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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

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')

Things not to forget about:

• 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 of 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

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

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

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

The typical voltage/current of your solar panel [5.5V, 540mA or ~3W] 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

You can find these in the Wikipedia article on solar panels.

What do we need to take care for?

• Assume worst condition for season (=1.18kWh/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% = 0.354W (avg); or 0.354Wh each hour during daylight in winter (1 day = 3.54Wh)
• 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 the energy generate by the panels 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

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