Power in x efficiency = power out - it is the power output into a specified impedance (ohms) that lets you drive a hungry speaker (high watts / low impedance). So you can have a high power input into an in-efficient amp that give a small output - typically this type of amp will get very warm.
The manual should give the output power (watts) into various impedances (typically 8 or 4 ohms), you can then match this to the speakers you are thinking of, but note speakers are typically marketed at a nominal impedance where in reality that impedance is frequency dependent and at certain point may dip quite low.