I have one solar related question, but please bare in mind that I am a complete newbie in this field so if my questions are stupid, I apologize.

I want to use a solar panel with this USB controller/charger for smartphone/tablet charging:

enter image description here

(source: https://www.aliexpress.com/item/OOTDTY-5V-2A-Power-Bank-Solar-Panel-Voltage-Controller-Charge-USB-Regulator-5V-20V-Input-5Vdc/32825541743.html )

I found a lot of different solar panels that match my needs (primarily, by their dimensions) but when it comes to watts, I dont know what to select (or better yet, I dont know what is enough). So, if it is possible, can you tell me how many watts is enough to charge today's smartphones or tablets?

  • 2
    \$\begingroup\$ 5 V x 2 A = 10 W is enough to charge Android devices over USB. You are not building a charger. You are building a power supply. The charge management circuitry is in the phone / tablet. The power supply just has to provide a constant 5 V. The phone will draw as much current as it requires. \$\endgroup\$
    – Transistor
    Oct 9, 2017 at 21:38
  • \$\begingroup\$ @Transistor, thank you a lot for your help. Is it a same case with iPhone or iPad? Also, if i use, lets say 15w, does that mean that the charging process will be faster or it does not matter? \$\endgroup\$
    – Libuk
    Oct 9, 2017 at 21:56
  • \$\begingroup\$ Read my comment again. What controls the battery charging? If I had a 5 V supply capable of 1000 W what would be the result? (Finally, and hypothetically, if my power supply was able to push 1000 W into the phone what would be the result?) \$\endgroup\$
    – Transistor
    Oct 9, 2017 at 22:01
  • \$\begingroup\$ It would burst into flames? Again, I really don't know anything about this, that is why I ask. I know that some manufacturers sell products which, when tested, show lower performances, that is why I asked about 15w. For example, this one (m.alibaba.com/product/60609273261/…) says that the max current is 1A, does that mean that it will take more time until the battery is full? \$\endgroup\$
    – Libuk
    Oct 9, 2017 at 22:20
  • \$\begingroup\$ Correct about the fire. I don't know enough about the solar cells. There are a few problems to watch for such as what happens when it gives less than 5 V, etc. Many people are doing what you wish to do so I think you will have to search around to see how they are managing it. \$\endgroup\$
    – Transistor
    Oct 10, 2017 at 5:53

1 Answer 1


Typically a smartphone will need around 1A, the newer fast charging cell phones need 2A. 1A should be find in phone charging application. USB runs at 5V so on the low end 5W on the high end 10W (1A*5V is 5W)

A good low number for a sunny day is 400w/m^2 (up to ~1000w/m^2 dependent on latitude, angle, and conditions), multiply this number by the panel size. If a panel is 0.04m^2 10W of solar energy on the face of the panel.

The panel can't convert all of the energy to electricity, so multiply the last number by the panel efficiency (if they don't have it listed, don't buy it). A typical number is 12% so you'd get roughly 1.2W out of a 0.04m^2 panel.

Which would only be 1.2W/5V = 0.24A, which would still charge the cell phone, but at a much slower rate. The solar charge controller will also take this number down as at most they are ~90% efficient.

That means a larger panel would be needed in this case, or two of them. Remember you need a solar charge controller with MPPT capability to convert the power at the highest efficiency.

  • \$\begingroup\$ Are you sure that 20cm^2 is 0.04m^2? I bet it is not. IMHO 20cm^2=0.002m^2 \$\endgroup\$ Oct 18, 2017 at 5:11
  • \$\begingroup\$ Yep, got it fixed \$\endgroup\$
    – Voltage Spike
    Oct 18, 2017 at 15:51

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