# iPhone with solar panel sanity check

An iPhone with a solar panel in the front and back intrigues me. No, it won't be able to charge the battery or power phone while the SoC, display and radios are on, but the intriguing thing to me is it may represent very long standby times and it may be able to power the use case that Apple is closely associated with. These things play a lot into how long you have between charges.

So, sanity check. I looked at the Anker 14W Solar Panel Foldable Dual-port Solar Charger. It has 4 panels (that can be folded to 8.5 x 11 for stowage) with panels that look to be about 7 x 10 inches in size. It outputs 10 Watts total which can be fed to 2 USB ports (2 ports at 5 W or 1 port at 10 W), so it basically can charge any cell phone and tablet in existence today at the same rates as their wall chargers can.

So, 10 W / (4 x 7 x 10) in^2 = 0.042 W/in^2 or 42 mW/in^2.

So, if the back of the iPhone was solar panel using these Anker cells, and was sized 2 x 4.5 = 9 in^2 of surface area. At 42 mW/in^2, this panel can generate 378 mW. 378 mW is enough to do some things. It can power the display at dim settings or the SoC at low settings even.

But, you'll have to knock down that number a bit because it's doubtful Apple would increase the thickness of their phones for it (if you increase the thickness of the phone, increasing the battery capacity would be more useful), and I have my doubts it would look like a photovoltaic cell we are are familiar with. It would be some kind of fancy super thin layer on the surface that is not visible to the naked eye. So, decrease that 378 mW in half to 190 mW.

Ok, what can 190 mW power? The iPhone 5S has a ~5.9 WHr battery or so.

WiFi browsing is advertised at 10 hrs. So WiFi browsing takes about 5.9/10 = 600 mW on average. The SoC goes up to 2000 mW or so, but it likely spends 90% of its time at an idle condition at 100 mW or so. The display is like at 400 mW or so, while the WiFi radio takes the rest. Everything else is negligible. Not possible.

3G Cellular talk time is advertised at 10 hrs. So 600 mW just to have a 3G radio call. That is frelling costly. The display is off during this test so the cellular radio, audio compression/decompression stuff takes a whole lot of power. And this is with good signal! At 1 dot, a phone call probably costs on the order of 1000 mW (6 hr talk time) or probably even more.

Audio playback is advertised at up to 40 hrs, 5.9/40 = 148 mW. Hmm...

Standby time is advertised at up to 250 hrs, 5.9/250 = 24 mW. This is with the cellular radio in a low-power standby state.

The biggest problem is that it is unlikely that Apple could design a solar panel that small that can generate 5 Volt differential, so it probably can't charge the battery. Even very very slowly. Don't know to really say actually. Maybe they can change the battery design. But if it could seamless switch between power and solar panel while the audio player is on or powering the radios and other chips while the phone is in standby, then I can see this being useful.

I spend a lot of time listening to the audio player. So, if I leave the phone on the table, it won't use any battery during this time? And if the solar panel can power the phone on standby, it can potentially double to standby time. Hmm. It could add an hour of screen on time or another hour of talk time for every 4 hours of audio I listen to? That sounds very intriguing indeed.

But that panel has to generate around 200 mW at voltages and currents that can power various components inside. No idea about that.