A good circular saw should have enough raw power to slice through everything from wet lumber to dense hardwood without bogging down. "When the motor slows, the blade heats up and dulls quickly," explains Tom Silva, This Old House general contractor. This not only produces a poor cut, it's dangerous because the blade can climb out of the kerf and push the saw back toward the user.
However, evaluating power from the motor ratings can be misleading. Amps indicate only the amount of electricity a motor draws, not the power it sends to the blade. Horsepower accounts for torque (rotational force), but not necessarily under working conditions.
In the end, the most reliable appraisal may be price. A dependable sidewinder—the more compact design, in which the motor sits alongside the blade—starts at around $100. There are many saws on the market under this price, but they're not as powerful, nor are they built for a lifetime's use. Professional-grade sidewinders, which run quieter and cut through dense wood better, cost between $125 and $150. TOH master carpenter Norm Abram prefers this tool, noting that buyers should choose one based on balance and maneuverability. "I'd never buy a saw I didn't have a chance to hold first," he says.
On the other hand, a good worm-drive saw, Tom's choice for framing because of its high torque output (its beefy spiral gear transfers power to the blade more efficiently), will set you back at least $200. Either way, a top-of-the-line saw, if treated with care, should still be cutting well when you're ready to hand it down.