What tolerance is acceptable? I never paid attention until I plugged in a surge protector with a display on it, 100 volts. Checked a few outlets around the house with a meter and all consistently 100 volts. I know a 10% tolerance can be normal, but this is almost strange to me.\n\nI hardly matters. AC is an efficient transport for electricity but most devices in your home will use a transformer to convert it to a [much lower] DC voltage. That voltage is then delivered to the respective devices at the voltage they specify.\n\nNow if you're talking about [class AB] amplifiers then P = IxE will always apply but you're still in tolerance and need not worry.