SOLAR TODAY - June 2009

readers respond

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Bubble Trouble

I thought the bubble chart on the annual energy bill for a typical home (“Home Energy Upgrades That Pay,” March issue) was a great graphic that I could use in teaching. But then I got to thinking about the graphic and what was portrayed, and it did not make sense to me. Knowing what I do about electric usage, I was stumped that the heating and cooling category was represented as such a huge part of the load. So I went to the source cited in the graphic, energystar.gov. Their graphic is a pie chart and shows that heating and cooling account for 49 percent of a typical home’s annual energy bill. Looking at SOLAR TODAY’s bubble chart, it certainly appears as though the heating and cooling bubble is at least three, maybe four, times greater than all the rest of the usages combined.

Revised graphic represents usage differences.

When I showed the documents to my daughter, a master teacher in math who specializes in graphics, she discovered the problem. While the diameters of each bubble are proportional, the areas are not. Increasing the diameter four times will increase the area 16 times.

So, the bubble chart should have represented the differences by area, not diameter. This would have made all the smaller bubbles, representing 51 percent of the typical home energy bill, about equal to the large bubble for heating and cooling, which represents 49 percent of the bill. This would have been a more accurate representation.

Mick Sagrillo

Forestville, Wis. msagrillo@wizunwired.net

10 June 2009 SOLAR TODAY solartoday.org

SOLAR TODAY, Kutscher Win Gold

SOLAR TODAY was recognized in April with a Gold EXCEL Award in the Society of National Association Publications annual EXCEL Awards program. The award, for the January/February editorial, “Tackling Climate Change: The Debate Is Over! (Or Is It?)?” by Chuck Kutscher, is SOLAR TODAY’s second Gold EXCEL in three years. The award will be presented in Washington, D.C., in June.

Corrections

In “Firefighters Learn To Work Around Rooftop PV” (April issue, page 12), we sought to clarify some issues regarding rooftop access and the potential for electric shock near photovoltaic modules. Our short article was illustrated by an out-of-date diagram from the Los Angeles Fire Department, indicating a 4-foot setback between the roof edge and the PV panel. In fact, this is a recommendation, not a code requirement, and the margin at the roof peak is 3 feet. Matt Paiss, whom we quoted in the article, writes, “It is our recommendation that all DC conduit be considered energized in daytime hours, instead of making a possibly fatal error assuming the conductors are de-energized when the disconnect is opened.”

The article “Integrated Module/Inverter Systems Produce Rooftop AC” (April issue, page 14) concluded with a reference to “a big red emergency cut-off switch for use by firefighters and utility company emergency crews.” John C. Wiles, program manager of the Southwest Technology Development Institute, writes that many utilities no longer require any such disconnect. He also points out that charge controllers, also mentioned in the article, are used only in off-grid, battery-backed systems and are rarely used where grid-tied systems have battery back-up.

In the “New Products” section of the March issue (page 48), our headline on the Electra Therm Waste Heat Generator implied that the machine is a cogeneration unit, which it is not. The generator produces electricity from waste heat.