How the cost is calculated
Energy is power times time: watts ÷ 1,000 gives kilowatts, times hours of use gives kilowatt-hours, times your price per kilowatt-hour gives money. A 100 W load running 10 hours a day uses 1 kWh daily — at 0.25 €/kWh that is 0.25 € a day, 7.50 € a month and about 91 € a year. Small always-on loads add up the same way: a 15 W router costs around 33 € a year.
Why estimates drift from bills
Three things push estimates off. Nameplate power is a maximum, not an average — cycling appliances draw far less over a day. Usage hours are usually guessed optimistically. And the price people remember is the bare energy price, while the bill adds transmission, fees and taxes. Measuring with a plug-in energy meter for a few typical days removes the first two errors; your bill fixes the third.
The standby tax
Devices that are never really off — routers, TVs on standby, chargers, smart speakers — often draw 20–50 W combined around the clock. At 0.25 €/kWh, every constant watt costs about 2.19 € a year, so a 30 W standby cluster is 65 € annually. This is usually the cheapest saving in the house: a switchable power strip pays for itself in months.
Methodology
The calculator UI, the appliance table and the displayed formula all call the same calculation function. For off-grid versions of the same question — how long a battery carries a load — see the Battery Runtime calculator.
- Energy cost = kW × h × price/kWh — Definition of the kilowatt-hour (SI-derived billing unit)
- Household electricity prices by country — Eurostat electricity price statistics
- Typical appliance average draws — Manufacturer energy labels and plug-in meter measurements — always prefer your own measurement
Assumptions last reviewed on 26 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
Which electricity price should I use?
The all-in price from your bill: energy plus transmission, fees and taxes. It is often 1.5–2 times the bare energy price quoted in the news.
Why does an appliance use less than its label says?
The nameplate shows maximum draw. Compressor and thermostat appliances cycle on and off, so a fridge labelled 150 W typically averages 30–50 W across a day.
How do I handle an hourly spot-price contract?
Use your average realised price for a first estimate, then remember that shiftable loads like laundry and charging can deliberately target the cheap hours.