SolarCity was the first company to provide U.S. homeowners, business owners, and nonprofits an affordable way to pay for solar installations through their innovative leasing program. And now, with the acquisition of Building Solutions, a leading energy efficiency services and technology provider, SolarCity is offering a wide range of energy saving services including home energy evaluations, efficient heating, cooling and lighting, water heating, and duct and building envelope sealing.

SolarCity’s new energy efficiency services will be available in all of the company’s current service areas, including Arizona, California, Colorado, Oregon, and Texas.

“We are currently rolling out energy efficiency services to our territories,” said Nina Oestlien, PR manager for SolarCity. “The rollout is going well and will continue into 2011.”

Upon completion of the acquisition integration, SolarCity expects to be able to complete hundreds of energy-efficiency projects each month and hopes to be the largest capacity provider in the U.S.

The acquisition is timely, as the U.S. House of Representatives voted to approve the Home Star Energy Retrofit Act, or “cash for caulkers,” which extends $5.7 billion in rebates to homeowners who invest in energy efficient improvements. In addition, fifteen states have passed financing legislation for Property Assessed Clean Energy (PACE) which will make energy efficiency projects more affordable for homeowners and businesses.

“We’ve been in the business of enabling customers to save money by generating clean energy, and now we can help customers save more money by using less energy,” said Peter Rive, co-founder and chief operating officer of SolarCity. “Our new services will help customers reduce their total energy bills by addressing natural gas usage as well as electricity.”

SolarCity software and Building Solutions software will be integrated to create a new web-based analytical tool for calculating and presenting to homeowners their efficiency needs by inputting square footage, home design, energy usage, ventilation, and other determining factors.

“Building Solutions has built the most advanced set of software tools for assessing energy efficiency return-on-investment that we’ve seen,” added Rive. “This integrated software platform, combined with the momentum from emerging financing mechanisms like PACE, will enable us to make energy efficiency services far more affordable and accessible.”

Pictured: SolarCity residential installation in CA, courtesy SolarCity.