Together with a sharp drop in the cost of solar power and corporate efforts to rack up green credentials, the expiring tax subsidies have large energy purchasers taking a hard look at buying solar under big long-term contracts. Apple on Tuesday said it would spend $848 million over 25 years to buy 130 megawatts of electricity from a 280 MW plant – the solar industry’s largest-ever corporate power purchase agreement, or PPA. Apple’s major financial commitment gives solar a level of mainstream credibility that should entice other new customers.