Total generating capacity is the sum of the capacity of the individual turbines in a wind farm. A wind farm with 10, one-megawatt turbines will have a total generating capacity of 10 megawatts. If it is operating at full capacity, in one hour it will generate 10 megawatt hours of electricity. If it is operating at 50% capacity, in one hour it will generate 5 megawatt hours of electricity.
The calculator works out electricity generation on an annual basis because wind generation varies on a daily and hourly basis (in New Zealand, hydro generation is used to balance variations in wind generation and ensure electricity supply meets demand). On an annual basis, wind generation is very reliable, typically varying between 5 and 10%. In contrast, hydro generation typically varies about 20%.
The figure given for electricity generation takes into account varying wind speeds throughout the year and the amount of time a turbine operates.
We aren't all electrical engineers who understand what a megawatt hour means, so the calculator provides a comparisons of electricity generation with household electricity use. Again, this information is provided as a guide, not as a precise figure.
The comparison is based on an annual basis, as over the course of a year household demand will vary and be influence by geographic location. Average household electricity demand is greater in the South Island than in the North Island, and demand in both islands increases during winter months. Over the course of a year, an average New Zealand home will use 8 megawatt hours (or 8000 kilowatt hours) of electricity.
Figures for avoiding emissions are based on data from the Ministry of Economic Development. Emissions are presented as carbon dioxide equivalent (CO2e) emissions from the different greenhouse gases (eg carbon dioxide, methane and nitrous oxide) based on their global warming potential. Emissions for electricity generated vary between plants, so the calculator uses the average for gas-fired plants (0.37 tonnes of carbon dioxide equivalent emissions per megawatt hour of electricity) and coal-fired plants (0.93 tonnes of carbon dioxide equivalent emissions per megawatt hour of electricity). In 2007, Electricity generation resulted in 6,639,000 CO-e emissions. 65% of these emissions were from gas generation, 35% from coal generation.
Detailed data regarding electricity generation and greenhouse gas emissions is available from the Ministry of Economic Development .