This glossary defines the terms shared across the whole Energy Systems discipline and, more importantly, points each one to the article that explains it in full. The definitions here are deliberately short — enough to get the concept and, where it matters, to avoid confusing two things that look alike. When a term controls a calculation, an equipment choice, an operating action, or a safety boundary, follow its link to the canonical article rather than acting on the one-line version.

Two companion glossaries go deeper on their domains: battery storage terms and solar and charging terms.

Quantities and units

  • Energy — the amount of work or stored capacity, measured in joules (J) or watt-hours (Wh); 1 Wh = 3,600 J. This is "how much."
  • Power — the rate of energy transfer, measured in watts (W); 1 W = 1 J/s. This is "how fast." → energy vs. power. Do not confuse energy with power — a watt is not a watt-hour.
  • Voltage (V) — electric potential difference, energy per unit charge.
  • Current (A) — the rate of electric-charge flow.
  • Ampere-hour (Ah) — a quantity of charge, not energy. Converting Ah to Wh needs a voltage and a valid boundary: Wh = Ah × V. → watt-hours vs. amp-hours. Do not confuse charge (Ah) with energy (Wh).

Electrical form and conversion

  • DC (direct current) — charge flows in one direction; PV panels and batteries are DC.
  • AC (alternating current) — the direction reverses periodically; utility and shore supply are AC. → AC vs. DC.
  • Inverter / charger / converter — devices that move energy between forms or voltage levels: an inverter makes AC from DC, a charger makes DC from AC, a DC-DC converter shifts DC voltage. → how inverters work.

System functions

Every small system is built from the same functions, even when one product fills several of them:

  • Load — what consumes energy. Source — what supplies it. Storage — what holds it (usually a battery). Conversion — DC↔AC and voltage changes. Control — regulating charge and flow. Distribution — carrying and splitting power. Protection — interrupting faults and enabling isolation. Monitoring — showing what is happening. → understanding small energy systems.

A product's label does not erase these functions; you still trace each energy and fault path separately.

Ratings and conditions

Nominal, rated, continuous, peak, surge, input, output, and usable are qualifiers, not synonyms. A rating means little without its side (input or output), duration, test conditions, and environmental conditions.

  • Continuous vs. peak vs. surge — a long-term load, the largest simultaneous load, and a brief startup spike are three different numbers. → continuous, peak, and surge.
  • Nominal vs. usable — a rating under declared conditions is not the portion you can actually use. → usable vs. nominal capacity.

Reading these off a specification correctly is its own skill — how to read an energy-system data sheet.

Performance and loss

  • Efficiency — useful output ÷ input, under stated conditions.
  • Loss — the portion not delivered to the chosen useful output.
  • Derating — reducing a rated or assumed value for real conditions or uncertainty.

All three need a declared boundary and operating point. One efficiency figure does not apply across every load, temperature, direction, or whole-system boundary.

Measurements, calculations, and estimates

Not every number carries the same weight:

  • Measured — a direct sensor reading, with its own accuracy.
  • Derived / calculated — arithmetic from measurements; it inherits their error.
  • Estimated / modeled — a model output with assumptions and, sometimes, memory.
  • Assumed — a value you chose; label it as such.

Extra decimal places never turn an estimate into a measurement, and no reading proves physical condition. → what a battery monitor measures.

Boundaries and comparisons

A system boundary states which components, energy paths, direction, interval, and losses a value includes. "Battery-side," "AC output," "load energy," "PV nameplate," and "utility meter" can all be correct while describing different scopes. Two numbers are only comparable when they share a boundary and interval — tag every calculation and comparison with its boundary.

Protection and safe state

Protection, isolation, disconnect, alarm, BMS, fuse, circuit breaker, and RCD/GFCI (earth-leakage protection) name different functions. Their meanings can overlap in products and codes, but they are not interchangeable, and no single one proves the system is fully protected or safe to touch. A glossary entry can name a function; it cannot certify safety. → why protection is part of system design and safety fundamentals.

A definition is not an instruction. Where a term affects conductor or protection sizing, isolation, grounding, exposed work, an abnormal battery, or code interpretation, the short definition here is a starting point only — follow the canonical article and your equipment's manual.

Key takeaways

  • Definitions here are short on purpose; follow the link when a term drives a calculation, purchase, action, or safety boundary.
  • Keep the classic pairs distinct: energy vs. power, charge (Ah) vs. energy (Wh), nominal vs. usable, measured vs. estimated.
  • A rating needs its side, duration, and conditions; a value needs its boundary before it can be compared.
  • Protection terms name functions, not proof of safety — route them to the safety cluster.

Where to go next