The will burn for billions of years, it won't run out any time soon. This is sustainable because it will not deplete in the near future.
The harder question is if the materials that go into a solar cell are sustainable. They use primarily silicon, which is one of the most abundant materials on earth. But they can also use other things, like metals (copper, silver, gold), and some toxic chemicals (arsenic, cadmium) etc. The sustainability of these materials is questionable. These materials are recyclable, and it is thought 99% of a solar cell can be recycled. But the production of them does has some toxic site affects, which means we have to balance the 'clean' energy production of the cells with the bad by products of their manufacturing process.
"One of the most important is that wind power is the least expensive of all other forms of alternative energy. Wind turbines generate electricity at around 5 cents per kWh (Kilowatt Hour), which is comparable to the new coal and/or oil burning power plants. The costs are projected to decline even more as technology improves, and this is very important because most of the cost with wind power is in manufacturing. Once the wind turbines are in place there is little cost to maintain and wind power is free."
