Silver Spring Networks and Chicago-area utility Commonwealth Edison have finally set the scope of their smart grid partnership  -- and it’s a big one.

IPO hopeful Silver Spring announced Thursday that it will deploy its smart grid networking platform to nearly 4 million homes, businesses and distribution grid devices over the coming years. That’s a big new deployment, compared to the 8 million smart meters the startup has connected in the field to date, and the roughly 17 million it previously had under contract.

Growing its customer base will be important for Silver Spring’s ambitions to raise up to $150 million in an initial public offering. The company filed its S-1 in July, but has since remained quiet on its IPO plans, though it has raised $24 million from new investor (and partner) EMC, on top of the roughly $275 million it had previously raised.

The Redwood City, Calif.-based startup relies on a few large smart meter deployments for the majority of its revenues, and with some of those winding down, it’s been under pressure to land new business.

The ComEd project is also noteworthy in the scope of devices, or “endpoints,” that Silver Spring will be networking. The company mainly connects residential smart meters right now, but it’s been seeking to expand its reach into distribution grid sensors and controls, demand response platforms, plug-in vehicle charging and other smart grid devices.

Silver Spring has been working on a distribution automation pilot with AEP in Ohio, and recently announced a project with Progress Energy that centers on commercial and industrial customers, rather than residential. Thursday’s announcement didn’t provide details of just how the 4-million endpoint deployment would be split between homes, businesses and grid devices.

We’ve been waiting since January to hear how big Silver Spring’s project with ComEd would be. That was when the two said they’d collaborate on the utility’s $2.6 billion smart grid deployment plan. Roughly half of that amount is aimed at adding new smart grid technologies, which would include smart meters, grid sensors and other devices Silver Spring might manage.

It will be interesting to see how ComEd and Silver Spring decide to use smart meters to connect to the utility’s 3.8 million customers The two started working on a Department of Energy-funded pilot project in 2009, aimed at connecting 141,000 homes to a variety of consumer services, including alternative pricing plans, web interfaces, in-home displays, home area network control systems and programmable thermostats.

Another question for Silver Spring and ComEd is how they might use cellular networks to support their planned 4 million endpoints. Silver Spring announced last month a new set of cellular communications capabilities to augment the wireless mesh networks it’s relied on for its previous deployments. Smart meter maker Itron’s decision to buy cellular smart meter startup SmartSynch for $100 million this week has cemented the role of public wireless networks in the smart grid hierarchy.