Rated watts are not a daily electricity bill
An aquarium heater's wattage describes power, while a bill charges for energy over time. A 200 W label does not by itself say how many hours the element heats during a day. A thermostatic heater can cycle rather than draw its full heating power continuously. Its actual operating pattern depends on the device and conditions, not just aquarium size or the printed wattage.
To estimate running energy, identify rated or measured heating power, the period considered and how much of that period the heater is actually on. Keep those assumptions explicit. The Electricity Cost Calculator does not infer a duty cycle from room temperature or use the Heater Size Calculator's wattage chart as an automatic energy-consumption prediction.
Convert on-time into kilowatt-hours
Energy in kWh = watts × heating hours/1,000 for a constant-power on-state model. If a heater is available for H hours and heats for a fraction d of that period, heating hours = H × d. Express a percentage duty cycle as a decimal in the equation: 30% is 0.30, not 30. The EIA's distinction between power and energy explains why multiplying by time is necessary.
For a hypothetical 200 W heater connected across 24 hours with an observed 30% on-time, equivalent heating time is 7.2 hours. Energy is 200 × 24 × 0.30/1,000 = 1.44 kWh/day. This example excludes any separately significant standby electronics and assumes full rated draw while heating. It is not a claim that every 200 W aquarium heater has a 30% duty cycle.
Calculate period cost using an explicit tariff
At an invented tariff of $0.25 per kWh, 1.44 kWh/day costs $0.36 in variable energy charges. Over a stated 30-day period, the same unchanging pattern uses 43.2 kWh and costs $10.80. A 365-day extension gives 525.6 kWh and $131.40. The annual figure is a constant-pattern illustration, not a forecast that winter and summer heating demand match.
Currency labels do not supply a local tariff. If the bill quotes cents per kWh, convert cents to the currency unit before entering the rate: 25 cents corresponds to 0.25 dollars. Fixed daily charges, taxes and other billing components may not be represented by multiplying heater kWh by one variable rate. Label what the estimate includes rather than presenting it as the whole household bill.
Count runtime only once
For the example, enter 24 hours with a 30% duty cycle, or 7.2 actual heating hours with 100% duty. Both represent 7.2 equivalent on-hours. Entering 7.2 hours and also 30% counts cycling twice, producing 0.432 kWh instead of 1.44. That smaller answer does not show a more efficient heater; it represents a different and incorrectly reduced time assumption.
Also distinguish a daily total from a multi-day observation. If a meter recorded 4.32 kWh over three days, the average is 1.44 kWh/day, not 4.32 each day. The period input then multiplies the daily estimate. Taking a monthly observation as a daily amount and applying another 30-day factor can create an error far larger than display rounding.
Use sensitivity scenarios instead of a universal monthly price
Hold 200 W, 24 hours, 30 days and $0.25/kWh fixed. At an assumed 15% duty cycle, period energy is 21.6 kWh and cost $5.40. At 60%, energy is 86.4 kWh and cost $21.60. These scenarios show the consequence of different on-time assumptions. They are not low and high confidence bounds or published typical consumption for an unspecified tank.
A continuous 100% on-state would use 4.8 kWh/day in this simplified model. That is a comparison endpoint, not proof the installed heater never operates differently or a diagnosis if measured use is high. Check actual operating instructions and observations. Do not alter required heating or electrical equipment merely to make a modeled cost smaller.
Measure energy under representative conditions
Where the equipment and installation permit safe use of an appropriate energy meter, a measured kWh interval can be more informative than guessing duty cycle. Record the heater or equipment actually included, observation duration and relevant conditions. A meter covering a complete aquarium power strip cannot isolate the heater unless the other loads are separately accounted for.
Follow electrical safety requirements and manufacturer instructions rather than improvise wet-area connections. This guide does not approve a particular meter, extension lead or wiring method. If the reading is inconsistent with the model, inspect the measurement boundary, period and assumptions first. A consumption difference alone cannot establish that a thermostat or element has failed.
Keep two-heater setups and sizing separate
For several heaters, estimate each independently unless measured evidence supports combining their on-time patterns. Two 100 W heaters both operating 30% across 24 hours would total the same 1.44 kWh/day in this invented case. If one cycles differently, their shared nameplate total no longer supplies the complete energy model.
Higher installed wattage does not necessarily mean proportionally higher daily consumption when the heating duty changes. It also does not guarantee lower use through shorter operation. The Heater Size Calculator provides a source-bounded capacity reference; it does not solve heat loss or forecast duty cycle. Do not use a cheaper energy scenario as evidence that an underspecified heating arrangement is suitable.
Reproduce the example and label its limits
In NumUtility's Electricity Cost Calculator, use one device row with 200 W, 24 hours and 30% duty, then enter rate 0.25 and 30 days. Read daily kWh, period kWh and cost separately. Additional filters and lights belong in their own rows. The result is an estimate based on entries, not a smart-meter import or a real-time tariff quote.
This guide is educational energy arithmetic, not electrical, installation or animal-care advice. Keep watts, on-time method, observation interval and tariff together so that a later cost change can be traced to its cause. The useful answer is conditional: given this power and heating time, the calculated energy follows. Choosing suitable temperatures or maintaining equipment safely requires separate appropriate guidance.