roof and our customer base consists of homes that have South, West, East and everything-in-between orientations.  Occasionally we even install on North-facing roofs if the pitch of the roof is low enough that panels are close to flat, or can be tilted southerly.

For homes that face East-West, you may be wondering which roof would best suited for solar.  This is a good question given the fact that the output of your solar panels is directly related to your Return on Investment and how quickly the panels can pay for themselves.

If either East or West favors a more Southerly angle, then that would likely be a more favorable roof.  Assuming that there aren’t issues related to shadingor obstructions caused by chimneys, vents, skylights and other roof-placed items.

If the house has a perfectly split East-West orientation, with all things equal – the next consideration would be roof angle; the lower the tilt (i.e. closer to horizontal) – the more solar energy will be generated over the course of the day.  If the tilt on either side is the same then we would usually favor the West facing side.   Here in Maryland, DC and Virginia we tend to have cloudier mornings, and sunnier afternoons going into dusk.  Therefore we want to capture the late afternoon sun (west facing) more than early morning sun (East facing).  Of course, should you happen to have a tree, chimney or other obstructing factor(s) on the West roof – we’d favor the East.

The Economics

Homeowners looking at an East-West installation often have concerns as to whether or not their system will be profitable enough, compared to its south-facing counterparts.    Disqualifiers for cost-effective solar systems include shading and limited available roof space.  Rarely, however, is a home found unsuitable due to a Non-Southerly facing roof alone.

To illustrate, following is a comparison of a 10kW system’s output respective to East, West and South facing orientations.  Data compiled using the National Renewable Energy Laboratory (NREL) weather data patterns for Baltimore, MD –

10kW system installed on a 20 degree pitched roof with zero shade

	SOUTH (180 degrees)	WEST (270 degrees)	EAST (90 degrees)
ANNUAL OUTPUT	13,224kWh	11,389kWh	11,328 kWh
*Annual $avings	$1853 per year	$1594 per year	$1586

*Savings based on a conservative $3.00/watt installation, and $0.14/watt BGE rate

Data from PV WATTS

As illustrated, although perfectly South would be ideal, the East and West orientations provide a competitive amount of solar and would add only a few months to the payback period.  If you were choosing between East and West (as opposed to installing on both), the difference is nominal.  The choice of which roof may come down to aesthetic preference, distance to utility meter and regional weather patterns.

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Ways in which solar panel arrays can affix to a commercial building are as many and varied as the buildings they serve.  In this article, we’re focusing on one popular method of attachment – the Ballasted roof mounted system.

Many commercial solar prospects in and around the Maryland/Washington DC area have buildings with flat or low-sloped roofs.  These are generally defined as a roof with a 3:12 pitch or less.  For these applications, ballasted systems can offer a way of attaching solar arrays without any (or few) roof penetrations.   Many building-owners prefer this method of attachment as it negates the risk of leaking, particularly on flat roofs that may collect water.  With a ballasted system, the arrays are held down using the weight of the existing solar panel arrays, racking and – most importantly – concrete blocks.  These blocks are placed methodically throughout the system design to secure the arrays in place, resisting wind loads or other types of disturbance.

The addition of the ballast blocks to the solar system arrays adds additional weight to the roof.   As with any commercial solar project, this weight load is analyzed and approved by a licensed structural engineer as required by the permitting jurisdiction – with some differences in each locale.   Ballast racking manufacturers also specify the amount of blocks required throughout a design.  The number of blocks per panel typically varies throughout design due to array proximity to building edges, parapets, or other inconsistencies that can affect wind and snow loads.

Although ballasted systems have many advantages, as with any design, they have their disadvantages and are not compatible with every type of commercial building.   Let’s look at a generalized comparison scenario between a roof penetrated system and a Ballasted System:

	Roof Penetrated System	Ballasted System
Labor Costs and Criteria	HIGH:  Penetrations require flashing and sealing techniques – sometimes requiring coordination with a roofer.	LOW.   Labor skills required for installation are fairly straight-forward and require no roofer coordination.
Panel Count	INCREASED:  Systems attached to roof substructure require less roof space for mounting.	DECREASED:  Ballast-blocks require roof space and can limit the available space for solar panels
Roof Loading	DECREASED.  No concrete/ballast block	INCREASED:  Ballast blocks add weight to the roof
Design Criteria	FLEXIBLE:  Attached racking systems can usually negotiate hatches, HVAC equipment	LESS FLEXIBLE:  Because of the increased space required per panel – designs may be more rigid.  However most ballast racking does have reasonable flexibility.
Roof Pitch	5 – 45 degrees	Generally less than 10 degrees

 

Multi-roofed commercial applications may have a variety of racking types for both sloped and pitched roofs. There are other options for flat roofs also, including attached and hybrid solutions that use a combination of both penetrated and non-penetrated techniques.

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