doing the math on solar

[JamieJohnson]

I have been doing some maths on batteries and solar and i think this is correct. Please help and advise if i am right

My raspberry pi consumes approx 2000ma with attached devices and camera at 250ma

a 12v 7ah battery provides 84 watts of power

5v at 84 watts gives us 16.8ah

running 2000ma from 16.8ah gives us 8.4 hours running

to run a pi for 24 hours would require 3 x 7ah 12v batteries (21ah)

Lets say we have 10 hours of sunlight to replenish the batteries

21ah ÷ 10 hours gives us 2.1a per hour charge required

12v at 2.1a is 25.2 watts

A 12v 25w solar panel is needed to maintain the battery for permanent non stop power supply

i hope my calculations are correct and if not please advise before i go buing batteries and panels

thanks

[joan]

Perhaps it would be simpler if you standardised on one unit, perhaps Watt Hour.

If the Pi takes 2 amps at 5V that is 10 watts. Over 24 hours you need 240 Watt Hours.

Assuming the solar panel will deliver 85% of its rating into the batteries and the batteries will supply 85% of their rating into the Pi.

You'd need 340 Watt Hours from your solar panels (340 *0.85 * 0.85 = 246).

[Ravenous]

i hope my calculations are correct and if not please advise before i go buing batteries and panels

Those calculations are for too optimistic - you're assuming the solar panel will produce its full rated power for 10h per day, every day.

I heard an estimate (can't remember where) that even under clear skies you can only rely on around 4 hours' worth - because all morning and afternon the sun is at a reduced angle.

Several people have built systems using panels around 80-100 Watts. Even then I can't find them and aren't sure what performance they reached. A 12V/25W panel will give you several hours per day, but not full operation.

[hampi]

I agree that real life can be quite complicated and is rarely described by one simple equation. So there are two possible approaches depending on how big investment the solar operation is likely to be. If you think it is going to be expensive for your budget, then do careful calculations and build a computer model to simulate the energy production.

The other possibility is to build something not too expensive and test it in real conditions. I have chosen this way and I am still learning from the experiment. There are issues like WiFi dropping too often because the installation is outside and the correct monitoring of the lead acid battery voltage. An other issue is the cyclic rebooting and writing data on the SD, which could corrupt the disk in some cases or after many rebooting cycles.

There are different strategies that can be used in running the solar powered system. First I tried a system that was running at scheduled times, but that lead to overcharging the battery. Then I choose to wake up the system in the morning some hours after the sun rise because the solar power is more likely to be available in the morning and the late afternoon brings often rains. If the battery capacity drops below 50 % the system goes to sleep and wakes up next morning again. This should extend the battey life compared to the case that you drain the battery every time below 30 %.