it depends on how hard the appliance is used.after the storms of 2004 I used these tools exclusively and intensly-- as ther was no power at the marina----and as the battery lost power the tool would loose power too.this is why more than one battery is good to have--as one wears out----use the one on the charger ---the catch 22 is -----if the power is out----how do you recharge the battery----usually a remote generator or other power supply can be and was located for this----and the portable tools transported to the job site---in my case----a destroyed marina-----what a scene---