This used to trip me up all the time, but the best way to remember the difference is–Power (rate); hours (time); and Energy (work) OR (power x time = energy). MW (power) references the rate at which a power plant can generate MWh (energy or work).
A follow-up to the fancy barrel diagram from last year–barrel of oil equivalent (or BOE) is a unit measure of unused energy resources. BOEs are defined in financial statements of most energy companies as 1.7 MWh or as 5.8×106 British Thermal Units (BTUs). A BTU is the amount of heat energy needed to raise the temperature of 1 pound of water by 1 degree F. I know–kind of confusing. This is used frequently when referencing steam, which most coal, gas, and CSP projects use as an intermediate step to generate electricity.
For some context, in 2009, the average household used approximately ~1 MWh a month or 11 MWh a year.
http://www.eia.gov/cneaf/electricity/esr/table5.html
