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Electrical calculator

Energy Consumption Calculator

Estimate energy use in kWh from device watts and hours of operation. Useful for appliance and load planning. Runs locally in your browser. Try it free.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · core energy calculation (W × h / 1000 → kWh)
Input interpretation
Enter values to calculate.
Result
—
Verified scope
core energy calculation (W × h / 1000 → kWh)
Assurance
Engineering
Declared partition coverage
PASS · 3/3 declared partitions (nominal-positive, zero, invalid-domain) · Matrix
Deferred
UI daily/month/year projections (×30 / ×365)
Known limitations
  • Core CVP does not include live graph, viewport, or pointer interaction.
Model
Electrical energy in kWh from power (W) and time (h), either for one period or as daily use with month/year estimates.
Scope
CVP verifies E(kWh)=P(W)×t(h)/1000. Daily month/year figures are UI projections (×30 / ×365), not engine outputs.
Verification
Engine tested · Source checked · v1.4.2 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance · core energy calculation (W × h / 1000 → kWh)· View Manifest · CVP overview · Specification
Versions
Calculation 1.4.2 · CVP protocol 1.0.0-proposed · Evidence 2026-09-13.negative-input-contract
Verification revision
2026-09-13.negative-input-contract · 4/4 property · digest 9f36849cd3cd
Legacy regression
16/16 tests · Production surface contract 4/4
Trust layers
Verification VERIFIED · Production CURRENT · overall VERIFIED
Reference
O1 model · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session.
Supplemental domain review
Internal · Pass · electrical-engineer
Named expert review
Not performed
CVP suite
2/2 golden · 3/3 CVP boundary · 5/5 invalid · 4/4 property · 2/2 cross-interface · 4/4 CVP contract · Manifest
Sources
Sources
Evidence
9 legacy golden · 3 legacy boundary · legacy regression suite · 2/2 oracle-backed golden · 5/5 invalid · Artifact integrity PASS · CalculatorX electrical review
This calculator CURRENT · Public schema 1.4.2 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

EnergyE(kWh) = P(W) × t(h) ÷ 1000
DailyE(kWh/day) = P(W) × t(h/day) ÷ 1000
Month / year≈ E/day × 30 · E/day × 365
Power ↔ energyP(kW) = E(kWh) ÷ t(h) · P(W) = 1000 × E(kWh) ÷ t(h)
i1 kWh = 1000 W for 1 hour = one “unit” on many bills. Nameplate watts are often maximum; refrigerators and HVAC cycle, so real energy is usually lower than nameplate × clock hours. Month/year use ×30 / ×365 as simple UI averages — leap years and seasonal use differ. Those projections are presentation-only; CVP verifies the core E = P×t/1000 result.

How to use

1

Choose One period or Daily use

One period = total hours in that window. Daily use = hours per day, with approximate month/year projections.

2

Enter power in watts

Use the label rating or a meter. Convert kW → W by ×1000.

3

Enter time

Read kWh (and for daily mode, approximate month/year totals labeled as UI projections).

Example calculations

Common configurations with formula and result.

ϟ

Space heater, one evening

1500 W · 3 h

1500 × 3 ÷ 1000
4.5 kWh
ϟ

LED, daily

9 W · 8 h/day

9 × 8 ÷ 1000
0.072 kWh/day · ≈ 2.2 kWh/mo
ϟ

Laptop workday

60 W · 5 h/day

daily mode
0.3 kWh/day · ≈ 9 kWh/mo · ≈ 110 kWh/yr
ϟ

From kW rating

1.5 kW · 2 h

1500 W × 2 ÷ 1000
3 kWh

Example daily use → monthly kWh

Common values at a glance.

LoadPowerh/daykWh/day≈ kWh/month
LED bulb9 W80.0722.2
Laptop60 W50.3009.0
Ceiling fan75 W100.75022.5
Microwave*1200 W0.250.3009.0
Portable heater*1500 W34.500135
Window AC*1500 W812.000360
i *Highly duty-cycle dependent. Short bursts (microwave) still add up; heaters and AC dominate many bills.

Energy Consumption calculator specification

Version 1.4.2 · Engine tested · Supplemental domain review · Internal · 2026-09-13

Calculation status

Review policy · Evidence · Reviewed by CalculatorX electrical review (electrical-engineer)

Definition
Energy consumption is how much electrical energy a load uses over time. Power (watts) is the rate; energy (kWh) is power × time. This calculator converts watts and hours into kilowatt-hours for a single period or for average daily use with month/year estimates.
What it calculates
Electrical energy in kWh from power (W) and time (h), either for one period or as daily use with month/year estimates.
Inputs
  • Power P in watts
  • Time t in hours (total or hours per day)
Outputs
  • Energy in kWh (and Wh in explainer for one period)
  • For daily mode: kWh/day (engine), plus approximate UI month/year projections
Formula
E = P×t/1000
Assumptions
  • Constant power over the stated hours.
  • Month = 30 days, year = 365 days in daily UI projections (presentation-only).
  • User converts kW→W when needed.
  • W ≥ 0 and hours ≥ 0 (consumption, not signed energy flow).
Units
  • W, h → kWh
Boundary conditions
  • Zero watts or zero hours → 0 kWh — soft boundary
  • Negative W or hours → VALUE_MUST_BE_NON_NEGATIVE
  • UI daily projections (month/year, ×30 / ×365) are presentation-only; engine returns period kWh
  • Daily UI: 0 ≤ hours/day ≤ 24 for a single load; period mode accepts any hours ≥ 0
Example
1500 W × 3 h = 4.5 kWh
Validation cases

12 published on this page · 16/16 tests · Production surface contract 4/4 · View evidence

  • 1500 W, 3 h (period) → 4.5 kWh
  • 9 W, 8 h → 0.072 kWh
  • 1000 W, 1 h → 1 kWh
  • 60 W, 5 h → 0.3 kWh
  • 3000 W, 3 h (2× of 1500) → 9 kWh (linear in W)
  • 1500 W, 6 h (2× hours) → 9 kWh (linear in hours)
  • 0 W, 3 h → 0 kWh
  • 1500 W, 0 h → 0 kWh
  • 2000 W, 24 h → 48 kWh
  • −500 W, 2 h → error VALUE_MUST_BE_NON_NEGATIVE
  • 500 W, −2 h → error VALUE_MUST_BE_NON_NEGATIVE
  • 500 W, 2 h → 1 kWh
Sources
Last reviewed
2026-09-13
Reviewed by
CalculatorX electrical review (electrical-engineer)
Calculation version
1.4.2

Background

Interpretation and common distinctions.

Power vs energy

Quantity Unit Meaning
Power W, kW How fast energy is used right now
Energy Wh, kWh How much was used over a period

Relationship:

P(kW) = (E(kWh))/(t(h)), P(W) = 1000 × (E(kWh))/(t(h))

Electricity meters and bills track energy in kWh. This tool turns appliance watts and hours into that number.

Units worth knowing

Unit Relation
1 kWh 1000 W for 1 h = 3.6 MJ = one bill “unit” in many places
1 Wh 0.001 kWh
1 kW 1000 W
BTU ≈ 0.293 Wh (energy); BTU/h is a power rate
ton (refrigeration) ≈ 3517 W cooling capacity (≈ 12,000 BTU/h)
metric hp ≈ 735.5 W

AC labels in “12,000 BTU” usually mean BTU/h cooling rate, not electrical input. Electrical watts depend on efficiency (EER/SEER/COP).

Typical appliance wattages (ranges)

Typical illustrative ranges; not part of the verified calculation model. Nameplate values vary widely; treat as order-of-magnitude only.

Appliance Typical W
LED bulb 3–25
Incandescent bulb 15–200
Laptop 35–150
Desktop PC 100–250
TV 25–500
Refrigerator 100–1000 (peak)
Microwave 750–1500
Dishwasher 1200–2000
Clothes washer 400–1500
Clothes dryer 1800–5000
Portable heater 750–2000
Window AC 1500–5000
Central HVAC 2500–10,000+
EV charger 1500–20,000

Insight: Monthly kWh is dominated by (1) high watts and (2) long hours. A 9 W LED left on is cheap; a 1500 W heater for a few hours often is not.

Practical tips

  • Sum daily kWh of major loads to sanity-check your meter.
  • Cut hours on big loads before obsessing over tiny standby watts — both matter, but hours × kW wins.
  • Swap incandescent → LED for an easy, permanent drop.
  • For cost: cost = kWh × $/kWh (+ standing charges on the full bill).
  • Seasonal heating/cooling: compute summer and winter days separately, then average.

Supported and not supported

Supported — E(kWh)=W×hours/1000 · API electrical.energy_consumption

Not supported — Efficiency, standby cycles, TOU costing (use Energy Cost / Electricity Bill)

Agent / API notes

Capability id: electrical.energy_consumption · tool id: energy-consumption · pin 1.4.2.

Engine contract: W ≥ 0, hours ≥ 0 (JSON Schema minimum: 0). Daily month/year figures are UI projections, not engine outputs. Daily UI also requires hours/day ≤ 24 for a single load.

Stable errors: MISSING_REQUIRED_INPUT, INVALID_NUMBER, VALUE_MUST_BE_NON_NEGATIVE.

Other calculators in this family: Amps to kW Calculator, Amps to VA Calculator, Amps to Volts Calculator, Electric Power Calculator, Energy Cost Calculator, eV to Volts Calculator, kVA to Amps Calculator, kVA to kW Calculator . Explore all Power & Energy.

→ Amps to kW Calculator Convert amps and volts to real power in kW for DC, AC single-phase, and balanced AC three-phase line-to-line. Runs locally in your browser. Free online tool — no sign-up. › → Amps to VA Calculator Convert amps and volts to apparent power in VA or kVA for single-phase and three-phase circuits. Also convert VA back to amps. Fast and free. › → Amps to Volts Calculator Solve voltage from current and resistance or power using Ohm’s law and P = V × I. Supports resistive DC and RMS AC estimates with clear formulas. › → Electric Power Calculator Calculate electrical power in watts from voltage and current for DC, single-phase AC, or balanced three-phase AC, from current and resistance, or from energy over time. › → Energy Cost Calculator Estimate electricity usage cost from watts/hours or kWh and your unit rate. Supports custom billing periods; standing charges and taxes are excluded. Runs locally in your browser. › → eV to Volts Calculator Relate electronvolt energy to electric potential difference through ΔE = qΔV. eV and volts measure different physical quantities. Runs locally in your browser. › → kVA to Amps Calculator Convert kVA and voltage to line current for single-phase and three-phase systems. Uses √3 for three-phase line-to-line volts. Runs locally in your browser. › → kVA to kW Calculator Convert apparent power (kVA) to real power (kW) with power factor. States when PF is required and when loads are assumed balanced. Try it free. ›
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Frequently asked questions

Key distinctions behind the calculation.

What is the difference between watts and kWh?

Watts (W) measure power — how fast energy is used. Kilowatt-hours (kWh) measure energy — how much was used over time. Analogy: watts are like speed; kWh are like distance traveled.

How do I calculate energy consumption?

E(kWh) = P(W) × t(h) ÷ 1000. Example: 1500 W for 3 hours → 4.5 kWh.

How many kWh per day / month / year?

Compute kWh/day with hours per day. The page then shows approximate UI projections (×30 for a 30-day month, ×365 for a year). Those month/year figures are not part of the verified engine output. Seasonal loads (heating/cooling) need separate summer/winter averages.

Should I use nameplate watts?

Nameplate is usually maximum. Fridges, ACs, and washers cycle or vary. For estimates, use average power if you have a meter; otherwise treat nameplate × hours as an upper bound.

What if my appliance is rated in kW or BTU/h?

1 kW = 1000 W. Roughly 1 ton of refrigeration ≈ 3517 W; 12,000 BTU/h ≈ 3517 W cooling capacity (electrical input may differ by efficiency).

How do I get cost from kWh?

Multiply kWh by your $/kWh rate, then add standing charges if needed — use Energy Cost or the Electricity bill calculator.

Why is household energy billed in kWh not Wh?

Scale. A home can use thousands of kWh per year; kWh keeps the numbers readable. 1 kWh = 1000 Wh = 3.6 MJ.