Transparent math
How watts become dollars
Annual kWh = watts ÷ 1,000 × hours per day × days per week × 52 × number of units.
Annual cost = annual kWh × your rate per kWh.
Worked example
A 1,500-watt space heater used three hours a day, five days a week, uses an estimated 1,170 kWh per year. At $0.17 per kWh, that is about $198.90 a year or $16.58 per average month. Reducing use to two hours on those same days lowers the straight-line estimate to 780 kWh and $132.60 per year.
Find a useful wattage
Look for a label on the appliance, power brick, underside, inside a door, or in the manual. Match the exact model number. If a label provides volts and amps but not watts, volts × amps gives a rough upper-level estimate for some devices, but motors and electronics may not behave like a simple resistive load. A plug-in energy meter can provide a more useful average for compatible appliances.
Use the rate from your bill
Your bill may separate energy, delivery, taxes, adjustments, and fixed customer charges. A listed per-kWh energy rate works for a basic comparison. An all-in variable rate can be approximated by dividing usage-based charges by billed kWh. A fixed monthly fee generally should not be assigned to one appliance because removing that appliance usually does not remove the fee.
Where estimates go wrong
- A refrigerator, air conditioner, dehumidifier, or heat pump cycles instead of drawing full power continuously.
- The product label lists maximum rather than average power.
- Standby consumption is omitted.
- Time-of-use or tiered pricing changes the effective rate.
- Weather and settings change how often equipment runs.
Safety matters more than a small saving
Do not bypass thermostats, guards, ventilation, or other safety controls to lower use. Follow manufacturer instructions. Have damaged cords, outlets, and electrical equipment evaluated by a qualified professional.