# What size solar panel to charge a smart phone?

13 December 2016

## Question

How big would a solar panel have to be to charge your phone?

We asked Kerstin Goepfrich how big a solar panel would have to be to charge a phone... Kersten - Well I guess this depends on where you are. I brought with me my phone charger because I think we can assume we want to charge our phone as fast as we can do. With this thing which plugs into the socket on a wall. So Chris, can you read off the output - it says on here which power output this can give us?

Chris - I reckon I need glasses or laser surgery or something to see. Oh dear, it's really indistinct. It's 1500 milliamp at 5 volt DC. And I almost needed the Hubble space telescope to see that. It's nearly as small as Georgia Mill's mince pie that we got before the programme!

Kerstin - 5 volt / 1.5 amp, if you multiply the two values that gives you the power output, which is 7.5 watts. So if we want a solar cell to produce 7.5 watts, how large would it have to be? Well, the best solar panels that have been made on a sort of laboratory scale had efficiencies up to 46 percent, so that's pretty good. However, the ones you would put on your roof, they typically have efficiencies of about 15 percent. So what does that mean? 15 percent of the energy that comes from the Sun is actually converted into electricity and in the areas of Northern Europe, we can assume that about 1,000 watts per square metre of sunlight hit the Earth. So, 1,000 watts per square metre, 15 percent, that means 150 watts per square metre.

So, we don't quite need 150 watts to charge a smartphone. We said we'd need 7.5 watts, that means we don't need a full square metre to charge our phone, but about the size of an A4 paper would be enough.

Chris - There you go. So that seems eminently doable. Andrew...

Andrew - Although, presumably, that would be at midday in full sun. When it's cloudy or dusk or something you're.

Chris - What about at nighttime?

Kerstin - You've exactly got the point there. I mean, there's not always sun. About 1,500 hours of sunshine is what we get here in the UK, for instance, out of 8,000 hours. So, yes, that's assuming perfect conditions so it might not be as practical. However, if you want to make this last vital phone call, it would save your day. There are these gadgets out there if you're still looking for Christmas presents.