For cordless tools, power is determined by battery voltage: the higher the voltage, the more power and torque the tool has available. While more power and torque may seem like like something you want, there are trade-offs to be made. With high voltage cordless tools, along with the additional power, comes additional cost and additional tool weight.
Here is a typical comparison to illustrate the point. A typical 9.6 Volt cordless drill will typically weigh in the neighborhood of 3-1/2 pounds and cost anywhere from $35 - $150. As a comparison, a 19.2 Volt cordless drill can weigh up to 10 pounds and cost in the range of $125 - $300. If the increased torque and power of a 19.2 Volt cordless drill is really needed, the added cost may be worth it. But if you are using your cordless drill to occasionally drive screws in your home, a 19.2 Volt cordless drill is probably not worth it since it is harder to use (since it is heavy) and money not well spent. If you really do not need the additional power, a cheaper, light-weight cordless drill will probably suit your needs much better.
-- Number of Batteries --
Another point to consider when buying a new cordless tool is the number of batteries that come included with the tool. If you plan on using your tool for extended periods of time or often, you will want to only consider tools that come with two batteries. It is very frustrating when your batteries dies in the middle of something and you have to wait a couple of hours for the battery to charge before you can get back to what you were doing. When you are purchasing a tool, make sure it comes with two batteries. If it does not, look up the cost of an additional battery and include that when you are comparing costs between tools.
-- Battery Type --
Two types of batteries are for sale for cordless tools: Lithium Ion (Li-Ion) and Nickel Metal Hydride (NiMH). Do yourself a favor, do not purchase a cordless tool that uses NiMH batteries. Yes, it may be cheaper, but it is cheap for a very good reason. NiMH batteries have issues with memory problems (they need to be completely discharged before they are recharged or they lose charge capacity), they do not last for as many charge cycles (the batteries die quicker), and they are lower power than equivalent voltage Li-Ion batteries.
-- Battery and Tool Storage --
Even when your cordless tool is off, it will draw a small amount of energy from the battery. While the amount of energy being drawn is slight, it can add up to a significant amount over the course of days, or weeks. So when you put away your cordless tools, be sure to remove the battery; this will help ensure that the battery is charged when you need it. Do not keep your battery in the charger for extended periods of time. While there are some chargers that claim to be "smart" about this, but as a rule storing your cordless tool batteries in the charger will lead to them dying sooner than if you do not. When storing your batteries avoid storing them in heat and cold. Cold weather will not harm your battery long-term, but it will decrease the amount of available power when the tool is being used. High heat can permanently ruin the batteries of your cordless tools. Avoid high heat, and do not charge your batteries in an area that is overly hot, this can permanently damage the battery.
--Charging Your Batteries--
Many higher-end cordless tools now come with advanced chargers that allow the battery to be charged in a very short period of time; often in about 15 minutes. For comparison, a standard charger can take up to 3 hours to charge the battery. If you need a quick-charging battery for your tool, look into the charging options that are available prior to getting a cordless tool.
-- Pick a Brand --
An easy way to avoid problems with battery availability is to decide on a battery size and brand and stay with it. If you consistently purchase the same brand of tool with the same battery size when buying drills, trim saws, reciprocating saws, screwdrivers, etc, you will be able to share batteries and chargers between all of your tools. This will increase the number of available batteries and it ensures that you will never run out of charged batteries.
