Electricity is the term people either leave out entirely or overstate by a factor of ten, and both mistakes come from the same place: using the wrong wattage number. This is the method in full, with the traps named.
The arithmetic
Energy in kilowatt-hours is average power in watts, multiplied by hours, divided by one thousand. Cost is that figure multiplied by your price per kilowatt-hour.
A machine averaging 100 W over a 12-hour print uses 1.2 kWh. At a United States average residential price of about 17.5 cents, that is 21 cents. At a German average of about 41 US cents it is 49 cents. At an Indian domestic rate of around 9 US cents it is 11 cents. Same print, same machine, more than four times the spread — which is the entire reason the rate has to be an input rather than a constant.
Trap one: the power-supply rating
The single most common error is taking the printer's power-supply rating as its consumption. A machine with a 350 W supply does not draw 350 W while printing; it draws whatever the heaters and motors need at that moment, and once the bed and hotend are at temperature that is usually somewhere between 60 and 130 W on a mid-sized machine.
The gap is not small. Published figures in the region of a kilowatt circulate for machines that meter at around a hundred watts, and estimates built on them make printing look ten times more expensive than it is. Every printer page on this site records three separate numbers — idle, steady-state printing, and heat-up peak — and labels each as metered, manufacturer-rated or estimated from machines of the same class, precisely so this substitution cannot happen silently.
Trap two: averaging in the heat-up peak
A bed coming up to temperature can pull several hundred watts, and on some enclosed machines the first thirty to sixty seconds are close to a kilowatt. That is real, and it is also brief.
On a twelve-hour print, a one-minute 850 W transient adds about 0.014 kWh — a quarter of a cent at the United States average, and under six tenths of a cent even at the highest rate in our table. On a fifteen-minute print it is a meaningful fraction of the total. So the honest treatment is to use the steady-state figure for the bulk of the print and only worry about the peak when the print is short, or when you are running dozens of short jobs a day.
What actually dominates: bed area
Compare the profiles on this site and the pattern is unmistakable. The strongest predictor of how much electricity a print consumes is not the hotend, the motion system, the print speed or the material. It is the area of heated bed being held at temperature, and whether anything is enclosing it.
A 180 mm bed is 324 cm². A 420 mm bed is 1,764 cm² — more than five times the area, radiating and convecting to the room continuously for the whole print. That is why a small enclosed machine can be cheaper to run than a large open one despite the enclosure, and why bed temperature is the setting with the largest energy consequence on the whole profile.
An actively heated chamber adds a second continuous load on top. A passive enclosure does not; it only slows the loss from the bed you were already heating, which makes it energy-positive rather than negative.
Choosing your rate
Use your own bill. Failing that, the regional rate table gives published national averages with the date they were compiled.
Two adjustments are worth making if they apply to you. If you are on a time-of-use tariff, an overnight print runs at the off-peak rate, which in several markets is substantially below the headline figure. If you have household solar and print during the day, the correct number is your export rate — what the electricity would have earned you if you had not used it — rather than your import price, and those can differ by a factor of three.
Do not include the daily standing charge. You pay it whether or not you print, so it is not a cost of the print.
Putting it together
The method, end to end: take the steady-state printing wattage from your machine's profile, multiply by the print's hours, divide by a thousand, multiply by your own rate per kilowatt-hour, and add a rounded allowance if the print is short enough for the heat-up transient to matter.
Then check it against reality once. A cheap plug-in energy meter costs less than a spool of filament, and half a day of measurement will tell you your machine's real average for the materials you actually print. After that you are not estimating any more, and every cost figure you produce is grounded rather than inherited.
Worked example, end to end
Take a 12-hour print on a 256 mm open-frame machine metering around 100 W in steady state, with a one-minute heat-up peak of 350 W.
Steady state: 100 W for 12 hours is 1.2 kWh. Heat-up: 350 W for one minute is 0.006 kWh, which rounds away. Total 1.21 kWh.
At the United States average of 17.5 cents, that is 21 cents. If the same job runs on a 350 mm enclosed machine at 220 W it becomes 2.64 kWh and 46 cents — more than double, for the same part, purely because of bed area and chamber load.
Against that, 200 g of PLA at twenty dollars a kilogram is four dollars of filament. So on this job electricity is between five and twelve percent of the material cost. That is the honest proportion for a typical hobby print, and it is worth knowing before deciding whether to chase it.
What to do with the answer
If electricity turns out to be a few percent of your total, stop optimising it and go and look at your failure rate instead, which is almost certainly larger. If it turns out to be a quarter of your total — a large bed, a hot chamber, a long print, an expensive market — then bed temperature, print time and machine choice are all genuinely worth revisiting, in that order.
The point of computing the term is not to reduce it. It is to find out whether reducing it is worth your afternoon.