Power = Watts
Higher voltage means more power, right? Not quite…
Power is measured in watts (W), and watts are calculated by multiplying voltage (V) by the current (amps, A). It’s possible to get the same power, for example, from an 18V battery and a 54V battery—but the 54V pack will do it with 2/3 less amps than its little brother.
Runtime = Watt-hours
Watt-hours (Wh) simply refers to how many watts a battery can put out for 1 hour. Just like power, watt-hours is calculated by multiplying voltage by the rated amp-hours (Ah) of the pack. This will be how you can calculate run-time of your tool.
What is “Nominal”?
The battery packs that you use on your tools are made up of many cylindrical cells, similar in shape to the AAs in your TV remote but much bigger and packed with a lithium-ion chemical cocktail to give them much higher energy density instead of the low energy, but very cheap, alkaline type that you buy in the check-out line of your local supermarket.
Battery cells are chemistry in a can, not perfectly identical production machines. The term nominal is an industry standard used by manufacturers of lithium-ion battery cells to rate the approximate mid-point voltage of their batteries. Nominal voltage ratings are, essentially, the average rating of the cells produced with a certain chemistry. In general, all commonly used lithium-ion battery cells are 3.6V nominal voltage. Beyond voltage, there are many options for cells with different amp-hour ratings as well as different maximum discharge current ratings, depending on how long a manufacturer wants the battery to last on a specific tool and how fast the pack is able to use its energy (i.e. higher power) during operation, respectively. Not all cells are created equal.
You are surely more familiar with the battery packs itself than the actual cells that these packs are made up of. Battery packs are made of stacks of cells assembled in tight clusters and connected in different series or parallel configurations to produce some combination of higher voltage or higher amp-hours, depending on the design and application of the tool it is intended to power.
When assembled in series, cell voltage (3.6V nominal) is multiplied by the number of cells in the series. For example, a 10S (10 series) configuration is built with 10 cells * 3.6V/cell, resulting in a 36V pack.
To increase amp-hours (Ah), a battery pack assembler will connect cells—or series of cells—in parallel to achieve their run-time targets. For example, 2P (2 parallel) configuration, using 4.0 Ah cells, is built with 2 cells * 4.0 Ah/cell, resulting in an 8.0 Ah pack.