How much would you pay today to cut your energy bills in half for the next  ten years or so? Or, how much money per month would you accept to let someone else do the cutting for you?

Those are two question the green building industry has to answer -- if it’s going to capture a $1 trillion energy-saving opportunity. Deutsche Bank and The Rockefeller Institute reported last week (PDF) that the United States could save that much over the next ten years by making an investment of $279 billion on green energy retrofits in U.S. buildings, from homes and storefronts to office buildings and factories.

That’s a pretty big return, compared to the slower uptake expected in residential smart appliances, energy efficiency standards and other home improvements. They’re only getting us to $1 trillion by 2035, according to a new study by the American Council for an Energy Efficient Economy.

But what about energy efficiency as an investment? Sure, it remains the cheapest way to spend money to save energy, with proven savings you can take to the bank. Even so, energy efficiency retrofits aren’t the sexiest way to invest the money you've got, compared to other opportunities that may present themselves to property owners, managers and tenants.

Incentives are split, particularly in the commercial building space, where property changes hands and occupants more frequently. Tenants tend to be insulated from energy price fluctuations, and owners may want to sell the property before they can realize their full return on capital investments.

That’s why financing models that take costs off property owners’ balance sheets are all the rage. SCIEnergy, a building energy software and engineering services startup, just bought Transcend Equity, which offers no-money-down retrofits to customers by owning the energy bills, promising the customer lower rates over a fixed term, and taking the remaining savings as revenue. Transcend is working with Japan’s Mitsui & Co. as financial backer.

Serious Energy, a green building startup with $140 million in venture capital financing, came out with its own Serious Capital financing model last year, built along similar lines. They, in turn, are adaptations of the energy services (ESCO) models common in government, educational and institutional buildings, where giants like Johnson Controls, Honeywell, Chevron Energy Services, Schneider Electric, Siemens, GE and others hold sway.

A rough guide of savings has to include, first off, the planning to decide which buildings can give you the biggest bang for your buck. Efficiency project planning and data analysis are the specialty of startups like Retroficiency and FirstFuel, and can shave significant upfront costs in project planning costs, as well as tracking the ongoing performance of the upgrades you choose to make.

The first wave, of course, comes as low-tech light bulb replacement and energy-efficient windows. Next up tends to be boilers, chillers and other HVAC improvements like variable-speed fans and the like. Of course, a building management system (BMS), running on a number of standard protocols, is a must for more modern buildings. Cypress EnviroSense installs wireless sensors on the old pneumatic thermostats common to buildings built before the '90s.

But every energy retrofit tends to see its performance slip over time, until it’s close to where it started -- a natural consequence of people fiddling with dials and thermostat settings, equipment failure, windows being left open, and the like.

Fixing those problems can gain you back all the efficiency you would have lost. Continuous commissioning and fault detection, as they’re known, can yield 10 percent to 20 percent energy bill reductions, and they’re persistent over time. That’s where SCIEnergy’s software expertise lies, as does that of Valence Energy, the software startup that Serious bought in 2010.

Beyond that, sensors can turn off lights and fans when rooms are empty, set cubicles to turn off when employees swipe out at night, and other such automation features that add energy savings of their own. Measuring them is hard, though, since it gets into offices where discrete energy use isn’t really metered by anything. Energy-smart office equipment could help with that equation. Cisco’s EnergyWise program was meant to connect office IT (computers, printers, etc.) to building management systems, but it’s backed off that effort, though the EnergyWise protocol it developed for the task is still in use by partners like Schneider Electric.

Adding power prices into the mix yields new applications for technology. Ice Energy builds rooftop air conditioning units that make ice with cheap nighttime electricity, then use it to reduce power during hot afternoons when prices are at their peak. BuildingIQ puts servers in buildings to pre-cool offices before a peak price event hits, then lets the AC drift to lessen its draw on the grid.

Yet deeper retrofits are available -- at a price. Facebook is putting $1 billion into its super-efficient data center in Oregon, and plenty of demonstration projects, like the Empire State Building in New York, are showing that big investment yields big improvements. The Obama administration’s $4 billion Better Buildings Challenge initiative is asking private partners to build showcases, but also to develop technology that the broader market can use.

The process should reveal some interesting contenders for the next new thing in marrying sensibility with profit.

Tags: cisco, demand response, efficiency, energy efficiency, general electric, green building, honeywell, ibm, johnson controls, policy, schneider electric, scienergy, serious energy, siemens, smart grid