Solar power egg incubator
There is only one answer - you will need a LOT more solar power generation than simple calculations will suggest:

48 watts  for 15 hours (assuming a 60% on time averaged over night and day) is ~700 watt hours. With nominal 12V batteries call that 60 amp-hours capacity.

Don't forget that in the day you need to be able to charge the battery and run the incubator at the same time so a nominal output of 100W minimum is needed (unless the incubator heater is off most of the time).

1 Solar cells rarely achieve their full rated output which assumes they are exactly normal to the incident light (pointing straight at the sun, in English)

2 The batteries won't be 100% effceint in taking or yielding up their charge

3 You will need lead acid batteries designed for deep discharge - i.e. leisure batteries. Even then remember you won't be able to use part of their capacity

4 Even in Ethiopia you can't guarantee day-long sunshine

Though you may get away with one, I would suggest using two 60Ah leisure batteries (to be on the safe side). If you have 10 hours sun and achieve 50% of nominal output you will need ~1400 watt-hours or 140 watts of panel capacity. So go with the BP 160W panel here or similar:

http://www.windandsun.co.uk/Solar/BP_Solar_modules.htm

So even if the sun isn't full on to the panel it should make 100W - worst case 50W incubator and 50W for the battery.