Step 1: find the electrical input
Look for watts (W) on the appliance label or manual. Be careful not to confuse output capacity with electrical input — this matters especially for heaters, air conditioners and chargers.
Step 2: calculate kilowatt-hours
Use watts ÷ 1,000 × hours. For example, 2,000 W for 3 hours equals 6 kWh if the device draws 2,000 W continuously.
Step 3: multiply by your tariff
Multiply kWh by the energy price from your tariff. At €0.30/kWh, 6 kWh costs €1.80 in usage charges.
Step 4: improve the estimate
For thermostatic or variable-load devices, use measured average power or measured kWh over a representative session. For time-of-use tariffs, split usage across each price period.
What not to mix into an appliance comparison
Fixed standing charges or subscription fees usually do not change when one appliance runs for an extra hour. Keep them separate when comparing the variable cost of two devices.
Working through a realistic multi-appliance example
Most people don't want to price just one appliance — they want a rough sense of a whole day or week. Suppose a household runs a 2,000 W heater for 3 hours, a 120 W fridge continuously for 24 hours, and a 1,200 W washing machine for 1.5 hours on one day, all on a €0.30/kWh tariff. The heater uses 6 kWh (€1.80), the fridge uses 2.88 kWh (€0.86), and the washing machine uses 1.8 kWh (€0.54) — a combined €3.20 for that day's usage from just those three appliances. Repeating this appliance-by-appliance approach, rather than guessing at a whole-house total, is the most reliable way to build an accurate personal estimate.
Common mistakes to avoid
The most frequent error is using an appliance's peak or maximum wattage instead of its typical running wattage, which can overstate cost significantly for appliances that cycle on and off, like fridges, washing machines and dehumidifiers. The second most common mistake is forgetting that a bill also includes fixed standing charges and taxes on top of usage cost, so an appliance-level estimate will never exactly match a full bill on its own.
Frequently asked questions
Can I calculate a whole month's bill this way?
You can build a reasonable usage-cost estimate by totalling your significant appliances' individual kWh over a representative day and multiplying by 30, then adding your bill's fixed charges and taxes separately.
What if two appliances run at the same time?
Simply calculate each appliance's kWh independently and add them together; running simultaneously does not change each appliance's own energy use.
Should I use my supplier's estimated or actual meter readings?
Actual meter readings, where available, are always more accurate than a supplier's estimate, since estimates are based on assumed usage patterns rather than your real consumption.
How accurate is this method compared with a smart meter's own app?
It should be broadly consistent, since both rely on the same watts-to-kWh-to-cost arithmetic; a smart meter app has the advantage of measuring actual draw rather than relying on assumed wattage.
Does this method work the same way outside Europe?
Yes, the underlying watts-to-kWh-to-cost formula is universal; only the currency and typical unit price change by region.
