Noted short seller Carson Block is betting against shares of Sunrun , saying the residential solar company is worth 90% less than its current market valuation. Block takes issue with the way Sunrun accounts for tax credits, telling CNBC's "Squawk Box" on Tuesday the company is "bamboozling" the Internal Revenue Service. The Muddy Waters founder announced his short position last Thursday in a report titled "Sunrun: 'ESG' is for Everybody Screws the Government." That day, shares of Sunrun surged 30% on the heels of the surprise announcement of a deal between Senator Chuck Schumer, D-N.Y., and Senator Joe Manchin , D-W.Va., on climate funding . "We're still short," Block said Tuesday. "We think the value for equity holders is probably 80% to 90% lower than what the company tells the Street" he said. Block argued the company uses "highly aggressive modeling" to arrive at its valuation, including around how it accounts for government tax credits. Sunrun CEO Mary Powell refuted Block's claims. "Muddy Waters' 'report' is flat-out wrong, a classic 'short and distort' tactic," she said in a statement to CNBC. "It is an attempt to mislead with grossly false and inaccurate assertions in a brazenly self-serving effort to mitigate recent losses in their short position at the expense of Sunrun's existing investors." Powell added that the company's tax and valuation procedures are "closely diligenced." Across Wall Street, 19 firms have a buy-equivalent rating on shares of Sunrun. Five firms are neutral, with no analyst recommending investors sell the stock. The average price target is roughly $45 per share, or about 50% above its $30.21 Monday closing price. "Bottom line is, we think the stock should be a lot lower than where it is. We thought it should be a lot lower when it was at $22, $23, so certainly around $30, yes, it should be a lot lower," he said. Shares of Sunrun slid 2.5% during premarket trading Tuesday. Block's latest short comes just a few weeks after he took aim against Hannon Armstrong , which finances clean energy projects. "Climate change is real. Climate change is manmade. I want something to be done about it, but it's actually pretty disheartening when you get into these companies," he said. Earlier this year, the Justice Department launched an investigation into short sellers , including Block.