2 pentillion watts of energy released
Energy is NOT measured in watts. Energy is measured in joules. Watts measure power. Power is the rate at which enegy is converted to another form. Therefore, watts are joules per second. A joule is also the same as 1 watt second. You buy energy from electric utilities, not power. We call them "power plants" but they aren't. They are energy conversion plants. You can't buy power. Power just tells how much energy you are using each second (day, month, etc.). Power is to energy as speed is to distance. Power and speed are time rates of change.
Your electric energy usage is measured in kWh (killowatt hours). When converted to watt seconds or joules, we get 1 kWh = 3.6 million joules. Say a US family uses 1000 kWh of energy per month, which comes out to about 3.6 billion joules. IF this quake released 2 x 10^18 joules as someone stated, it could support 50 million US familes for about a year if its energy could all be convert to electrical energy.