When Siemens bought eMeter in December, one of the biggest questions left unanswered was for what price, and thus, what return for the investors that have collectively put about $70 million into the company.

We may have an answer. According to two sources with knowledge of the matter, Siemens paid between $180 million and $220 million for eMeter, representing a 2.6x to 3.1x multiple on investors’ stakes in the San Mateo, Calif.-based startup. Siemens and eMeter executives contacted this week declined to comment on whether or not those price estimates were accurate.

If they are, however, that means eMeter’s investors didn’t see the 3.65x return that most Silicon Valley software startups have realized over the past decade and a half, according to Silicon Valley Bank (PDF), never mind the 10x return that qualifies a breakout success in VC terms. That return will differ by investor, of course, with earlier investors expected to reap greater rewards. Foundation Capital and Sequoia Capital led eMeter’s Series A round of about $12.5 million in 2007, Siemens led a $12.5 million Series B round in 2008, Foundation and Sequoia invested $32 million more in July 2009 and the two brought in Northgate Capital on a $12.5 million round in 2010.

But there’s another way to measure the price Siemens paid for eMeter. According to our two sources, eMeter had about $35 million in annual revenues at the time of the purchase. That would mean that Siemens’s $180 million to $220 million represents a 5x to 6x multiple on revenues -- and that, one of our sources told us, seemed like an impressive deal in today’s smart grid M&A world.

Think of it as the buyer’s return on its investment, in terms of the years it will take to earn back the money it spent. Of course, both parties in the transaction are hoping those revenues will grow -- but sometimes it takes a partner that’s in the business to open up the field.

These two viewpoints underscore a divide between the expectations of traditional VC investors seeking internet-style returns and the realities of doing business in the smart grid world. Utilities may be big and reliable customers, but they’re also cautious, burdened by regulations and very slow moving.

To grow beyond that slow and cautious pace, startups will have to find a way to expand the scope of what they do for the utilities they serve -- or perhaps, reach beyond the utility to capture the same mass-market dynamics that have made the likes of Google (and now, perhaps, Facebook) hits for their investors. But that hasn’t happened for the smart grid, at least yet.  

Even during the pre-recession high point in smart grid investment, we were hearing warnings from certain high-profile green technology investors, including Vinod Khosla, that the sector was likely to disappoint anyone who lacked a lot of patience. Smart grid has traditionally lagged VC investment seen forsolar biofuels, transportation or other hot greentech sectors.

The billions of dollars in government stimulus directed at smart grid projects in late 2009 and through 2010 helped boost investment, but last year saw a big dropoff. Mercom Capital reported this month that smart grid venture investment fell to $377 million in 2011, nearly half the $769 million raised in 2010.

Instead, smart grid exits have increasingly been inside jobs. Acquisitions in the space boomed to $4.6 billion in 2011, up from $1.34 billion the year before, Mercom reported. Big deals included Schneider Electric’s $2 billion purchase of Telvent and its $268 million acquisition of Summit Energy, and GE and ABB also buying. Siemens leaped into the grid communications and networking space on Monday with its $382 million purchase of RuggedCom, a maker of hardened grid routers and switches that’s competing with Cisco to link up substations. 

But industry observers agree it’s likely that the best return on a smart grid sale in 2011 was for Bayard Capital Group, the private equity firm that sold Landis+Gyr to Toshiba for $2.3 billion in May. Bayard bought the conglomerate in 2004 and watched it grow to about $1.5 billion in 2010 sales. While Bayard didn’t disclose its purchase price back in 2004, industry observers believe that it saw a handsome return on its investment.

Measured against $1.5 billion in annual revenues, however, Toshiba’s $2.3 billion seems like it will pay off much faster for it than Siemens’ rumored $180 million to $220 million for eMeter. Siemens has been relatively quiet on the smart grid acquisition front, but has a lot of cash in reserve to buy more, and seems to be willing to pay more than the competition. GTM Research smart grid analyst Ben Kellison noted that Siemens paid $104 million more for RuggedCom than Belden's offer in early January, or a 38 percent premium.

Of course, companies like RuggedCom and Landis+Gyr are established players in the field of building hardware for utilities, with growth patterns that closely match utilities slow and ponderous advances -- and retreats -- in the face of market and regulatory demand.

But eMeter is in a different field, one where software is king, and while it had to build its product from scratch, it’s also building a market that could soon exceed equipment sales for utility smart grid projects.

It’s made an impressive debut. Big utility customers include Alliant Energy, Texas utilities CenterPoint Energy and Bluebonnet Electric Cooperative, and Canada's Toronto Hydro. It’s working with Siemens on a stimulus grant-funded smart grid project with utility Kansas City Power & Light, linking distribution grid and demand response into the mix. On the other side of the scale, it has tested its Energy Engage customer platform with utility Pepco in Washington, D.C.

Even so, it would appear that its chosen field of smart meter data management is not going to be one that supports a raft of IPOs. Rival MDM provider Ecologic Analytics was bought up by Landis+Gyr earlier this year, and smart meter kingpin Itron and software behemoth Oracle own the largest shares of the MDM market.

As for public exits in the smart grid, the list is short. Foundation Capital presumably saw a return from the IPO of portfolio company EnerNOC, which along with fellow demand response aggregator Comverge could be considered the first pure-play smart grid firms to go public in 2007.

Foundation is also a key investor in Silver Spring Networks, the smart grid networking company that filed plans in June to raise up to $150 million in an initial public offering, but has yet to pull the trigger. The Redwood City, Calif.-based startup has raised about $300 million from investors, and reports say it is seeking a $2 billion to $3 billion market capitalization in its IPO.

It also has yet to post its first quarterly profit. Forbes wrote an interesting IPO head-to-head comparing Silver Spring to headphone maker Skullcandy in terms of market share, market potential and the all-important question of profitability, which Skullcandy had and Silver Spring lacked. It's worth reading, if only to remind ourselves of the gulf between the smart grid and the mass consumer market.