How the running cost is worked out
Electricity is billed by energy, not by power. Power, measured in watts, is the rate at which a device draws electricity at any instant; energy is power accumulated over time and is what shows up on your bill in kilowatt-hours (kWh). One kilowatt-hour is a 1000-watt load running for a full hour.
To get there, the wattage is multiplied by the hours it runs each day and divided by 1000 to convert watts into kilowatts. That gives the daily energy in kWh. Multiplying by the days in the period scales it up, and multiplying energy by your per-kWh rate turns it into money.
Reading the numbers
The headline figure is the total cost across the period you entered, which is handy for sizing a monthly or yearly line on your bill. The per-day cost is the same calculation at a daily scale and is the easiest figure to compare between appliances.
The cumulative-cost chart shows the running total climbing one day at a time, so you can see how a small daily draw quietly compounds into a meaningful sum over a billing cycle.
Trimming the bill
Because cost scales directly with watts, hours and rate, lowering any one of them lowers the bill proportionally.
- Cut runtime first — a device left on standby for 24 hours can cost more than one used briefly at high power.
- Replace the highest-wattage, longest-running loads (heaters, dryers, old fridges) before chasing small electronics.
- Shift heavy use to off-peak hours if your utility charges time-of-use rates.
- Check the nameplate wattage rather than guessing; an efficient model can use a fraction of the power for the same job.
Things to keep in mind
Nameplate wattage is usually the maximum draw, so a device that cycles on and off (like a fridge or air conditioner) will average less than the label suggests. Real bills also fold in taxes, fixed service charges and tiered or seasonal rates that a single flat per-kWh figure cannot capture. Treat this as a solid estimate for comparing appliances, not an exact prediction of your statement.
Formula
kWh/day = watts·hours/1000; cost = kWh × rateFrequently asked questions
- Where do I find an appliance’s wattage?
- Check the nameplate or label on the device, or its manual. Many list watts directly, or amps and volts you can multiply.