Two numbers hide behind this question, and they differ by more than an order of magnitude.
Electricity: about 1.8 cents an hour for a mainstream 100 W machine at the US national average of 17.5 cents per kWh. An hour of printing is 0.1 kWh — the same energy a 100 W lamp uses in the same hour, because it is the same 100 W.
Everything else: roughly 30 cents an hour, and that is a conservative allowance for nozzles, belts, plates, hotends and the machine's own finite life.
The same hour in different countries
| Region | Rate per kWh | One hour at 100 W |
|---|---|---|
| India | 7.5 INR | about 0.75 INR |
| United States | 17.5 cents | 1.8 cents |
| United Kingdom | 27.7 p | 2.8 p |
| Germany | 38.35 cents | 3.8 cents |
The spread across the twelve regions in the rate table is roughly four to one, so an hourly electricity figure quoted without a rate is close to meaningless. Nothing else in this calculation varies that much.
Wattage is not the printer's rating
The 100 W above is a steady-state draw with the bed already hot. It is not the number on the power supply, which is a ceiling and typically several times higher. A 350 W supply on an open-frame 256 mm machine sits near 100 W once it is printing; using the rating instead would overstate the hour by more than three times.
Big machines really do cost more per hour. A 250 W large-format printer uses 0.25 kWh in the same hour — 4.4 cents at the US rate, and 34 cents an hour once the same machine-time allowance is added.
So what is the honest hourly rate?
If you are asking because the electricity bill worries you, the answer is that a 3D printer is a minor appliance and you can stop worrying.
If you are asking because you are quoting a job, use the second number. Machine wear, not power, is what an hour of printing actually costs you, and it is the term that disappears from every "it only costs pennies" estimate on the internet.
Set your own wattage, rate and hourly allowance in the print cost calculator.