the electricity supply to every customer a power company

Maintaining the voltage level of the electricity supply to every customer a power company has is a tremendously difficult task (think about the famous "brownouts" and "blackouts"). Thus, even by established standards, mains supply voltages have a fairly generous leeway before they're considered out of bounds. A nominal 115-volt supply, for example, is better described as one that might vary from 105 to 125 volts; a nominally 230-volt supply might possibly vary from 214 to 246 volts. Moreover, the "nominal" value can vary somewhat from region to region: some places specify 115 volts, where others require 120 volts; likewise, some specify 230 volts, others 240 volts, yet others 220 volts. (The specifying is done by whatever governmental body regulates--or operates--power companies.) Tolerated variations from the nominal can range from ± 7% to ± 10%.

None of this means that power companies' supply voltages do normally or frequently swing to the limits of their tolerance--most keep the mains voltages not far from their nominal value most of the time. But swings can happen, and we don't want to be in a marginal condition where our cooking-unit breaker regularly trips.

It is hard to make flat statements with certainty in this area, but it seems--from looking at owner manuals--that fortunately for us most or all modern units are now designed so that lower supply voltages simply lower the maximum power output of the unit. They do not, that is, try to "make up" for lower supply voltages by drawing more current. We are thus usually sufficiently safe to proceed by reckoning the current draw for the specified unit total maximum power (about which see the paragraphs below) and a nominal supply voltage of 240 volts.