GreenBuild, the country’s biggest building efficiency and sustainability design and technology conference, hit San Francisco last week. As one might expect, the conference was accompanied by a flurry of news on green building projects and partnerships.  

One of the more interesting items comes from San Francisco-based startup Stem and electric vehicle and battery startup Coda. The two announced Thursday that they’ve deployed a building energy optimization system at two San Francisco hotels -- the InterContinental in SoMa, opened in 2008, and the InterContinental Mark Hopkins, first opened in 1926.

You’d be hard-pressed to find two more dissimilar properties in terms of energy technology and design than the LEED-certified SoMa hotel and the Jazz Era Mark Hopkins, which still uses steam boilers to run its plant. But Brian Thompson, Stem’s founder and CEO, said in an interview last week that the startup can handle both types of challenge -- the building that’s so new it’s hard to find any more efficiencies to wring out of it, and the building that’s so old it’s hard to know where to begin.

That’s because Stem doesn’t touch the actual building systems it optimizes, he said. Instead, it installs a battery and control systems at the building, connects it to the building power line via an inverter, and then uses that battery to store grid power when it’s cheap and discharge it when prices peak -- all without asking the building owner to do a thing. Typical energy bill reductions are in the 10 percent to 20 percent range, with ROI predicted in under three years, but often faster.

Can batteries actually be cost-effective in this way? Wouldn’t it be cheaper to just turn down the lights and raise the thermostats during peak events to save money? Not necessarily, Thompson contends. First of all, most building owners and operators care far more about keeping occupants comfortable and safe than shaving money from their power bills, which puts practical limits on how much spare energy they can dial back at any given moment.

Secondly, Stem’s databases and cloud-based optimization platform can fine-tune the battery’s operation to maximize economic return, compared to most other building energy storage or power generation systems, Thompson said. That, in turn, is enabled by the big data analytics and real-time software that builds energy profiles of its buildings, using one-second power data, and then incorporates some 50 other variables into the equation.

That’s a similar kind of building energy analytics that we’ve seen deployed by startups like First Fuel, Retroficiency, Viridity Energy, BuildingIQ, SCIenergy and SkyFoundry, to name a few players in the building energy efficiency space. Stem differs from those in that it’s tapping an actual power source, rather than the end systems that consume power.

Of course, other green building projects are also using batteries as part of their mix. But Thompson contends that they’re not optimizing their use. Instead, they’re typically setting them up either for emergency backup power, or setting them on daily schedules that don’t necessarily reflect the best economic return, he said.

Indeed, one of Stem’s core economic advantages is that it can cycle its batteries far fewer times, by picking and choosing which times yield the highest economic impact. That extends battery life, and thus the working lifetime of the asset, improving ROI, he said.

In the case of the San Francisco projects, Stem is using 60 kilowatt-hour lithium-ion batteries from Coda, which has its own grid energy storage line of business. But Stem is battery-agnostic, Thompson said, and is working with other battery partners with unnamed retail and commercial property customers, he said.

At the same time, the complexity of commercial power rate plans plays to Stem’s benefit, Thompson said, since the software can analyze that complexity and pick out best options at a fraction of the speed and cost of hiring humans to figure it out.

“We have a real-time scalable data processing engine in the cloud that allows us to make decisions on the fly for our systems on the ground,” he said. “Most systems that use data collect data and store it -- and then it has to go read all that data,” a process that doesn’t lend itself to real-time management of building assets like batteries or automation systems.

Thompson and colleagues developed the technology at the Wharton School of the University of Pennsylvania, and launched the company, formerly called Powergetics, in 2009. The 30-employee startup has raised more than $10 million in VC funding from the Angeleno Group and Greener Capital, as well as investor David Buzby, a former SunEdison board chairman.

In other GreenBuild news:

- Last week saw a new development on the front of property assessed clean energy (PACE) financing -- putting the cost and payback of retrofits onto property tax bills. On Tuesday, San Francisco became the latest municipality to launch its own commercial PACE program, bringing building systems giant Johnson Controls in on a $1.6 million project to retrofit building property management firm Prologis’ headquarters at the city’s historic Pier 1 building. (Johnson Controls also launched new partnerships for its Panoptix cloud-based building management platform last week, including a set of apps partners like BuildingIQ and First Fuel).

About 90 percent of the project cost is being covered by 20-year, low-interest bonds backed by the property tax assessment created by the PACE program. That sets up a payment stream that doesn’t change when buildings change hands -- an important barrier to investing in efficiency in the commercial real estate world. In the case of the Pier 1 project, the bonds were bought by Clean Fund of San Rafael Calif., a specialty PACE finance provider.

PACE financing is one of several new financial models being tested in the commercial building efficiency retrofit market. As for the residential market, PACE schemes have suffered setbacks under federal rulings that barred Fannie Mae and Freddie Mac from backing mortgages that included solar or efficiency property tax assessments, though we’re seeing new home PACE programs emerge that could help surmount those problems. 

- Can networked building lights save enough energy to make them worth the cost? We’ve seen a growing chorus answering that question in the affirmative, with startups like Adura, Redwood Systems, Daintree Networks, Digital Lumens and others announcing some big energy savings from their deployments.

The City of San Jose and light networking startup Enlighted joined that chorus on Wednesday, announcing that they’ve cut lighting bills at San Hose City Hall by 53 percent since deploying Enlighted’s distributed intelligence lighting control nodes in the building. The project wasn’t just aimed at saving energy -- it was also done to respond to employees complaining about the quality of light in their workspaces, which had led some to ask building maintenance staff to remove ceiling fixtures and had then brought in their own lights.

San Jose’s City Hall was already LEED Platinum certified when Enlighted came in, showing that there’s still room for squeezing even more energy efficiency out of even the most shiny-new green buildings.