What is the difference between 110 volt RMS and 220 volt RMS?

To answer the exact question, 220 Volts RMS is the voltage of a supply of electric power which is twice as big as 110 Volts RMS. Note: ordinary electrical test meters (multimeters) normally measure the RMS value, not the peak value, when they are used to measure an amount of AC voltage or current. Scroll down to related links and look at "Difference between RMS voltage, peak voltage, and peak-to-peak voltage". Further notes: RMS is an acronym for 'Root Mean Square'. In essence, it's an overall average voltage rating which tells you the 'real work' which can be done by a supply of power, or, in other words, it is a truer representation of the overall power profile delivered over time by an alternating voltage supply.

To determine an RMS voltage of a sine wave (as is used in AC mains power distribution), you measure its peak voltage and multiply it by .707, which will give you the RMS voltage.

So a 110 Volt RMS mains supply actually has a peak voltage of about 155.6 Volts and a 220 Volts RMS mains supply actually has a peak voltage of about 311.2 Volts.