Apple on Thursday unveiled an overhauled environmental Web page and green strategy, complete with some interesting new data. Most notably, the Mac and iPhone maker is now calculating what it calls its entire carbon footprint–including emissions generated by its products. As the company puts it, “what happens when we design them, what happens when we make them, and what happens when you take them home and use them.”

The approach is different from that used by PC rivals HP and Dell. Apple puts its greenhouse gas emissions at 10.2 million metric tons–a total that includes energy used by folks typing away on on their Macs.

In fact, by Apple’s calculations a majority of the company’s environmental footprint — 53 percent — comes from users plugging in Apple devices and using them. An additional 38 percent comes from manufacturing, with 5 percent from transportation.

Given that half its carbon footprint comes from the juice used by its products, Apple is keying on energy efficiency as the main challenge. It touts innovations — such as a Mac operating system that regulates the machine’s processor between keystrokes, reducing power — that Apple says separates it from Windows-based PCs.

Green has emerged as new area of competition among technology companies, particularly PC makers, and Apple’s new approach adds an intriguing component to the rivalry.