Do solar energy units pay for themselves?

I got an add from Amazon.com on a sale of various Solar energy units. Normally this 60 Watt solar charger goes for $600. Right now it is on sale for $300. A kWh reportedly costs about $0.12, so running a 60 watt light bulb for an hour should cost about $0.0072. At that rate, running the solar unit sixteen hours a day and assuming no rain or clouds, you would have to run it for 7.135 years before it would pay for itself -- that assumes a zero interest rate. At its normal price, it would take 14.27 years to pay for itself, again assuming no interest. The problem is that even these are very optimistic assumptions. It doesn't include the costs of the battery or how the battery's efficiency will decline over time. I assume that one would need to replace the batter a couple or a few times over this time period. Those costs would have to be added to the total and it means that the hour calculations shown above will be too favorable to solar power. I assume that there are also additional costs of installation.

See a related story available here on a half billion dollars in government loan guarantees for Solyndra.