Operating Energy & Cost

Estimate annual energy and tariff cost from operating hours and active/standby electrical power.

How this calculation works

This annual estimate separates electrical use during active operation from standby use during the rest of each operating day. Enter actual electrical input powers, active hours per day, operating days per year and a single energy tariff. Standby is applied to 24 minus the active hours on those operating days only; every unlisted day is assumed off. Multiplying each power by its annual hours gives the active and standby energy totals in kWh. Their sum times the tariff gives energy-only cost in the currency used for that tariff. The schedule and rates are held constant and a 365-day planning year is used. Demand charges, fixed charges, taxes, tiered prices and time-of-use rates are excluded, so the result is not a complete bill forecast.

Inputs and units

Electrical power while active (kW)
Electrical power while on standby (kW)
Active hours per operating day (h/day)
Operating days per year (days/year): Standby applies only to the rest of each operating day; other days are assumed off
Energy-only electricity tariff (currency units/kWh): Use one consistent currency throughout

Method and formula

Eactive = Pactive × active hours/day × days/year; Estandby = Pstandby × (24 − active hours/day) × days/year; cost = (Eactive + Estandby) × tariff.

Worked example

Example inputs

Electrical power while active: 10 kW
Electrical power while on standby: 0.1 kW
Active hours per operating day: 8 h/day
Operating days per year: 250 days/year
Energy-only electricity tariff: 0.15 currency units/kWh

Calculation steps

Calculate annual active energy: 10 × 8 × 250 = 20000 kWh/year.
Calculate standby hours on each operating day: 24 − 8 = 16 h/day.
Calculate annual standby energy: 0.1 × 16 × 250 = 400 kWh/year.
Add both modes: total energy = 20000 + 400 = 20400 kWh/year.
Apply the energy tariff: 20400 × 0.15 = 3060 currency units/year.

Example results

Annual active energy: 20000 kWh/year
Annual standby energy: 400 kWh/year
Annual total energy: 20400 kWh/year
Annual energy-only cost: 3060 currency units/year

Assumptions

Entered powers are actual electrical input, not rated useful output
Unlisted days are off; tariff and daily schedule remain constant

Limitations

Excludes demand charges, fixed fees, taxes, tiering and time-of-use tariffs
A 365-day planning year is used; actual bills may differ

Sources

U.S. DOE: Energy Efficiency at Home: Science Activities p. 11, energy equals electrical power × operating time