Powering a solar powered plane

Hi, I am trying to design a concept for a solar powered plane and I have decided to use lithium sulfur batteries during night time operation.

I have estimated the power required to be 15kW and he duration of the night to be 12 hours.

To calculate the mass of battery required do I use the specific energy density (500Wh/kg), and so (12*15000)/500

or do I use the nominal voltage (2.5) and capacity (700mAh/g) to calculate the mass?

Thanks

Might I suggest moving to a different concept?

It is going to be a little more complex than that. You need to work out the curve for system efficiency (motor drivers, battery internal resistance, etc) for various battery potentials, then combine the discharge curve for the battery with the efficiency curve to work out the actual amount of work you can do over the entire discharge curve (integrate the combined curve over the allowable discharge interval). You will no doubt want a safety factor and a contingency plan for a cloudy sunrise. Also remember that winter nights are longer than 12 hours.

If you are just going a swag you can probably use the energy density (15Kw/12 hours sounds like a swag) but you are going to want to derate the number you get a bit.