MPPT monitors the conditions and changes the resistance to ensure the maximum possible power output at any given time.

The innovation lies in how the stored power is released to charge a phone. A customer sends a text message, which in Uganda costs 110 shillings, to the device. Once it receives the message, an LED above a socket on the battery lights up, indicating that it is ready to charge a phone.

At the Konokoyi coffee cooperative in Uganda, each text message allows a phone to be charged for 1.5 hours. A fully charged Buffalo Grid unit can last for three days, has up to 10 charging points and charges 30 to 50 phones a day.

To bring the cost down further, Buffalo Grid hopes to co-opt the cellphone network operators into subsidising power for charging the phones, or even making it free.

"When you bring power to phones that don't have any, people will use them more," said Buffalo Grid's Daniel Becerra.

"Instead of paying for the charge, people will spend more on airtime," Becerra said.

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