How to Use This Tool
Pick an appliance for sensible defaults, then correct the watts from your own label. The duty cycle is the input that most calculators leave out entirely.
Duty cycle: the reason the simple sum is wrong
Anything with a thermostat cycles. A fridge compressor runs, reaches temperature, stops, and starts again when the cabinet warms. Over a day it might be running 30 to 40 per cent of the time. The nameplate watts are what it draws while running, not an average.
Appliances without a thermostat — a kettle, a TV, a router, an LED lamp — draw close to their rating the whole time they are on, so their duty cycle is effectively 100 per cent. For those the naive sum is right, which is part of why the mistake survives.
naive 150 W x 24 h x 30 d / 1000 = 108.0 kWh -> $18.36 real 108.0 x 35% duty cycle = 37.8 kWh -> $6.43 annual 1.26 kWh/day x 365 = 459.9 kWh -> $78.18 a 2 W standby, if it were ever off = 17.5 kWh -> $2.98 a year
Where to find the real numbers
Three sources, in order of reliability. The energy label gives an annual kWh figure measured under a standard test, which is the best single number available. The nameplate gives running watts. A plug-in energy monitor left for a week gives your actual duty cycle in your kitchen, which is better than any table.
If the label lists both starting and running watts for a motor, use the running figure. Starting watts can be three times higher but only last a second or two, so they matter for sizing a generator rather than for a bill.
Standby is small per device and not small per house
Two watts of standby is 17.5 kWh a year. On its own, about three dollars. The point is that standby runs 8,760 hours a year with no exceptions, so twenty devices at 2 W is 350 kWh — comparable to running the fridge for nine months.
Tariffs are not one number
Many bills have a fixed daily charge, tiered rates above a threshold, and time-of-use pricing. This calculator uses a single marginal rate, which is the right basis for the question "what does this appliance add to my bill". If you are on a time-of-use tariff, put in the rate for the period the appliance actually runs in.
What this is not
This estimates energy cost, not whether replacing an appliance pays for itself. That needs the new unit's annual kWh, the purchase price and the remaining life of the old one.
