Buying a solar system requires that you:

• Manage the procurement and installation of the system.
• Hire a consultant to ensure the solar equipment supplier’s proposed solution is in your best interest.
• Use your own money or borrow from a bank to purchase the system.
• Allocate staff and money for insurance, repairs, cleaning, and replacing defective parts over the system’s useful life; installer's warranties are normally for 1-2 years.
  

Entering into a lease or power purchase agreement (PPA) transfers the above activities and risks to the solar developer. The lease or PPA price includes the cost of the system, regular cleaning, maintenance, repairs, parts replacement and insurance. Developers normally provide a guaranteed level of energy generation.

Companies normally perform a discounted cash flow analysis (NPV) for the Buy vs. Lease scenarios. Major variables analyzed are:

Buy Scenario:

• Cost of the system
• Opportunity cost of capital (what return can we achieve if we invest the required money in our core business)
• Annual costs over the life of the system: cleaning, repairs, maintenance, replacement of failures (e.g. inverters, panels), and asset & 3rd party insurance
• Offset by the savings on energy

Lease (or PPA) Scenario

• Annual lease costs
• Offset by the savings on energy
  

Failure rates are influenced by many factors, such as the quality of the manufacturing process, the quality of the installation process, long-term weather (e.g., dust, humidity, heat, ice), and the quality of the lifetime maintenance routine.

It's important to purchase products from approved suppliers with consistently high quality performance tests, use an experienced installer offering a thorough maintenance and inspection routine, and to replace faults and degraded components promptly. An online search will show that estimates of system lifetime costs (especially replacement of major parts and repairs) are routinely grossly underestimated.

Below are some suggested sources of independent analysis and studies of solar systems and their lifetime performance:

National Renewable Energy Laboratory

Nature Magazine

Peer-reviewed journal articles

PV testing and calibration lab