But over the past few years, the rapid expansion of low-cost Chinese manufacturers like Suntech, Yingli and Trina has sent the cost of conventional solar panels plummeting.
Birns says that as Africa's cities get more populated and as demand for business is growing, the expansion of the low-cost model should revolutionize the continent's air transport in the same way that it did in North America, Europe and now in Asia.
Critics complain that the scheme's total costs have been higher than expected, but the flip side has been a dramatic expansion of coverage at a relatively lowcost per head.
The expansion at Stansted arguably fostered the low-cost airline industry by offering it cheap landing rights there but it failed to relieve congestion at Heathrow.
But the new budget office analysis does not include the cost of a proposed expansion of the federally funded Medicaid system for low-income families, a proposal expected to add several hundred billion to the tab.