Energy Use Calculator (kWh from Watts and Time)

This calculator converts watts, quantity, hours per day, and days into electricity use in kWh. Example: a 100 W load running 24 hours for 30 days uses 72 kWh.

Energy use is not the same as circuit ampacity, feeder demand, or peak connected load. A branch circuit may have sufficient conductor ampacity and overcurrent protection for a 600 W connected load, while its monthly energy use depends entirely on how long the load operates. The calculator separates those two questions by reporting both connected power and accumulated energy.

The calculated kWh value can support:

• Utility cost estimates after a local energy rate is applied separately.
• Load-operation comparisons between lighting, plug loads, heaters, equipment, and other electrical loads.
• Energy-budget estimates for a tenant space, equipment schedule, or small project.
• Preliminary review of whether runtime reductions, controls, scheduling, or equipment replacement may reduce energy consumption.
• Documentation of expected load operation before a broader electrical load review.

It does not determine branch-circuit conductor size, AWG or kcmil conductor selection, ampacity, overcurrent protection, voltage drop, feeder demand, raceway fill, or utility billing.

Calculator Inputs

The calculation uses four operating inputs:

Input	Electrical meaning	Unit
Power per item	Nameplate or expected real power for one identical load	W
Quantity	Number of identical loads operating under the same assumptions	items
Hours per day	Average operating time per day	h/day
Days	Number of days included in the estimate	days

Power per item should represent watts per individual load, not the total for all loads. For equipment with a nameplate input in amperes rather than watts, do not assume watts from amperes without accounting for voltage, phase configuration, power factor, and actual operating characteristics.

Quantity applies when each item has the same power rating and approximately the same daily runtime. If loads have different wattages or schedules, calculate each group separately and add the resulting kWh values.

Hours per day is an average runtime. A load controlled by occupancy sensors, timers, process controls, thermostats, or duty cycling may have a lower actual average runtime than its scheduled availability.

Energy Calculation

The calculator first finds total connected power:

\(\displaystyle \text{Total connected power (W)} = \text{Power per item (W)} \times \text{Quantity (items)}\)

It then converts watts to kilowatts and applies runtime:

\(\displaystyle \text{Daily energy (kWh/day)} = \frac{\text{Total connected power (W)}}{1000} \times \text{Hours per day (h/day)}\)

\(\displaystyle \text{Total energy (kWh)} = \text{Daily energy (kWh/day)} \times \text{Days (days)}\)

The calculator also reports the average power spread across a full 24-hour day:

\(\displaystyle \text{Average power over full day (kW)} = \frac{\text{Daily energy (kWh/day)}}{24}\)

That value is not the same as the instantaneous connected load. It expresses the daily energy use as a continuous 24-hour equivalent load.

Calculation Example

Six identical 100 W loads operate for 8 hours per day over 30 days.

Field	Entered value
Power per item	100 W
Quantity	6 items
Hours per day	8 h/day
Days	30 days

The connected power is:

\(\displaystyle 100 \text{ W} \times 6 = 600 \text{ W}\)

Daily energy is:

\(\displaystyle \frac{600 \text{ W}}{1000} \times 8 \text{ h/day} = 4.8 \text{ kWh/day}\)

Total energy over 30 days is:

\(\displaystyle 4.8 \text{ kWh/day} \times 30 \text{ days} = 144 \text{ kWh}\)

The resulting outputs are:

Result	Value
Total connected power	600 W
Daily energy	4.8 kWh/day
Total energy	144 kWh
Average power over full day	0.2 kW

The 600 W connected load is present while all six items are energized. The 0.2 kW average power over full day does not mean the equipment draws only 0.2 kW while operating; it reflects 4.8 kWh spread evenly over 24 hours.

kWh, Demand, and Circuit Loading

Kilowatt-hours measure accumulated energy. Circuit sizing and electrical distribution design generally begin with current, voltage, load type, conductor ampacity, termination ratings, overcurrent protective device selection, and applicable adjustment or correction factors.

For example, a 600 W load may draw different current at different supply voltages. The kWh calculation remains based on power and runtime, but branch-circuit or feeder evaluation requires the actual electrical system characteristics. Motor loads, non-linear electronic loads, HVAC equipment, and equipment with variable duty cycles may also require a separate load analysis.

Utility bills can include more than energy use. A facility may have demand charges, time-of-use pricing, fixed customer charges, taxes, minimum charges, or other tariff components. The calculated Total energy is therefore suitable as an energy-use input before rate application, not as a final utility bill result.

Field Limits

Use actual or well-supported operating values where possible. Nameplate watts may not represent real operating input for equipment with variable output, motor loading, thermostatic control, standby modes, or electronic power supplies.

The calculation excludes:

• Demand charges and utility tariff rules.
• Duty-cycle variation and changing operating schedules.
• Power-factor billing.
• Standby or parasitic loads not included in Power per item.
• Voltage variation, voltage drop, and resulting equipment-performance changes.
• Final utility cost or billing results.

For an electrical installation decision, verify the actual branch circuit, feeder, voltage, connected equipment, conductor ampacity, termination rating, current-carrying conductors, protective device, and applicable AHJ requirements separately.