Viridity Energy, the startup that’s been building “virtual power plants” for customers in the United States, has just raised $15 million from Japanese conglomerate and big green investor Mitsui & Co. It looks like a trans-Pacific partnership that could open up markets to both companies.

The Series C round comes on top of a $14 million round in January 2011 and $10 million previously, as well as millions in state grants and investment, to bring total funds raised to about $40 million. Previous investors include Braemar Energy Ventures and Intel Capital.

As for Mitsui, it’s taking a board seat and hopes to see the investment “further internationalize” its “Smart Green Information Technology” line of business, which includes smart grid IT services, renewable energy development and CO2 mitigation, according to Tuesday’s release.

“Mitsui’s very interested in taking Viridity’s V-Power platform and developing businesses based on that capability, both in domestic markets and internationally,” Viridity CEO Audrey Zibelman said in a Tuesday interview. While she wouldn’t name any projects the two might be working on, she said targets could include dynamic load management for various industry sectors, as well as energy storage integration.

It will be interesting to see how Mitsui puts Viridity’s technology to work, particularly in Japan. The country is facing an epochal power crisis in the wake of the Fukushima disaster and nationwide nuclear power shutdown, and has started investing heavily in smart grid, renewables, efficiency and energy storage as a result.

Mitsui is taking part in that effort, joining such partners as Toshiba, Fuji Electric, Hitachi, Panasonic, General Electric and Accenture in a series of Smart City Projects being built across Japan. Last summer’s peak power crisis forced businesses in Tokyo to shut off air conditioners and power down data centers, making power-balancing capabilities like those Viridity promises seem very worth the investment.

Founded in 2008, the Philadelphia-based company is commercializing technology that turns offices, factories, train systems and other big power users into virtual power plants. On the building side, Viridity says its software can interface with building and energy management systems to fine-tune power loads like HVAC and lights to use less electricity when prices are high, such as during peak afternoons.

On the market side, Viridity’s V-Power platform verifies load reduction and manages its sale into various markets for demand response and grid power. Key Viridity executives, including Zibelman, are former executives of grid operator PJM, giving the company a lot of insight into that side of the business.

So far, Viridity is balancing and playing power into markets at several university campus microgrids, as well as a wind power storage management project in Pennsylvania. In February it partnered with ConEd Solutions, the energy services arm of New York utility ConEd, to deliver this energy efficiency and demand response service to commercial customers around the country, and the two have some projects they’re working on today, Zibelman said.

Viridity has also doing several projects that link batteries to the grid, both to cushion the customer during peak price times and to sell that power back to the grid. Battery supplier Axion Power is using Viridity’s system to sell battery power into fast-reacting frequency regulation markets, and the Southeastern Pennsylvania Transit Authority (SEPTA) has hired Viridity to manage an 800-kilowatt battery that captures train braking energy for its own use and for grid backup.

As for Mitsui, it’s working across the spectrum of green industries, from building 50 megawatts of solar power with Toshiba in Japan to investing in next-generation technologies from the likes of waste-heat-to-power startup GMZ Energy and networked LED startup Redwood Systems. On the building energy efficiency front, Mitsui is working with Transcend Equity, the efficiency services financing startup acquired by SCIEnergy earlier this year.

In December it put $12 million into the audacious Tres Amigas smart grid project, which plans to raise $1.5 billion or so to build a high-voltage direct current superconducting transmission hub to link the United States’ three separate grids into one. (Viridity is a partner on the Tres Amigas project, by the way.)