inputenergy

Input energy is the energy supplied to a system, device, or process from an external source over a given period. It encompasses all energy entering the object of interest, regardless of how efficiently that energy is used. Common units include joules (J), kilojoules (kJ), and kilowatt-hours (kWh) for larger time frames. The instantaneous rate at which energy is supplied is called input power and is measured in watts (W); integrating input power over time yields the input energy.

Input energy is a component of an energy balance. It must be considered together with the energy leaving the system (outputs), as well as losses like heat, friction, and radiation. The first law of thermodynamics formalizes this balance: the change in a system’s internal energy equals the input energy minus the work done by the system and minus losses. In electrical contexts, input energy is the integral of input power (voltage times current) over time.

Measurement and estimation. Electrical systems often use watt-hour meters or power analyzers to track input energy. Mechanical and chemical systems may rely on fuel flow meters, calorimetry, or process simulations to estimate the energy delivered to the system.

Examples. A 60-watt light bulb operating for 2 hours consumes 120 watt-hours (0.12 kWh) of input energy. In engines, the chemical energy in fuel constitutes the input energy; only part becomes useful work, with the remainder appearing as heat and other losses. In renewable and industrial contexts, managing input energy efficiently is central to reducing operating costs and environmental impact.

Applications. Input energy is a key concept in engineering design, energy management, and systems optimization, providing a basis for calculating efficiency, performance, and required energy supply for desired outputs.