Definitions of Terms

Units of Energy

Kilowatt-hour (kWh), Megawatt-hour (MWh), Gigawatt-hour (GWh)

Energy = power x time

Thus a kilowatt-hour is the energy of a 1kW device running for 1 hour or a 100W device running for 10 hours.
Megawatt-hour = 1000 kWh, Gigawatt-hour = 1000 MWh

Units – Greenhouse Gas Emissions

Tonnes of Carbon (tC)

Unit used to express the carbon content of greenhouse gas emissions that facilitates easy comparison of different greenhouse gases with different carbon content. One tonne of carbon is equivalent to 3.7 tonnes of carbon dioxide, the most important greenhouse gas.

See more at either:
The Carbon Trust
Sustainable Energy Authority of Ireland