# Am I Correctly Calculating mAh for the Pi?

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.

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?

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`).