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PSA: Ah vs. Wh and why you need to use Wh to compare apples to apples

upnorthandpersonal

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Ok, so, it seems there is quite a lot of confusion regarding this. I've spent the past day or two replying to several comments both here, on Reddit, and on YouTube, and Will made it a sticky on his latest video as well.

Only compare Ah ratings of a battery between those with the same nominal voltage. It makes no sense to take the Ah rating on its own as an indication of battery capacity without also taking voltage of said battery into account. In other words:

100Ah @12 V == 50A @24 V == 25Ah @48 V --> and they all have the same capacity of 1200Wh or 1.2kWh

When comparing capacity between different voltage batteries, use Wh/kWh. You get that by multiplying the Ah rating with the nominal voltage of the battery in quesiton.

The only reason I would use Ah is when testing capacity, since this rating is independent of the voltage and thus losses in cables/connections/etc. The current is the same in the entire circuit, thus this measurement is most accurate. Proof:

 
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Also note that Ah is "current capacity" and Wh is "energy capacity".

Didn't want to over-complicate things. People use 'capacity', and both are an indication of energy in the battery, it's just that Ah is incomplete without taking voltage into account. After all, energy E = V.I.t.
 
This is one of the most common mistakes/misunderstandings when it comes to units, along with confusing Amps and Amp-Hours,
 
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