Watt Hours vs Amp Hours - What’s the Difference?

Posted by Bixpy on Feb 6th 2017

Watt Hours vs Amp Hours - What’s the Difference?

Understanding Battery Power Vocabulary

Different batteries seem to have different ways of defining their power output and overall power capacity. Some use Amps, Amp Hours, or Watt-Hours.

What's the difference and what's the best way to actually understand and compare batteries?

For the purposes of this blog, we'll keep things simple and mainly focus on outdoor power banks and outboard trolling motors batteries.

Until very recently, only one kind of trolling motor was available for marine use (those old fashion trolling motors with the large propellers and throttle handles on top). Most marine electronics ran on 12V power, and power banks in their current format were not really a thing.

So aside from the batteries commonly found in flashlights and other small electronics, the only rechargeable batteries were those bulky 12V lead-acid batteries and, for a slightly more waterproof battery option, were their SLA (sealed lead acid) counterparts.

Because the most common voltage in these batteries was 12V, most people simply refer to the battery capacity using the Amperes (Amps) of these batteries, and the comparison was simple, the higher the Amps, the more energy a battery held.

Watt Hours vs Amp Hours

With the introduction of lithium batteries and a much wider range of electronics requiring different voltages, Amps are becoming less useful in comparing batteries. The problem with defining power in Amps is that it doesn't really take into consideration the voltage of the battery. It's all good if all batteries are 12V, but things fall apart quickly at different voltages.

Because of this, a simpler way to compare batteries is by knowing the battery's watt-hours since watt-hours take into consideration the voltage of the battery in question. It offers a simpler way to understand the overall capacity of the battery.

Here's a simple example comparing an old fashion SLA battery with the lithium batteries used in running the Bixpy J-2 motors in the J-2 Outboard kit:

  1. A 12-volt lead-acid battery with 30 Amps of capacity holds (12x30) 360 watt-hours of power.
  2. A 7-cell lithium battery at 25.2 volts and 15 Amps of capacity holds (25.2 x 15) 378 watt-hours of power.

So if you were to compare these two batteries simply using their amper values, you would think the lead-acid battery has a higher capacity, but that would be wrong. In fact, the 15Amp lithium battery holds 5% more power!

This also applies to power banks made for powering or charging your small electronic devices (like the PP-166 Power Station).

Here's another quick example to compare the power capacity of two power banks:

  1. ACME Power Bank is labeled as holding 13Amp (or 13,000 mAmp) of power, but because the manufacturer is labeling the power bank at 5V output (common for USB devices), the true power capacity of ACME Power Bank holds 13Amp x 5V = 60 Watt hours.
  2. The PP-166 Power Station by Bixpy holds 13Amp (or 13,000 mAmp) or power, but it's rated at 12.8 Volts, which is the nominal voltage of the battery. So the PP-166 Power Station holds 13Amps x 12.8V = 166 Watt hours.

So the Wh of power contained within the battery pack is generally the easiest way to compare batteries to each other. It's analogous to comparing gallons (or liters) of water in a bucket regardless of the shape of the bucket or the hose that filled it with water, or the hose you will use to empty it. Another lesson on that to follow soon ;-)