Although our goal is to make buying solar as simple as buying a household appliance, we know that things get confusing when you’re talking with multiple contractors who propose different system designs.
So . . .
- What should you ask your solar contractor?
- What should you look for in a well-designed system?
- How can you compare different proposals?
First, use the same judgement you would with any contractor and consider these questions throughout the process:
- Is your contractor selling you or informing you?
- Is your contractor helping you meet your goals or telling you what you want to hear?
The most common point of confusion: System size (capacity) versus actual output
Capacity refers to how much electricity a solar panel or system can generate in ideal conditions. A 1kW (kilowatt) system (consisting of four or five panels) will produce 1kW hour of electricity per hour under ideal conditions – enough electricity to power ten 100-watt light bulbs. Because conditions are seldom ideal, a panel’s capacity and the actual energy it produces are seldom the same. In the DC area, an optimally positioned 1 kW system will generate an average of 1292 kWhs of electricity in a year.
It’s critical to keep the difference between capacity and projected energy output in mind – and to understand the factors that affect that difference.
The Surprising Effect of Shading on System Output
Ralf J. Muenster, Director of National Semiconductor’s Renewable Energy Segment, writes “. . . many solar installation owners and customers remain unaware of the degree to which the array . . . might be failing to realize its full potential due to partial or temporary shading.
“In fact, whether caused by a neighbor's trees or their own chimneys, homeowners may be losing between 20% and 40% of the potential output of their solar installations because of shade. Just 10% shading of a solar array can lead to a 50% decline in efficiency and even, on occasion, total system shutdown.”
That’s why a smaller, well-positioned, unshaded system can produce as much electricity as a larger system that’s improperly positioned and partially shaded.
Your contractor should use one of two tools (Solar Pathfinder or Solmetric Suneye) to accurately measure how shading (if there is any) will affect energy production of the panels.
Each contractor should show you his or her proposed system’s projected output, not just the system’s capacity. With that, you can figure out how much real electrical bang you’re getting for your buck: divide the cost of the system by the annual output in kWh to calculate cents per kWh.
How Panel Tilt and Orientation Affect System Output
Solar panel output is maximized when the sun strikes the panel “head on” (at a right angle). In the DC area, panels should be mounted at a 39 degree angle from vertical to maximize output. For the same reason, your panels should (ideally) point due south (not magnetic south, which is about 10 degrees different in the DC metro area).
The Oregon Department of Energy adds these two angles together into a “tilt and orientation factor,” or TOF. The ideal TOF is 100%; a tilt and orientation factor of 70% indicates a loss of 30% in solar potential.
The biggest mistake we see in the DC area is when contractors mount solar collectors flat on a flat roof. According to the U.S. Department of Energy’s National Renewable Energy Laboratory, this cuts solar output by 14% -- not counting dirt accumulation over time, which is a big factor on flat mounted panels. ( As far as we know, the NREL numbers are a laboratory average.)
Google studied the real-world effect of tilt on the output of its very large solar panel installations. Google concluded that “Tilt plays a key role in solar output fluctuation. . .” -- affecting output by as much as 35% during some periods. The effect of tilt is very significant near horizontal (flat) mounting and much less significant near the ideal mounting angle for any given location. In other words, some tilt away from the ideal mounting angle doesn't have a large negative effect, but some tilt up from horizontal can provide significant benefits.
If your contractor plans to mount your panels at some less-than-ideal angle, he or she should explain why. Is there a cost consideration? And how will that cost consideration balance against the loss of efficiency?
How Tilt Affects Panel Cleanliness -- and System Output
Tilt also makes a significant difference in how often panels need to be cleaned to maximize output. The Google report concluded that “The difference in output between tilted panels that are cleaned professionally versus those that are cleaned by rain is [only] 12%. Dirt does remain in the bottom-most corner of the solar panels, however.”
As for horizontal panels, the report said “Rain does not clean them well . . . [ Manual ] Cleaning of non-tilted solar panels resulted in tremendous gains in output.” The actual losses in efficiency for dirty panels were 36% over 8 months and 50% over 15 months!
Taking these results into account, Google decided to clean the flat panels regularly, leaving the tilted ones uncleaned “because the difference in output is negligible.”
We recommend having your panels "squeegeed" whenever you have your gutters cleaned. Automatic cleaning systems are also available.
Properly Sizing the System and the Inverter
In some states, you can sell all the electricity you produce to the electric company. DC, MD, and VA are all subject to “net metering,” which means the electric company only has to pay you for as much electricity as you consume over the course of a year.
So if your system’s projected annual output is more than your annual electrical use, you’ll end up “giving” electricity to the electric company. So your installer should not only ask if your future electrical needs might increase, but if they might decrease (because of incremental energy efficiency changes, or large ones, such as installing a high-efficiency heat pump or air conditioner).
Another factor your contractor has to consider is how to size the inverter. The inverter changes the panels’ direct current (DC) output to the alternating current (AC) that we’re all familiar with. To maximize efficiency, your inverter should be sized as close as possible to the size of the system. Note, too, that inverters are priced by size.
Consider asking your contractor how he or she chose the size of the inverter.
Proper Grounding of Your System
Every system should have two grounds: one that ties the solar electrical wiring into the electrical panel box and one that separately grounds the system’s frame and hardware. We have seen systems without this second ground.
Consider asking your contractor how your system will be grounded.
References:
http://www.renewableenergyworld.com/rea/news/article/2009/02/shade-happens-54551
http://www.goodcleantech.com/2009/08/google_studies_how_dirt_affect.php