I am trying to figure out what size battery I will need to power my RPi and 2 components with it. I want to build a "tablet" of sorts, mainly because I don't want to spend $800 on a Wacom tablet when I can spend $200-$300 building my own (The point is to do digital sketches and take colorful and digital math notes).

I prefer for the battery to last for 10 hours as I don't want to have to constantly charge it. However, I don't need a $300 battery with a billion mAh on it either.

Since I have never calculated mAh before, I don't know if I screwed any of the math or formulas up, so I would like to know if I calculated it correctly and what pointers I could use to become more knowledgeable about building battery powered electronics.

The intended battery size I am looking for is under the Average Usage column.

Screenshot of Calculations

Edit: The 30 hours is supposed to be 10. I just lazily dragged the sum(...) command across the columns.

Also, I cannot seem to find any actual sources of power requirements for the touch screen. I am looking at the 7 inch official RPi touch screen screen. I just randomly found on Google that is may use around .5 amps. It is similar for the camera being 200-250 milliamps. The last thing is, would I be right in believing these devices around around 5 volts. If not, how would I find out what voltage they use?

1 Answer 1


The calculation looks good so far.

Please note that power banks give the capacity in mAh only for their internal voltage (most times). Best is to look at the capacity in (m)Wh.

The calculation for the capacity in Wh for your case would be

10h * 1.15 A * 5 V = 57.5 Wh (average usage)
10h * 3.25 A * 5 V = 162.5 Wh (peak usage)

So for an average usage you need a battery pack with at least 57.5 Wh.

To explain it further with the capacity: I have a power bank which states 20,000 mAh. Calculating with the output voltage (5 V) it should have 100 Wh capacity but it has only 74 Wh printed on it because the mAh are calculated for the internal voltage (typ. 3.6 V).

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