Do you know what's in your solar ETF? The Guggenheim Solar ETF (TAN) is up 76% YYD, outperforming the Market Vectors Solar ETF (KWT) by 2400 basis points. TAN is more concentrated than KWT - holding 26 stocks vs. 34 - meaning it's probably going to do better in the good times, but the key may be SolarCity (SCTY). The stock's nearly tripled this year and is included in TAN but absent from KWT. One wonders for how long. KWT's periodic review comes at summer's end.