Power is energy used per unit of time, so a 2,000 watt appliance running for three hours uses 6 kilowatt-hours of energy. Power, energy, and time are the same three-way relationship behind "what does it cost to run this heater" and "how long will my EV take to charge" - know any two, get the third, plus the real electricity cost.
Real charging isn't 100% efficient - some energy is lost as heat converting AC power from the wall into the DC the battery actually stores. Home AC charging typically runs 85-94% efficient, so a 7kW charger delivering to an 88%-efficient system is really only putting about 6.16kW into the battery. Skip that adjustment and every EV charge-time estimate comes out optimistic. The checkbox above is off by default since most power/energy/time questions - a heater, a kettle, a power tool - don't involve this loss at all; it's specifically for charging scenarios.
The electricity rate defaults to the 2026 US national average of about 18 cents per kWh, but real rates range from roughly 12 cents in North Dakota to over 40 cents in Hawaii - swap in your own rate from a recent bill for an accurate cost, not just a ballpark.
The "boiling water" helper above works out the real energy needed to heat a given amount of water from a given starting temperature to boiling (100°C), using the same physics as heating anything else: energy = mass × specific heat × temperature rise. It fills in the Energy field for you, so the usual power and efficiency inputs below still apply - a kettle typically runs 80-90% efficient, an open stovetop burner can be as low as 50-70%, which is exactly why a stovetop often feels slower than a kettle at similar wattage.
Multiply its power in kilowatts by the hours used to get kilowatt-hours, then multiply by your electricity rate. A 2 kW heater run for 3 hours uses 6 kWh.
A kilowatt is a rate of energy use; a kilowatt-hour is an amount of energy. A 1 kW appliance running for one hour uses 1 kWh.
Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.