What is the feasibility of solar powered drones?
Let's take the MQ-9 as an example...

The MQ-9 is powered by a 900 HP Honeywell TPE331-10GD engine. In order to generate 900 HP, it will take 2100 kVA (assuming 3-phase electric AC motor). 2100 kVA is about 1680 kW. That is 1.68 million watts. 

Now, the sun delivers (if you could get 100% of the energy sent to Earth) about 4.8kW per square meter (depends on Latitude, which can range from about 6kW to less than 1kW). This would mean you need about 350 square meters of 100% efficient solar panels.

The problem is that the current "world record" for solar efficiency is 46%, let's round up to 50% for easy math sake. So we have to double our solar panel area to 700 square meters. 

Now the wingspan of an MQ-9 is 66 feet (about 20 meters), and a length of 36 feet (11 meters). If the MQ-9 was box shaped, it has a surface area of 220 square meters. Lets assume that 20% of that is real "upward facing" surface area so in reality, if you covered the MQ-9 with the most efficient solar panels in the world, you would only generate (220 * .2 * (4.8 * .46)) about 97kW or about 6% of the power actually required to run the aircraft.

This is also assuming that the extra weight of the batteries/wiring/charge controllers don't make the aircraft heavier or require a larger power plant, and assuming that the 3-ph 2100kVA motor is the same weight as the TPE331-10GD.

This gets even worse for quad-copters because they just don't have the surface area, and don't benefit from aerodynamic lift to reduce the amount of power required to fly.

Sources: 
USAF MQ-9 Facts
Wikipedia Solar Cell Efficiency