Electricity where i live is 12p 1000wph
soo...
for 12/12h x 7d =
1kw x 12h x 7d = 84kw
84kw x .12p = £10.08 for the week, thats not including the ballast.
Hope that helps.
alsoo dont worry as far as a legal issue (suspect), it takes more than 6KW an hour to raise suspicion.. I have a friend who works for the utility company and has given me quite the legal run down, thats why so many states have gotten into helicopters for heat signatures etc.. your "big light" will raise no eyebrows in case you were wondering.