check your bill, I bet they came out and didnt estimate this time. I they probably misread by 1000kwh.
You need to factor in the filter (24hr/day) a heater (?hr/day) etc but the difference between a 15W and 36W lamp is not the sole factor in your increased bill.36W/1000 = 0.036 KW
0.036KW * 12 hr/day * 28 days = 12.096 Kwh
12.096Kwh * $0.144$/Kwh = $1.74