Honeywell, the building management giant, has gone shopping again.

Today, it announced that it will buy E-Mon, which makes sub-metering equipment for apartments and commercial buildings. Sub-metering essentially allows landlords to put tenants onto individual electrical meters and thereby encourage conservation. It is often cited as one of the more effective techniques for getting companies and individuals in shared buildings to think about power. (It came up during a panel discussion yesterday, in fact.)

E-Mon, an appellation that sounds like someone trying out a fake Jamaican accent, is no startup. It was founded in 1981 and works with 1,500 distributors. Along with meters, the company develops software and management tools.

Back in May, Honeywell bought Akuacom, which provides demand response services to independent service providers and building owners. Honeywell and Johnson Controls are the dominant names in building management. Chances are you're in a building right now controlled by their systems. With its two grid companies, Honeywell can, ideally, begin to better connect its traditional products and services to demand response services, grid management programs, etc. In other words, the bridge between the grid and what goes on inside your office is being solidified.

The deal also marks another step along the way in one of the big trends of 2009 and 2010: the rise of the green giants. Conglomerates have money, sales contacts, and worldwide heft. Smaller companies often have good technology. Acquisitions bring out the best in both. General Electric last year bought Naverus, a software developer that can curb airline fuel consumption. Since then, GE has landed deals to get the software deployed in U.S. and Europe.

Expect to hear a lot from Honeywell over the next few years. (We picked them as one of our Top Ten Green Giants recently.) The company works extensively on a variety of industrial and power projects. It is also minting executives. Jack Bolick, the CEO of Adura, came out of Honeywell. So did Robert Gillette, the CEO of First Solar. These guys have packed speed dials.