Having a browse around on Amazon, I saw what I thought to be a really good deal. A solar powered USB charger for £40. Thinking I'd struck it a great deal for free electricity for life, I went ahead and bought the unit, along with a battery pack (15000mAh).

After unboxing, I did a quick test (thank you overcast weather), to see if the solar panel indeed would work. I plugged my phone in, and it let out a promising beep saying "thanks for the juice!". I kept checking the battery percentage every few seconds waiting for it to go from 48% to 49%. A few minutes later, sure enough, I got my first free percent of battery on my phone, and didn't have to pay British Gas for it.

This got me to thinking "How much electricity does a mobile phone actually use to charge it from empty to full?". Doing a bit of research on the good old 'tinternet', I went calculating everything from mAh, to watts, kilowatts, kilowatt hours, cost of a kilowatt hour.... etc.

So here's how I worked it out (Warning: Math ahead, feel free to skip it, or critique where I've gone wrong!).

I chose 2 samples of phones as below
Mobile phone A: Samsung Galaxy S4
Mobile phone B: Samsung Galaxy S5

Samsung galaxy S4 has a 2600mAh battery.
Samsung galaxy S5 has a 2800mAh battery.

Great, let's work out how many Watts are required to charge said phone from empty to full.

S4: 2600 / 1000 = 2.6 Amps@ 3.8V
Watts: 2.6 x 3.8 = 9.88 Watts
Add in inefficiency coefficient of say 30% due to conductors, heat, conversions etc
9.88 x 1.30 = 12.84 Watts

My phone takes roughly 1.5-2 hours to charge from completely flat, and comparing it to the table here: http://standby.lbl.gov/summary-table.html it seems pretty accurate.

Ok, so we have a ballpark figure of 13 Watts (rounded up). Still here? Ok, let's work that out if I charged that "on-grid". First, let's change the watts to kilowatt hours. 13/1000 = 0.013kW/h. 1kW/h on average costs around 15 pence. So let's take that 15p and multiply it by the 0.013kW/h.

0.195p

So, I begin to scratch my head. Really? 2 tenths of a pence?

So I ramp it up, let's work out how much it would cost to charge my phone in a month.

0.195 x 30 = 5.85p

That's it?

A year? Why not.

5.85 x 12 = 70.2p

We haven't even broken £1....

So, we have 2 mobile phones in the house, one is an S4, another an S5. Let's just for arguments sake say they both were S4's (to save boring you with numbers again), that works out to £1.40 a year.

So then it got me thinking once again about how long it'd take for the solar panel to pay itself off (let's say I charge both phones every day).

£40/1.40 =  28.5 years....

Balls!

Let's ramp this up a little bit. Let's say I fully discharge and recharge my 15,000mAh battery pack every day.

Let's plug the numbers in, shall we?

15,000mAh / 1000 = 15Amps @ 5v (which is what the battery pack is rated at)

This gives me a grand total of  75 Watts per day harvested from the solar panel.

convert to kilowatt hours: 75/1000 = 0.075kW/h

Cost: 15p x 0.075 = 1.125p per day

Per month: 1.125 x 30 = 33.75p

Per year: 33.75 x 12 = 405p

Time required for panel to pay itself off:

40 / 4.05 = 9.87 years. (by which time, the battery pack will be completely useless from the full charge discharge cycles)

Balls again!

So....

Anyone want their phone charged? :(