When it comes to lithium ion batteries, voltage seems to be the sexy spec. But voltage is only one part of a more critical equation. The watt-hours spec determines actual power output over a given amount of time.
Watt-hours, or Wh, is calculated by taking the voltage times the ampere-hours, or Ah.
But we're still not done. Maximum voltage and nominal voltage are two separate things. For instance, a 40V max battery actually provides 36V of usage, because the second a tool begins drawing power from it, the battery loses about 10% of its voltage. So look for Nominal voltage.
Finally, the more voltage, the more weight. So evaluate a battery's Wh relative to the weight of the battery. In other words, think about the tools you’ll be using and in what applications. Just like a gas-powered engine, you want a battery that is right-sized for what you’re needing to do, providing the ideal power:weight ratio.