You can actually come up with an estimation of how much electricity the tank will use every day with a little more information and a little guestimation. First the easy stuff that does not require guessing. Take the wattage of your filter motors, which should be imprinted on the motor, and multiply by 24. Then take the wattage of your lighting and multiply by how many hours you will have it on. Add these two numbers together and divide by 1000. This will give you the KWh for running the filters and lights for one day. Multiply this by your utility rate to get the cost per day of running the filters and lights.

Now the guestimation portion... the heater costs are more difficult to calculate because it will depend on how often the heater comes on. The amount of time the heater is on depends on the room (ambient) temperature compared to the desired tank temperature. The closer these two are, the less often the heater will come on. If you want an upper bound on costs, then just take all the guesswork out and say the heater is on all the time and multiply the heater wattage by 24 then divide by 1000 to get the KWh. Even though this is unrealistic, you'll at least know the heater will never use more energy. Otherwise you can try to estimate how many hours in the day your heater will be on and multiply the heater wattage by that guess.

So here's an example. Let's say you have 80W of lights on 12hrs a day, an AC70 HOB with a 6W motor, a Eheim canister with a 25W motor and a 250W heater. Here's the calculations:

Filters: (6W + 25W) * 24h = 744Wh / 1000 = 0.744 KWh per day

Lights: 80W * 12h = 960Wh / 1000 = 0.96 KWh per day

So that's 1.704 KWh per day for the filters and lights. At a $0.11 rate (which is my current rate too), that's about $0.20 a day and $6 for a 30 day month. Now onto guessing on the heater:

Heater (max): 250W * 24h = 6000 Wh / 1000 = 6 KWh per day (max)

Heater (12h): 250W * 12h = 3000 Wh / 1000 = 3 KWh per day (12h)

Heater (8h): 250W * 8h = 2000 Wh / 1000 = 2 KWh per day (8h)

Heater (4h): 250W * 4h = 1000 Wh / 1000 = 1 KWh per day (4h)

So the heater at most uses 6KWh per day, but more realistically will use 1-2KWh per day unless the room the tank is in is very cold. Together with the calculations for the lights and filters, you'd use about 2.7KWh to 3.7KWh per day. That's about $0.30 to $0.40 a day or $9 to $12 for a 30 day month. At most if the heater was always on, it would be 7.7KWh per day, for a cost of $0.85 a day and $25 for a 30 day month.