Energy consumption is worth considering, but usually it's only an issue for larger tanks (planted, reef, etc.).
If the energy use is a limiting factor, you should be able to do some calculations based on the wattages of everything (usually, this is available on the box, or a sticker or something, but not always). Just remember to effectively 1/2 the lighting wattage (since it would only be on ~12 hours or less) and either make a guess as to the heater, or use one of those calculators that advises heater wattage based on ambient (room/air) temp and desired tank temp, compared to the size of tank.
Once you have the wattages all added up, figure out the kilowatt/hour rates (look online, see if you can check a bill, or something. round up.), and multiply the two numbers (watts x rate) for an estimate of monthly energy use hopefully this will give you a slightly high number, since there is only ~720 hours in a 30-day month, but I would think it would help you if you presented a worst-case scenario, and the actual results fell quite a bit below that.
anyways, good luck