Find the energy charge first
Look for electricity consumption in kWh and the price per kWh on a recent bill or tariff schedule. Some bills call one kWh a unit. Check the billing period and confirm that you are reading electricity rather than gas, water, or a combined total. The calculator asks for the price of energy consumed by an appliance, not the amount due for the entire household.
A UK bill may show pence per kWh while another bill shows pounds per kWh. Select the rate-entry unit that matches the printed number. For example, 24 pence per kWh and £0.24 per kWh express the same price; entering 24 into a pounds-per-kWh field would inflate the result one hundredfold.
Keep fixed charges separate
A standing, service, meter, connection, or minimum charge can appear even if the household uses very little energy. It is a property of the account or billing period, not of one television or fan. Add it when modeling the whole bill; do not multiply it by an individual appliance's kWh.
Suppose a 30-day bill has 320 kWh at 0.20 currency units per kWh and a daily fixed charge of 0.50. The energy charge is 64.00, the fixed charge is 15.00, and the subtotal is 79.00 before other adjustments. A new appliance using 20 kWh adds about 4.00 in energy charges at that flat rate, not 20 × (79 ÷ 320). Dividing the full bill by kWh would fold the existing fixed charge into every extra unit.
Check whether you cross a slab
With progressive slabs, extra energy may be priced partly in one band and partly in the next. Imagine a tariff where the first 100 kWh cost 0.15 each and the next 100 cost 0.22 each. If the household would use 95 kWh without an appliance and the appliance adds 20 kWh, its marginal cost is 5 × 0.15 + 15 × 0.22 = 4.05. Multiplying all 20 kWh by either single price would miss the boundary.
Confirm whether your supplier uses progressive slabs, a whole-bill band, or a different method. The word 'tier' alone does not tell you how the bill is calculated. Where the structure is unclear, label the calculator result as an approximation and check the supplier's published tariff.
Match peak and off-peak hours
A time-of-use tariff has different prices at different times. A water heater that runs overnight can cost something different from the same kWh used at an evening peak. Use the rate for the hours you expect to run the appliance, or split the energy across tariff periods in an advanced bill calculation.
Do not use a cheap off-peak rate for an appliance that runs during the day. If a bill shows only an average price across several periods, use it for broad planning and record that the schedule was not modeled.
Record taxes, discounts and dates
Check whether the printed unit rate already includes tax. Also identify fuel adjustments, rebates, subsidies and minimum charges so they are not counted twice. Save the tariff provider, region and effective date alongside a result; prices change and a past bill may not describe the current period.
When you cannot find a reliable current tariff, enter a clearly marked planning rate and treat the result as a range. The most useful next step is to replace that rate with the per-kWh price from your bill or supplier—not to add more decimal places to an uncertain estimate.
Use the value in a Wattwise calculator
Start with the closest appliance model, replace the preset with your measured or label value, and review the confidence and excluded charges before making a decision.
Browse appliance calculatorsOfficial references used in this guide
Label formats and bill terminology differ by market. Use the original source for your product or supplier when checking a number, then enter your own usage and electricity price.