Instead of plugging into an electric outlet, solar cellphone chargers use free energy and they will help with cost savings as well as be part of the green initiative. The chargers are of minimal cost and depending upon how many cellphones are charged the cost savings would be very substanitial year after year.
General Rule to estimate total hours of direct sunlights to full charge: (Amperer per Hour of the Battery / Amperes per Hour of the Charger) + 10%
Assuming You have a 5 Watt / Hour Cell Phone Battery and a 2 Watt output solar Charger, You need first to convert Watt in Ampere:
It’s about 5/12 = 0.417 Amperes per Hours for the batery and 0.167 Amperes for the charger
To Calculate how much time You’ll need to charge the battery with a 2 Watt solar charger You’ll have:
1° Calculate the Ampere per hour of the charger: 2 Watts /12 Volts = 0.167 Amperes
2° Divide 0.417 amp hours (cell phone batery) / 0.167 ampers (solar charger) = 2.50 Hours of direct sunlight to full charge You cell phone Battery
3° Add 10%: 2.75 hours
Result: You’ll need an average of 3 hours of direct sunlight to full charge Your cell phone battery using a 2 Watt solar charger or 30 minutes if You have one of 18 Watt.
An 18 watt charger is optimal to be used by many cellphones as the time for charging is faster.