One of the questions that often comes up when we show off CoBattery is “Why don’t you stick a solar panel on the back?”
I usually try to answer by explaining how adding a solar panel to a phone doesn’t make sense due to the physics of solar power (not to mention the dangers of leaving your phone in the sun). But I never have time to explain the physical analysis, so I’ll break it down here.
The sun emits a lot of energy!
The sun is a hot, fiery ball of plasma powered by nuclear fusion. And it’s really friggin’ powerful!
To put a number on it, the sun emits 8.602 * 10²¹ Watts. (A Watt is a unit of power, so it’s an amount of energy released every second.)
In more relevant terms, the sun is emitting the energy equivalent of 2 * 10¹⁷ exploding Samsung Note 7s each second [1]. That’s enough power to continuously run a gajabillion iPhones.
Most of the sun’s energy misses Earth, but a lot still hits us.
The sun emanates energy uniformly in all directions. So only a small sliver of that energy hits us here on Earth.
But if we captured all of the sunlight hitting Earth on one day, we would be able to power the world’s energy for the next three years. So we still have a lot to work with.
Today about 1% of the Earth’s electricity production comes from solar power. That’s enough energy to power 35 trillion phones.
The back of your iPhone is small
Solar panels work when they have large amounts of area that are in the sun all day. That’s why it makes sense to put solar panels on a large, flat roof in a sunny climate or spanning miles in the California desert.
Your phone which is in and out of your pocket all day, spends little time in direct sunlight. But let’s just say that you are able to leave your phone in the sun all day.
The sunlight incident on Earth comes in at a rate of roughly 1,000 Watts per square meter over the course of the day. The back of the iPhone 7 is 14.3 inches, so that comes out to 9.3 Watts hitting the back of your phone [2].
If a solar panel on the back of your phone was able to capture all of that energy, your phone would be charged from 0 to 100% in just 40 minutes.
So if we just add one more solar panel to the back of our phone, we should be all set, right?
Solar panel efficiency and those damned clouds
But again, that’s not the full story. The best commercial solar panels are operating at 20% efficiency which increases the time we need to leave our phone in the sun for 5x.
All of this analysis was assuming that we have clear skies and constant sunlight. In reality, we know that’s not the case. Clouds often reflect up to 50% of the sun’s energy. And the sun is hidden for an average of 12 hours each day, a phenomenon known as “nighttime”.
The bottom line
Our analysis suggests that for the average user, you will be lucky to add 10–20% of charge to your phone on an sunnier-than-average day. This is in good conditions. If it’s cloudy or you spend your time inside, the solar case will do nothing.
Unless you live in the Australian outback and want to leave your phone baking in the sun for hours each day, you should not buy a solar phone case.
At CoBattery, we have a lot of respect for solar power. But solar panels should be in large, flat land where they can soak in the sun for hours a day, not on the back of your phone.
If solar cases aren’t the answer, what is?
Ummm… well… CoBattery is! CoBattery is an iPhone battery case with a swappable battery and it comes with two batteries. So after a long day using your phone, you can simply swap your battery in seconds. No need to move to a sunnier climate or ever plug in your phone.
And with CoBattery you are still using solar power! You charge your batteries using the electric grid. So the solar panel that is fed into the grid ends up in your home powering your CoBattery batteries.
Get your CoBattery here: cobattery.com
[1] Assuming that the 3,500 mAh Note 7 battery is stored at 3.4 Volts and explodes in 1 second. Wolfram calculation link.
