Think about the last time you plugged in your phone to charge it up. Did you first pause to think about how much it would cost you?

If you're like me, you plug in your phone before you go to bed so that it's charged up and ready to go in the morning. Or you top it off throughout the day. And if you're like me, you give little thought to how much it actually costs to charge up your phone. That's because charging mobile phones and tablets doesn't cost all that much in the grand scheme of things - at least for those of us "on the grid". A typical electricity rate in the US or UK is 12 cents per kilowatt-hour (kWh).

But what about those without the convenience of centrally-produced electricity and a network of transmission lines and distribution stations and household wiring? How does one charge a mobile phone. And more importantly, how much does it cost?

As I found out during research for my article about pay-as-you-go solar energy, those who live off the grid pay an outrageous amount for their energy.

Let's do a quick comparison. For convenience, I'm going to use my iPhone 5. According to the folks at iFixIt, it uses a 5.45 watt-hour (Wh) battery. That is about the same as running a 5-watt CFL for a little over an hour. If we assume that I charge my phone every day (365 times a year) my total bill comes out to just 25 cents. What a steal! Nothing costs a quarter anymore. Except, apparently, charging my iPhone for an entire year.

Now, let's switch gears to rural Africa. If I take my iPhone 5 to a rural village in Kenya without grid access to electricity I can't plug it into the wall because there won't be a wall outlet. For nearly 5-in-6 people living in rural sub-Saharan Africa this is the norm. Instead, I would have to walk to a local shop and find a kiosk to charge my phone, where I might pay around 25 cents every time I need a top up - or $91 a year. That's 383 times more expensive than in the US!

I like the way Simon Bransfield-Garth puts it: "the poorest people in the world are not just paying a bit more for their energy, they’re paying a disproportionate amount."

As I write in my article, clever business models coupled with technology can bring electricity to rural populations at competitive rates. And with roughly 253 million mobile phone subscribers in sub-Saharan Africa, there appears to be a market ripe for disruption.