Start with the quantity you actually have
A watt rating describes power: the rate of electricity use. A kWh figure describes energy accumulated over time. If a product label already provides annual kWh, multiply that figure by your electricity price per kWh. Do not multiply it by operating hours again.
For an appliance with reasonably steady power, divide watts by 1,000 and multiply by hours. A 1,200 W appliance running for 15 minutes uses 1.2 × 0.25 = 0.3 kWh. At $0.20/kWh, that event costs $0.06. The arithmetic is precise for the inputs; the real uncertainty is whether the wattage represents actual operation.
Recognize appliances that cycle
Refrigerators, ovens, dryers and thermostatic heaters can switch components on and off or vary their power. A maximum nameplate wattage multiplied by the whole elapsed time can overstate energy. For these appliances, use cycle kWh, standardized annual label kWh, or a measured average input.
For a refrigerator, collect energy over several representative days rather than reading instantaneous watts. A short reading can miss the compressor or defrost cycle. Use only monitoring equipment rated for the appliance and electrical connection; high-power or hardwired appliances may need a suitable circuit-level monitor.
Match the tariff to your decision
The variable price per kWh is useful for estimating the extra cost of running an appliance. A fixed daily account charge does not usually change when you run one more cycle. If your bill has separate variable delivery charges, include them when calculating the marginal cost.
For time-of-use pricing, calculate each operating period separately. One kWh at a peak rate and one kWh at an off-peak rate do not have the same cost. A single average rate can help with rough budgeting, but it may conceal the benefit of moving a flexible cycle.
Compare the same useful task
A lower-power appliance can run longer. Compare energy to prepare the same meal, dry a similar load, or provide the same period of comfort. An oven and air fryer also differ in batch capacity, so the number of batches matters.
Keep a short record of the input source, program, quantity and tariff used in each comparison. Replace an assumed value with a measured one when possible. The electricity cost calculator is suited to steady power; the refrigerator and laundry tools accept assumptions that better match those appliances.
Sources & methodology
- Measuring electricity — U.S. Energy Information Administration
- Energy Saver: Tips on Saving Money and Energy in Your Home — U.S. Department of Energy
- Refrigerators: Buying Guidance — ENERGY STAR