Methodology

HomeBillLab calculators are designed to make the inputs, formula and assumptions behind a cost estimate visible.

Core running-cost calculation

For a simple appliance with a known input power, the basic energy calculation is:

If power is shown in watts, we convert watts to kilowatts by dividing by 1,000. For example, 2,000 W is 2 kW.

This is the same basic relationship used in Australian Government household energy guidance: input power in kW multiplied by the electricity price per kWh gives an hourly running-cost estimate.

When annual energy consumption is available

For appliances with an Energy Rating Label or another reliable annual energy-consumption figure, an annual kWh value can be more useful than pretending the appliance draws its rated power continuously. In those cases we may calculate:

Why real-world results can differ

Many household products do not draw their maximum input power continuously. Air conditioners, refrigerators, heat pumps and other controlled equipment cycle or modulate. Weather, thermostat settings, room size, insulation, equipment condition and user behaviour can all change actual consumption.

For that reason, a calculator may offer more than one method or scenario. Where we use a duty cycle, efficiency assumption or other modelled input, it should be labelled as an assumption rather than measured consumption.

Electricity rates

We prefer a user-entered usage rate from a current electricity bill or plan when the calculator allows it. A broad default can be useful for an example, but it is not a personalised tariff.

Electricity plans can include flat rates, time-of-use rates, controlled-load rates, demand components and daily supply charges. A simple appliance running-cost calculator normally estimates the variable usage component unless it explicitly says otherwise. We do not add a household’s daily supply charge to an appliance estimate because that fixed charge is generally paid whether or not that individual appliance is used.

Location and date

Australia does not have one permanent household electricity price or one set of rebates. Where location matters, calculators and guides should identify the jurisdiction or postcode assumptions they use. Time-sensitive defaults should include a source and review date.

Rounding

Calculations may use more precision internally than is displayed. We round the final result to a practical number of decimal places so that a display such as cents per hour remains readable. Rounding should not be used to imply greater accuracy than the source inputs support.

Source priority

For Australian energy topics we prioritise official Australian Government, regulator and market-body sources, followed by primary manufacturer specifications for product-specific data. See the Sources page for the current hierarchy.