Electrical Power Explained
The Power Wheel
The power wheel (or power triangle) shows all relationships between voltage (V), current (I), resistance (R), and power (P):
- P = V × I — Power from voltage and current
- P = I² × R — Power from current and resistance
- P = V² / R — Power from voltage and resistance
- V = I × R — Voltage from current and resistance
- I = V / R — Current from voltage and resistance
AC vs DC Power
DC power is straightforward: P = V × I. What you measure is what you get.
AC power has three components:
- Real power (W): The actual power consumed and converted to useful work
- Reactive power (VAR): Power stored and released by inductors and capacitors
- Apparent power (VA): The total power supplied, combining real and reactive
For purely resistive AC loads (heaters, incandescent bulbs), P = V × I works. For motors and other inductive loads, multiply by the power factor.
Watts vs Kilowatts
1 kilowatt (kW) = 1,000 watts. Electrical bills use kilowatt-hours (kWh), which measure energy consumed over time. A 1,000W device running for 1 hour uses 1 kWh. At $0.12/kWh, that costs about 12 cents.
Common Power Ratings
- LED bulb: 8-15 W
- Laptop: 45-100 W
- Microwave: 600-1,200 W
- Space heater: 1,500 W
- Electric dryer: 4,000-5,000 W
Electrical Power in Numbers
EIA 2023 data shows the average U.S. household consumes 10,500 kWh/year of electricity — roughly 878 kWh/month or 29 kWh/day. At the U.S. average residential rate of 16.21 cents/kWh (EIA November 2023), that's $142/month or $1,701/year. Peak demand runs 3-8 kW for typical homes, which is why service panels are commonly rated 100-200 amps (at 240 V, that's 24-48 kW capacity).
Appliance power draw spans six orders of magnitude. Phone charger (2-5 W), LED bulb (9-15 W), refrigerator (100-400 W), microwave (600-1,200 W), hair dryer (1,500-1,800 W), electric range burner (2,000-3,000 W), water heater (4,500-5,500 W), EV Level 2 charger (7,200-11,500 W). The National Electrical Code 2023 requires 80% continuous-load de-rating — a 30 A circuit supplies only 24 A of continuous load.
Power factor matters for industrial users but rarely for residential. Inductive loads (motors, transformers) with power factor 0.7 require 43% more apparent power (VA) than real power (W) for the same work — factories pay penalties for PF < 0.95 under many utility tariffs. A 100 HP motor (74.6 kW real power) at PF 0.8 draws 93.3 kVA, costing about 17% more per month if the utility charges kVA-demand.
Sources: EIA Residential Energy Consumption Survey, NEC 2023, IEEE Std 141
Methodology & Assumptions
This calculator implements standard formulas drawn from primary-source authorities. Values are point-in-time estimates; consult a licensed professional for high-stakes decisions. See the per-input definitions and source citations below.
How this works
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updated rates or thresholds we refresh defaults and update the page's
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| Input | Default | Source / authority |
|---|---|---|
| All inputs | Domain-typical defaults | Editorial methodology, CalcMesh 2026 |