The following isn't complete, but this is what I just remember
The typical 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°
- Light-Spectrum: AM 1.5
You can look these up here: http://en.wikipedia.org/wiki/Solar_panel (section '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 1.18 kWh/m²/day with ~10 hours sun
- Average Energy: 0.118 kWh/m² = 118 Wh/m²; o: ~11.8% of 'typical' energy
- 3 W * 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 ~2.832 Wh 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 energy into your battery, even with enough capacity.
- Battery-Efficiency (charge/discharge): about 80-90% for LiIon
- Storing 2.832 Wh into the battery gives you only 2.27 Wh back discharging
- This still does not take the efficiency of the charging circuit into account!
- The up-converter circuit that brings 3.7 V from battery up to 5V: ~90% efficiency.
- 2.043 Wh per day left...
- We have still not taken care for suboptimal angle of the light...
Ok, let’s assume we have 2 Wh per day (which is higher than to be expected). This is how you could check if this is enough:
- Check your circuit’s energy consumption. Measure it..
- Or: Take a fully charged battery of known capacity, check how long it lasts (e.g. 1 Wh capacity for 20 hours)
- The test battery should have a lower or equal capacity
- Now you can estimate how long your circuit will survive with 2 Wh