Here are some calculations and notes that should help people to understand what kind of Solar Power solution they will need to support their BeLL System.
"The terms power and energy are frequently confused. Power is the rate at which energy is generated or consumed. For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (W·h)..." -- http://en.wikipedia.org/wiki/Watt#Confusion_of_watts.2C_watt-hours.2C_and_watts_per_hour
"Starter batteries are designed to deliver short, high-current bursts for starting the engine, and are designed to discharge only a very small part of their capacity. A daily cycle of using most the capacity would corrode a car battery very quickly, the plates and the chemistry are designed to stay nearly 100% fully charged most of the time." -- http://voltaicsystems.com/blog/choosing-a-lead-acid-battery-for-solar-charging/
"Deep cycle batteries are designed with larger plates and different chemistry to avoid the corrosive effect of frequently using the full capacity. They are designed to be regularly deeply discharged using most of its capacity." -- http://voltaicsystems.com/blog/choosing-a-lead-acid-battery-for-solar-charging/
The 7" OLE tablet batteries have energy of 3.5Ah running at 3.7V.
3.5Ah x 3.7V = 13 Watt Hours per Tablet battery charge
13 Watt Hours x 35 tablets = 455 Watt Hours of energy required to charge 35 Tablets
Calculating the energy requirements of a parent battery that could fully charge the Tablet batteries
If the tablets are receiving 12V from a 12V battery, the energy capacity of that battery would require 38Ah.
355 Watt Hours / 12v = 29.6Ah
But this is not quite correct, there is efficiency loss in the Tablet batteries that must be taken into account. The tablet batteries may be able to deliver 13 Watt Hours but you have to give more power due to the efficiency loss.
The efficiency of the Lithium Ion batteries in tablets are... "Charge/discharge efficiency 80–90%[3]" -- http://en.wikipedia.org/wiki/Lithium-ion_battery
So the power capacity of a 12V parent battery would need to be ...
(355 Watt hours / .8 efficiency) / 12V = 37Ah
Again, this is not quite correct. There is efficiency loss in converting the 12V the battery provides to the 5V the Tablets require to charge. I'm guessing for now that there is an efficiency of .8.
37Ah / .8 efficiency = 46Ah
So in conclusion, a parent battery @12V that can charge 35 tablets would need 46Ah of energy capacity. Luckily, the deep cycle (not a starter battery as found in cars) lead acid battery recommended in the blog post by Voltaic Systems is a 12V 51Ah battery. This battery sells for around $500. -- http://www.xantrex.com/power-products/backup-power/xpower-powerpack-1500.aspx -- http://www.amazon.com/Xantrex-802-1500-XPower-Portable-Powerpack/dp/B00005RHQQ
Let's say we get 8 hours of direct sunlight and the lead acid battery operates at 80% efficiency. How many Watts would a solar panel need to provide for the battery to have potential energy of 46Ah?
"If lead acid batteries are maintained properly, they will function at 80-90% efficiency." -- http://voltaicsystems.com/blog/choosing-a-lead-acid-battery-for-solar-charging/
(46Ah / .8 efficiency) / 8 hours = 7.2A @ 12V = 86.4 Watts
Here's an example of a 100 Watt solar panel for $380. -- http://www.amazon.com/Instapark-PowerBox-Fold-n-Go-Solar-powered-Controller/dp/B007VWW8X2/ref=sr_1_8?ie=UTF8&qid=1359556099&sr=8-8&keywords=100watt+solar+panel
Total up front cost of power supply for 35 tablets: $880
Power supply cost per tablet: $25
Tablet price: $65 for a OLE 7" Tablet
Total up front cost of one tablet: $90
- Recurring fee of $500 depending on the battery life (2 years?)
- This calculation assumes the Nook Simple Touch has the same battery as the OLE 7" Tablet but only has to be charged once a week.
Let's say we get 40 hours of direct sunlight and the lead acid battery operates at 80% efficiency. Usage of this would mean you get to charge the parent battery during the weekdays and then the parent battery powers the tablets over the weekend. How many Watts would a solar panel need to provide for the battery to have potential energy of 46Ah?
(46Ah / .8 efficiency) / 40 hours = 1.4A @ 12V = 16.8 Watts
Here's an example of a 16.8 Watt solar panel for $160. -- http://www.voltaicsystems.com/16wattkit.php
Total up front cost of power supply for 35 tablets: $680
Tablet Price: $80 for a Nook Simple Touch
$680 / 35 tablets = $19 cost of power per tablet
Total up front cost of one tablet: $99
- Recurring fee of $500 depending on the battery life (2 years?)
Solar Power Example 3: Calculating the power requirements to charge tablets directly in 16 hours (over the weekend with no parent battery)
- This calculation assumes the Nook Simple Touch has the same battery as the OLE 7" Tablet but only has to be charged once a week.
Because we are bypassing a parent battery, we can take out one of the efficieny losses.
46Ah / 16 hours = 2.875A @ 5V = 14.3 Watts
Here's an example of a 16.8 Watt solar panel for $160. -- http://www.voltaicsystems.com/16wattkit.php
Total up front cost of power supply for 35 tablets: $160
Tablet Price: $80 for a Nook Simple Touch
$160 / 35 tablets = $4.50 cost of power per tablet
Total up front cost of one tablet: $84.50
"Most United States, Canada and Mexico domestic power suppliers run at 120 V. Household circuit breakers typically provide a maximum of 15 A or 20 A of current to a given set of outlets."
120V x 15A = 1800 Watts
How much power is being provided to outlets is important to know. At some of the schools where we observe "low power", this may be the case because of how many devices they are trying to charge (light bulbs, fans, refrigerators, computers, etc.). For example, if the tablets can charge their battery in an hour, then the max number of tablets you can charge in a house in North American would be...
1800 Watts in a house / 13 Watts per tablet = 138.5 tablets can be charged in one house. In Ghana, I'd assume that power capacity would be much lower than 1800 Watts. This is something to keep in mind.