What Is Electrical Power?

Electrical power is the rate at which energy moves through a circuit, measured in watts (W). One watt equals one joule of energy transferred per second. Every appliance, motor and light bulb in a home or facility is rated in watts because that number tells you how much energy it draws while running, which is the foundation of any power consumption calculator or load calculation.

In practical terms, power is what determines your electric bill, what size wire or breaker a circuit needs, and what capacity a generator, inverter, or solar array must have to run a given load.

Power Formula Explained

The three core equations behind this electrical power calculator are:

Each rearranges Ohm's Law (V = I × R) into a power equation. For example, a 120V circuit drawing 10A delivers 120 × 10 = 1200 watts. A 10-ohm heating element carrying 5A dissipates 5² × 10 = 250 watts. And a 120V element with 14.4 ohms of resistance draws 120² ÷ 14.4 = 1000 watts.

AC vs DC Power

Direct current (DC) flows in one constant direction, as in batteries, solar panels and USB electronics, so power is simply P = V × I. Alternating current (AC), used in U.S. homes at 120V/240V, reverses direction 60 times per second and introduces a power factor (PF) for inductive loads like motors, so real power becomes P = V × I × PF. This is why an AC power calculator and a DC power calculator use slightly different formulas.

Single Phase vs Three Phase

Most U.S. homes run on single-phase power at 120V/240V, which is simpler and sufficient for household loads. Commercial and industrial facilities typically use three-phase power at 208V or 480V, which delivers power more efficiently and smoothly for large motors and heavy machinery using the formula P = √3 × V × I × PF.

How to Calculate Power Consumption

To estimate energy use, multiply a device's wattage by the hours it runs, then divide by 1000 to get kilowatt-hours (kWh) — the unit your utility bills you for. Example: a 1500W space heater run for 6 hours uses (1500 × 6) ÷ 1000 = 9 kWh per day.

Generator Power Calculation

Generator sizing starts with adding the running watts of everything you'll use simultaneously, then adding the single largest surge (starting) watt requirement on top, since motors and compressors can briefly draw 2-3x their running wattage at startup. Always size the generator above this total, and account for the generator's own efficiency rating.

Solar Power Calculation

Estimate daily solar output with: panel wattage × average peak sun hours × system efficiency. A 400W panel with 5 peak sun hours and 80% system efficiency produces roughly 400 × 5 × 0.8 = 1600 watt-hours (1.6 kWh) per day.

Battery Power Calculation

Battery runtime is calculated as (battery capacity in amp-hours × battery voltage) ÷ load in watts. A 100Ah, 12V battery running a 100W load lasts roughly (100 × 12) ÷ 100 = 12 hours before full discharge (ignoring depth-of-discharge limits).

Appliance Power Usage

Typical running wattages for common household appliances, useful for load planning, generator sizing and solar system design:

Example Calculations