I'm building an off-grid security system, based on a smartphone as a 24/7 security cam and an arduino nano for several sensors. A pretty interesting build in my opinion, maybe I'll post about it sometime. But I'm struggling trying to calculate the required values for my system. The three things I need to find are:

  1. Battery capacity
  2. solar-panel wattage
  3. charge-controller

Information I have:

  • The smartphone running in camera mode is the main load, the arduino is pretty negligible. I'm using a Samsung Galaxy S4 with a battery of 2600 mAh capacity. I tested it and found that it lasts 3 hours for a full charge. The phone's regular wall-charger is 5v\2A.

  • I actually tried to run it with an old solar panel system I got from a friend, which uses a 12v 7Ah lead battery and this set of 10W panel and a 3A pwm charge controller. I used a car-usb-charger (to convert from the 12v on the load output pins on the charge controller) and it didn't hold very long, only for half a day or so.

  • I live in Israel so at the very least 6 good strong sunlight hours a day.

Here is my attempt for some calculations: 2600mAh battery consumed in 3 hours can be rounded up to a 1Ah load (or, since it is in 5v, to a 1Ah*5v=5Wh load), so overall the system consumes 24A daily (or, 120W). Therefore a battery with 30-50Ah capacity is suitable, and a 30W-40W solar panel will do (since 30W*6h = 180Wh daily output). If this is correct, I'm thinking about buying a 30W solar panel, a 10A PWM charge controller (with 5v/3A usb outlet which is really good for my needs) and a 12v 40Ah Deep Cycle Lead Acid Battery.

Am I doing it right? Is this correct? Please share your knowladge.

All the best, Roei.

  • 2
    \$\begingroup\$ 1) You have A where you should have Ah, this is WRONG, one is current, the other capacity, and the implicit times to convert one to the other are not everywhere obvious. 2) It's better when your voltages are different at different points in the system to use W and Wh throughout, than A and Ah. The latter are very confusing unless you are very thorough and systematic in your description, which you are not. I'm not going to attempt an answer until you use units of the right dimension. \$\endgroup\$
    – Neil_UK
    Jun 26, 2018 at 6:44
  • \$\begingroup\$ Well, that's part of my problem.. This is why I put the "dry" information and specs before (and separate from) my attempt for calculations, because I'm afraid they're wrong. Anyway, I edited it a bit, hope it's better now. \$\endgroup\$
    – Roei
    Jun 26, 2018 at 8:36
  • \$\begingroup\$ First learn the difference between A and Ah. Then you can start to make sense of the numbers. \$\endgroup\$
    – user16324
    Jun 26, 2018 at 11:15

1 Answer 1


If you are going to run external power to your phone, you need to know how much power the external port consumes.

Your present estimate of 5W based on the internal 3.7v 2600mA battery capacity lasting 3 hours may or may not be good. It's possible for a phone to be quite wasteful in its use of cheap external mains (what it thinks is mains) power.

If we look at your 'half a day' exercise (let's call that 12 hours), then we have a 10W solar panel illuminated for a strong 6 hours, would supply 60 Wh. It's not clear how much energy your battery started the exercise with, but if it didn't last, then at the end it was zero. A 12v 7Ah battery stores 84Wh, and you've used somewhere between 0% and 100% of this. This gives a range of 60 to 144 Wh used by your phone in 12 hours, or a power of 5 to 12 watts. I've neglected power conversion inefficiency through charge controllers and USB supplies, as they're generally quite efficient, and the uncertainty in your 5-12W range dwarfs the odd 90% efficiency figures.

Let's pick a round figure at the top of that range, of 10W consumption.

As your solar panel supplies power for 6 hours of a 24 hour day, you need a minimum of a 40W panel. Going larger increases the tolerance to misalignment, ageing, dirt.

As your batteries supply power for 18 hours a day, you need a minimum of 180 Wh. With a 12v battery, that's 15Ah. Going larger reduces the depth of discharge, so improves the cycle life. If you ever have a dull day, then you would need to run for 24+18 hours, rather than 18, so you'd need a basic 35Ah. A battery of that size would offer you good cycle life on a daily basis, and have the emergency capacity to take you two days from time to time if you needed it.

The charge controller has to recharge the battery in 6 hours, so would need to be rated at a minimum of 30W (the 30W * 6 hours charge time to supply the 10W * 18 hours use). At 12v, 30W is 2.5A. It doesn't hurt to have it rated a little more, but the panel power capacity would limit it.

Once you've measured the power consumption of your phone properly, you can scale the other power and capacity numbers appropriately.


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