Electrical Power
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Power (in watts) is the rate at which electrical energy is used or delivered at a given instant, while energy (in watt-hours or joules) is power multiplied by the time it's sustained. A 100 W bulb left on for 10 hours uses 1,000 watt-hours (1 kWh) of energy, but its power rating never changes.

All three are equivalent and come from combining Ohm's law with the basic power formula — use whichever one matches the two quantities you already know. P = VI is simplest when you have voltage and current; use P = I²R or P = V²/R when only resistance and one other value are available.

Because power depends on the square of voltage when resistance is fixed, so doubling voltage doesn't just double the power — it multiplies it by four (2²). This squared relationship is why small overvoltages can cause disproportionately large heating in a fixed-resistance device.

Not necessarily — wattage indicates how much energy the device draws or converts, not how well it performs its task. A higher-wattage heater will warm a room faster, but a higher-wattage light bulb doing the same job as an LED just wastes more electricity as heat.

Multiply the power in kilowatts by the number of hours the device runs to get kilowatt-hours (kWh), then multiply that by your utility's price per kWh. For example, a 1,500 W space heater run for 4 hours uses 6 kWh, which might cost around $0.90 at $0.15/kWh.