Quick answer #

For constant input power, energy (kWh) = input power (kW) × elapsed time (h). A measured 100 kW load operating for 8 hours uses 800 kWh. At a flat energy rate of $0.12/kWh, the energy-charge screen is $96—before demand, time-of-use, reactive-energy, fixed, tax or other tariff components.

Open Energy Estimator — 100 kW · 8 h · $0.12 →

Best for: equipment or facility kWh baselines, operating-schedule scenarios, and first-pass flat-rate energy cost.

Not suitable for: utility-bill disputes, revenue-grade metering, tariff reconstruction, transformer/feeder sizing from energy alone, or proving savings without a defined baseline and measurement plan.

Industrial energy consumption formula #

For constant input power:

E = P × t

For readings or operating modes over several intervals:

E_total = Σ(P_i × Δt_i)

For a continuously varying load, the exact relationship is the time integral E = ∫P(t)dt; a meter or interval-data sum performs that integration in practice.

Where:

  • E is electrical energy in kWh.
  • P is electrical input real power in kW, averaged over the interval.
  • t is time in hours.
  • P_i and Δt_i use consistent interval boundaries.

Do not substitute motor shaft horsepower, cooling tons, compressed-air capacity or transformer kVA directly for electrical input kW. Those are different quantities and require efficiency, power factor or equipment-performance data as applicable.

Step-by-step calculation #

  1. Define the boundary: one machine, line, building, meter or whole facility.
  2. Choose a period and time basis, including shift changes, weekends, shutdowns and seasons.
  3. Obtain input kW from a suitable meter or a defensible operating-mode estimate. Nameplate data are a screening fallback, not measured consumption.
  4. Split variable operation into intervals or modes and sum P_i × Δt_i.
  5. Reconcile the subtotal with the boundary meter or utility kWh for the same timestamps.
  6. Apply the actual tariff only after separating energy, demand and other billed quantities.

Worked monthly factory example #

Assume the listed kW values are average electrical input power while operating:

Load group Input kW Schedule Monthly energy
Production 50 8 h/day × 22 days 8,800 kWh
Lighting 10 10 h/day × 30 days 3,000 kWh
HVAC 20 12 h/day × 30 days 7,200 kWh
Total — — 19,000 kWh

At a hypothetical flat energy rate of $0.12/kWh:

Energy-charge screen = 19,000 × $0.12 = $2,280/month

This does not estimate a complete bill. The three schedules may also overlap, but overlap does not change the kWh sum; it changes the time profile and may affect billed demand.

How to calculate variable-load energy correctly #

Use interval data when available #

If a meter reports average kW every 15 minutes, multiply each interval by 0.25 h and sum it. Confirm whether the export already reports interval kWh; if it does, sum kWh directly and do not multiply by time again.

Use operating modes when interval data are unavailable #

For a machine with 6 hours at 40 kW, 1 hour at 15 kW and 3 hours at 2 kW standby:

E = (40 × 6) + (15 × 1) + (2 × 3) = 261 kWh
Average power over 10 h = 261 ÷ 10 = 26.1 kW

Do not generally estimate average power as (peak + minimum) ÷ 2. That shortcut is correct only for a time-symmetric linear profile; it can be badly wrong for stepped, cyclic or skewed industrial loads.

Use duty cycle only for a defined repeated state #

If an on/off load draws P_on while on, P_off while off, and spends fraction D on:

P_avg = D × P_on + (1 − D) × P_off

The simpler P_avg = rated power × duty cycle assumes zero off-state power and that rated power equals actual on-state input power. State those assumptions.

Nameplate, measured power and meter data #

Evidence Appropriate use Main limitation
Revenue/submeter kWh Baseline and reconciliation Boundary, timestamp, multiplier and data-quality checks still matter
Interval kW/kWh Load profile and mode analysis Confirm interval length, units and missing-data treatment
Portable power analyzer Equipment study Installation, CT orientation/range, safety and sampling period
Electrical input kW from OEM curve Scenario estimate Must match load point, voltage, controls and conditions
Nameplate output/rating Inventory screen Not automatically actual electrical input or average power

For a three-phase circuit, use an appropriate three-phase power meter when accuracy matters. A current-only spot reading plus an assumed voltage and power factor can miss unbalance, harmonics and operating variation.

Power factor: what changes and what does not #

A kWh meter integrates real power, so do not multiply measured kW by power factor again. For the same delivered real-power load, lower PF increases current and kVA and can increase upstream losses; a utility may also bill kVA demand, reactive energy or a PF adjustment. PF therefore can affect system losses and the bill even though the core measured-energy equation remains kWh = ∫kW dt.

Use the Power Factor Guide and the applicable utility tariff before adding any PF charge.

Energy, demand and the electricity bill #

Energy and demand are not interchangeable:

  • kWh accumulates real energy over time.
  • kW demand is computed over the utility's defined demand interval and tariff method, not necessarily an instantaneous peak.
  • kVA or reactive billing depends on the tariff and metering quantities.

A general bill framework is:

Bill = Σ(interval kWh × applicable energy rate)
     + billed demand quantity × demand rate
     + tariff-specific adjustments, fixed charges and taxes

The Energy Estimator models constant average kW, hours/day, days/month and one flat $/kWh rate. It does not model demand intervals, ratchets, time-of-use periods, tiered rates, PF/reactive charges, taxes or production-dependent load variation.

Monthly and annual projections #

Annual energy = representative monthly energy × 12 is valid only when that month represents every month. Otherwise calculate each month or season separately:

E_year = Σ(E_month,j)

Record production volume, weather, operating days, outages and boundary changes. When comparing years or claiming improvement, use an energy baseline and an Energy Performance Indicator (EnPI) that accounts for relevant variables; total kWh alone may simply track more or less production.

Useful industrial energy metrics #

Metric Formula Use
Energy intensity kWh ÷ production units Compare energy per unit when product mix is sufficiently comparable
Specific energy consumption kWh ÷ tonne, batch, m³, etc. Process-focused normalization
Electrical load factor average kW ÷ peak/billed kW for the same period/basis Describe profile utilization; not equipment efficiency
Model variance measured kWh − modeled kWh Find missing loads, schedules or bad assumptions

Keep metric boundaries and denominators consistent. A lower kWh/unit can coexist with higher total kWh when production rises.

Reconciliation checklist #

  • Match model and meter start/end timestamps and timezone.
  • Confirm meter multipliers, CT/PT ratios, units and import/export sign.
  • Include standby, weekends, auxiliaries, HVAC, compressed air and shared services inside the boundary.
  • Flag missing or estimated intervals instead of silently replacing them.
  • Compare modeled and measured kWh, investigate the largest residuals, then update documented assumptions.
  • Keep the original baseline when reporting savings; do not rewrite it after seeing the result without an approved adjustment method.

Frequently asked questions #

Is kW the same as kWh? #

No. kW is the rate of real-energy use; kWh is energy accumulated over time. A constant 20 kW load for 3 hours uses 60 kWh.

How do I calculate power consumption? #

If energy and elapsed time cover the same interval, average real power is P_avg = E ÷ t; for example, 60 kWh over 3 hours averages 20 kW. For instantaneous or equipment input power, use a suitable power meter or the correct single-/three-phase real-power relationship with measured voltage, current and true PF—do not divide a monthly kWh total by only the machine's scheduled hours unless the meter boundary matches that machine.

Can I calculate industrial kWh from a motor's horsepower? #

Not directly. Horsepower normally describes mechanical output rating. Electrical input depends on actual shaft load, motor efficiency, controls and operating mode. Use measured electrical input kW or applicable performance data.

How should I estimate mixed operating modes? #

Multiply each mode's average input kW by its duration and sum the interval energies. Include standby/off-state power when it is nonzero.

Does this calculation reproduce my electricity bill? #

Only for a simple flat energy-charge screen. A real tariff may add demand, time-of-use, tier, reactive/PF, fixed, tax and other rules.

Technical sources #

Next step #

  1. Run a constant-load scenario in the Energy Estimator.
  2. Compare it with the same-period meter total and document the variance.
  3. Use Load Factor for a defined average/peak period, then review the industrial energy cost guide for tariff components.
  4. Browse the Power Calculator hub for related workflows.