It’s the big smart grid IPO of the year. Silver Spring Networks, the most successful smart meter-smart grid networking startup out there, has filed for an initial public offering to raise as much as $150 million on the New York Stock Exchange. We’ve been waiting for this one for a long time. SSN’s Internet protocol-based networking and 900-megahertz radios are inside about 8 million smart meters now deployed and a total of 17 million under contract, mostly in the United States, but also in Australia and potentially in Europe and South America as well. Unlike some of its fellow greentech IPO candidates in industry sectors such as biofuels and solar power, Silver Spring has revenues — $70.22 million in 2010 and $46.69 million so far this year, although it’s still not a profitable company. Then again, SSN works with slow-moving utilities as its clients, meaning it can take years for projects to move from initiation to revenue-generation. Indeed, beyond the potential good news this IPO represents to Silver Spring’s investors — including big shareholder Foundation Capital with 41.5 percent of shares, but also Kleiner Perkins Caufield Byers, W.R. Holdings, NCD Investors, Contra Costa Capital and JVB Properties — there’s the value in finally seeing a smart grid company take itself to the public markets. The smart grid sector’s exits have almost exclusively been via acquisitions. While some pretty big payouts have been happening there, a successful Silver Spring IPO could prove that startups in the sector can make it in the utility-driven smart grid world on their own.