Commodity prices have been rebounding this year, which bodes well for energy ETFs, such as the natural gas ETF: United States Gas Fund (UNG). It doesn't matter if its fossil fuel or alternative energy ETF like Guggeinheim (TAN), higher energy prices would drive both up.Fellow blogger Bruce Jefferson notes that both natural gas and renewable energy investors want higher energy prices:Why Natural Gas Investors And Renewable Energy Investors Are Hoping For The Same Thing UNG tracks natural gas prices.UNG Daily Chart 5/16(click to enlarge) UNG has been bearish, but appears to have flattened since March after it broke below 6.00 briefly. It has came up above 7 since, but still remains bearish. 1) Price failed to climb above the 100-day simple moving average and is now returning below the 50-day SMA.2) The RSI remains below 60 outside of a brief crack above it. With natural gas prices still pressured, we should probably refrain from investing in TAN and similar renewable energy funds. The long-term outlook could still be bullish, but we might have to tie up cash in the short to medium-term into a choppy, maybe bearish sector. As you can see in TAN, the 21.50 common support pivot is broken, and price is looking at 20.00 as the next support pivot. TAN Daily Chart 5/16(click to enlarge)Below 20, TAN opens up the 2012-2013 lows between 12.43 and 15.00.TAN Weekly Chart 5/16(click to enlarge)I did want to take a stab at TAN at 20.00, but not if UNG is falling. The good thing is, oil prices are rallying, but if we see this stall as well, we should really keep some distance from TAN. USDWTI Daily Chart(click to enlarge)Oil prices have enjoyed a run up since it was tagged 26. However, as price approaches 50, we should consider a possible retreat. If so, there could be pressure on other types of energy investments as well.