Worked example: why switched-on time is different
The default motor runs for 0.5 hours during each two-hour session. Its energy use is 1.5 kW × 0.5 hours = 0.75 kWh. At the sample tariff, that is $0.15 per day of use, or $30 over 200 use days.
| Observed motor-on share | Annual electricity | Annual energy cost |
|---|---|---|
| 10% | 60 kWh | $12 |
| 25% | 150 kWh | $30 |
| 75% | 450 kWh | $90 |
The percentages describe observed use. A compressor’s published 50% permitted duty rating is an operating limit, not proof that it actually ran half of every session. Keep that distinction when entering your data.
Two power states, one energy total
Daily kWh = session hours × [running kW × on fraction + remaining-state kW × (1 − on fraction)]Annual energy cost = daily kWh × use days × tariff per kWhIf needed: electrical input kW ≈ shaft HP × 0.746 ÷ motor efficiency fractionThe ORNL MEASUR method treats loaded and unloaded energy separately. Here you enter their electrical input power directly, or estimate the loaded input from shaft horsepower and efficiency. If the compressor truly stops and its off-state draw is negligible, zero is a reasonable scenario input; it is not a universal idle-power assumption.
For an illustration of idling, keep the default values but enter 0.3 kW during the remaining 75% of the session. Annual energy rises from 150 to 240 kWh, and the example cost rises from $30 to $48. The graph shows which state contributes that energy.
Choose inputs that answer your question
A representative measured kWh total already includes the real session’s cycling and starting energy. A nameplate running value is an estimate of one operating state. Shaft-output horsepower needs an efficiency adjustment; a measured electrical-input value does not. Dividing measured kW by efficiency again would count motor losses twice.
Use the applicable per-kWh rate from your bill, in the selected currency. The currency label changes the display only. Commercial demand charges, fixed fees and time-of-use periods need their own billing calculation; they are not folded into this energy-only total.
For a meter or logging installation, follow the equipment ratings and use qualified electrical help where required. This page does not size wiring, breakers or extension cords. Do not make temporary electrical connections to obtain a number for the form.
Use a baseline before estimating savings
Compare observations over similar work and output. A lower bill during a quiet week is not evidence that the machine became more efficient. Record operating pressure, task, loaded time and any auxiliary equipment included in the measurement.
If the motor cycles while no air is intentionally used, the pressure-decay guide can help organize an observation. Read the duty-cycle guide for the separate operating-limit question. Fixing a leak may change several power states, so its estimated CFM is not automatically a measured electricity saving.
Questions before you decide
How much electricity does a two-horsepower compressor use?
Horsepower alone is insufficient. Establish running shaft output and motor efficiency, or measure electrical input; then include actual loaded time and any unloaded draw. Do not use a peak marketing horsepower figure as a constant wall-power reading.
Can I use volts multiplied by amps as watts?
For an AC motor, that product does not by itself establish real input power. Power factor, operating state and the rating basis matter. Use a suitable real-power measurement or confirmed input-kW data.
Is the annual result a prediction of my exact utility bill?
No. It is an energy estimate using your operating schedule and tariff. Fixed charges, demand billing, taxes, maintenance and equipment outside the chosen measurement boundary are excluded.