The formula
watts × hours ÷ 1,000 × rate = cost. The division by 1,000 converts watt-hours to kilowatt-hours, which is what you are billed in.
A 1,500 W space heater running 4 hours a day uses 6 kWh — about $1.02 a day at $0.17/kWh, $30.60 a month, or $372 a year. A 60 W lamp running 6 hours a day uses 0.36 kWh, costing roughly $22 a year. Same house, seventeen times the cost.
Watts are not all equal
Nameplate wattage is peak draw, not average consumption. Two common traps:
- Cycling appliances. A fridge rated 150 W does not draw 150 W for 24 hours — the compressor runs perhaps a third of the time. Enter effective hours, not hours plugged in.
- Thermostatic devices. Heaters and air conditioners modulate. A 1,500 W heater on a thermostat may average well under its rating once the room is warm.
Standby power
Devices drawing 1 to 5 W around the clock look trivial and add up. A 3 W standby load runs 26 kWh a year — a few dollars each, but a typical home has dozens of them. It is usually a smaller prize than people expect, and far smaller than a single electric heater or dryer.
Where the money actually is
The ranking above sorts by annual cost, and the pattern is consistent: heat is expensive. Anything that heats or cools — heaters, air conditioning, water heating, electric ovens, tumble dryers — dominates, because converting electricity to heat takes kilowatts rather than watts. Electronics and lighting rarely move a bill much once LEDs are in.
If you are sizing solar to offset these loads, panel sizing starts from the same kWh figures — and this guide explains how monthly consumption turns into system size.