Manufacturers rate vac power by many measurements, each of which means something different.
Power draw in amperes. More is better, up to 12 amps; more than that can trip a typical household circuit.
Peak horsepower. Measures maximum power. Not a good indicator of performance, but sometimes the only spec available. Look for 1.5 to 2 hp on small machines; at least 3.5 on larger ones.
Sealed suction or sealed pressure in inches of water lift. Measures the motor's suction power with no air flow. Look for 50 inches or more.
Air flow in cubic feet per minute (cfm). Tells how powerfully the motor draws in air, and factors in restrictions like filters and bags. Best comparison between similar machines. Anything less than 90 cfm isn't very effective.
Air watts, which combines air flow with sealed suction. Useful for comparing two similarly configured machines; 250 air watts and up is good.
Hooking Up Power Tools
Many power tools come with exhaust ports for attaching a vac hose to directly suck away dust or wood chips. However, unless your power tools and vac are made by the same manufacturer, you'll probably need a universal adapter (less than $10) or a roll of duct tape to connect the hose.
Better still is a vacuum that has an outlet for plugging in the tool. The outlet is wired with a delay switch so the vac starts up when you turn on the tool and stays on for several seconds after you turn it off, to clear the hose and tool.