Solar PV Installations: Beyond ‘measure twice, cut once’

A successful solar photovoltaic installation is one in which the customer is happy and has received what was expected. Some would say that a proper installation is the first step. While a proper installation is of crucial importance, the most important first steps usually occur during the selling process where information is gathered, a proposal is generated and where the customer expectations are set.

A good first step is to explain to the customer how solar panels are rated and how efficiency is related to the larger picture. Once a customer (and the salesperson!) understands this basic information, most of the additional information that we will recommend sharing will be more easily understood.


Solar PV panels are rated in Watts-peak. This means for example, a solar panel rated as “200W” really means that (±3-5%) this is the maximum or peak instantaneous wattage that can be produced under STC (standard test conditions). A 200W panel will not produce 200Wh (watt-hours) each hour all day long, rather, it will produce up to 200Wh per each hour of full strength insolation. In many parts of the US, the adjusted hours of insolation will be between 5-6 hours per day in the summer, and 3-4 hours per day in the winter. The table  provides a snapshot of insolation values for a few major U.S. cities.


The efficiency of a PV panel is often misunderstood and misused in the sales process. A “high efficiency” panel is usually not the best value for the customer. Efficiency of a typical poly- or multicrystalline silicon PV panel may be in the range of 14%-16%. Efficiency of a more expensive monocrystalline panel will usually be in the range of 15%-18%. Some very expensive panels are reaching the 22% range. At a practical level, efficiency only affects (in a relatively small way) the amount of roof-space that a panel will need.

Power production

When a PV system does not produce the expected amount of power, the cause is often traceable to the original system specifications and design that were proposed during the sales process.

Most power production problems seem to occur not as a result of faulty installation, but rather as a result of faulty expectations or faulty design. These kinds of problems are mostly created during the specifications and design phase leading up to the sales proposal, and are most often the result of cost-cutting mixed with an effort to indulge the customer’s aesthetic considerations, without full disclosure of the ramifications.

Partial shading of a panel or string of panels can dramatically reduce the performance of the array. Within each panel, if a single cell is shaded, its lowered performance will cause a bottleneck effect that lowers the output of all other cells to match it. Within a string of panels, a shaded or partly shaded panel has the same bottleneck effect on all other panels.

Mismatched orientation causes the same effect. If some of the panels in a string face in different directions or have differing tilt angles, all of other panels in the string will drop production in real time to match the lowest performing panel of the moment. There must be a separate string used for each angle and tilt used.

It is important to show the customer much more than the rated output peak of the system. It’s far better to do a conservative calculation of average output considering all of the factors including temperature, sun angles, transient shading, inverter inefficiencies, etc. A proper calculation will ultimately be confirmed by measured production just a surely as an overstated capability will be noticed and challenged.

[A full version of this Photovoltaics World article can be found at]

John Williams is COO of Solar Panels Plus, 533 Byron Street, Suite E, Chesapeake, VA 23320 USA; 757-549-1494;



Previous articleThere Goes China and Here We Sit Bickering
Next articleSNL Energy’s Renewable Energy News Round-Up

No posts to display