Where the power actually goes
Heaters carry the biggest rating (100–300 W) but cycle on and off—actual duty varies from 10% in a warm room to 50%+ against a cold wall in winter. Lights run at full power for their whole photoperiod, so a 40 W fixture on 10 hours a day uses 12 kWh a month, every month. Filters and air pumps are small (3–20 W) but run 24/7, adding a steady 2–15 kWh monthly. CO₂ solenoids, UV sterilisers and chillers add more for specialised setups—chillers can dwarf everything else.
Worked example: a 180 litre community tank
A 150 W heater at 30% duty in a 21°C room: about 33 kWh/month. A 35 W LED for 8 hours: 8.4 kWh. A 10 W canister filter: 7.2 kWh. Total ≈ 49 kWh/month—at $0.20/kWh that is about $9.80 a month or $118 a year. The same tank in a 17°C room in winter can double the heater's share. Plug your own wattages and local price into our electricity cost calculator for a personalised figure.
Cutting the bill without hurting the tank
The cheapest wins: put a lid on the tank (evaporation is a huge hidden heat loss), move the tank off exterior walls and out of drafts, and insulate the back and sides with foam board if looks allow. Trim the photoperiod to 6–8 hours—plants don't need more and algae loves extra. Replace old T8/T5 tubes with LED. Do not undersize the heater to "save power": a smaller heater running constantly uses the same energy as a bigger one cycling, with less safety margin.
Measuring beats estimating
A $10–15 plug-in energy meter between the wall and your power strip tells you the tank's true consumption in a week, including real heater duty cycle—something no calculator can know exactly. Measure winter and summer separately; the difference is often 2× and it is almost entirely the heater. That number also tells you whether a lid or insulation actually paid off.