some SI prefixes:
Code:
Factor Name Symbol
10E24 yotta Y
10E21 zetta Z
10E18 exa E
10E15 peta P
10E12 tera T
10E9 giga G
10E6 mega M
10E3 kilo k
10E2 hecto h
10E1 deka da
10E-1 deci d
10E-2 centi c
10E-3 milli m
10E-6 micro µ
10E-9 nano n
10E-12 pico p
10E-15 femto f
10E-18 atto a
10E-21 zepto z
10E-24 yocto y
so to answer your question, a terawatt is 1000 gigawatts; or 1,000,000,000,000 watts (12 zeros).
edit: typo