This list isn't complete, but this is what I just remember 

The typicap voltage/current [5.5V, 540mA or ~3Wp] is for standard conditions, which are:

* Light intensity: 1000 W/m² (!!!)
* Temperature: 25°C
* Angle of light beam: 90 degree
* Ligh-Spectrum: AM 1,5 (http://en.wikipedia.org/wiki/Air_mass_%28astronomy%29)

You can look these up here: http://en.wikipedia.org/wiki/Solar_panel#Module_performance_and_lifetime


What do we need to take care for?

* Assume worst condition for season (=1.18 kWh/m²/day, winter)
* You worst case (winter) is 1.18kWh/m²/day with ~10 hours sun
* average Energy: 0.118kWh/m² = 118Wh/m² => or: ~11,8% of 'typical' energy
* 3W * 11,8% = 0,354 W (avg); or 0,354 Wh each hour during daylight in winter (1day: 3,54 Wh)
* Know that panels are aging: At ~80% of original efficiency it should still work.
* At ~80% efficiency your panel will give you ~2,832Wh per winter day
* Note: at night it's dark, your battery should be able to hold enough energy that period
* Also: These are averages. There may be some longer period of rainy or cloudy days..
* You will not be able to store all enery into your battery, even with enough capacity.
* Battery-Efficiency (charge/discharge): about 80-90% for LiIon
* Storing 2,832Wh into the battery gives you only 2,27Wh back discharging
* This still does not take the efficiency of the charging circuit into account!
* The up-converter circuit that brings 3,7V from battery up to 5V: ~90% efficiency.
* 2,043Wh per day left...
* We have still not taken care for suboptimal angle of the light...


Ok, lets assum we have 2Wh per day (which is higher than to be expected):

* Check your circuits energy consumption. Measure it..
* Or: Take a fully charged battery of known capacity, check how long it lasts (e.g. 1Wh capacity, 20 hours)
* The test battery should have a lower or equal capacity
* Now you can estimate how long your circuit will survive with 2Wh