Well if Lights are on 18/6 and you meant 23.926 cents/kWh
18(hrs) * 30(days) = 540hrs.
540(hrs) * 0.6 kW = 324 kWh
180 * $.23926 = $43.0668
+
144 * $.10475 = $15.084
(324 kWh - 180 kWh = 144 kWh)
=$58.15/month
at 12/12 it would be much less
12 hrs * 30 days = 360hrs
360 hrs * 0.6kW = 216 kWh
180 * $.23926 = $43.0668 (same as first time for first 180 kWh)
36 * .10475 = $3.771
= $46.84/month if you were using 12 hours.
What I would say is that it's going to be a little over 50$/month even with 12/12 Not just because of taxes and fees but also because the ballast probably grabs more than 600W @ 5amps... that's what it outputs to the light, since no conversion process is 100% efficient you have some loss (usually as heat)
You will be paying approximately $50-$65/month depending on taxes/fees/overage charges and actual kWh drawn.
I was really stoned and spent way too much time on this response