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How the calculation works
For a device, daily energy in kWh is watts × operating hours ÷ 1,000. Light and pump hours are entered separately. Heater energy uses rated watts × 24 hours × duty-cycle fraction, so a thermostat-controlled heater is not assumed to draw full power continuously.
Add daily device energy, multiply by days in the period, then multiply by your per-kWh tariff. The answer uses the same currency as your input without assuming dollars, euros or a location. This is an energy-cost estimate, excluding standing charges, taxes, billing tiers and time-of-use price changes.
A plug-in energy meter can improve the estimate for variable-speed equipment and thermostatic heating. Heater duty varies with room temperature, evaporation, insulation and water temperature. The example 30% duty cycle is adjustable and is not a recommended heater size or a prediction for every aquarium. Add other equipment to your own budget separately.
Worked example
A 30 W light for 8 hours, a 10 W pump for 24 hours, and a 100 W heater at 30% duty use 0.24 + 0.24 + 0.72 = 1.20 kWh/day. Over 30 days at 0.20 per kWh, estimated energy cost is 7.20 in your billing currency.
Common questions
What does 30% heater duty mean?
The heater draws rated power for an equivalent of 7.2 hours per day. Use measured runtime or energy where possible; the thermostat can cycle many times during that period.
Can the estimate match my full utility bill?
Only the entered device energy is modeled. Fixed fees, taxes, household use and changing tariff bands need separate billing calculations.