Example 1: W versus kWh
A 1000 W heater is a 1 kW load. If it runs for 5 hours, the energy is 5 kWh. The first number describes load size; the second describes how much energy you paid for.
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Electrical Units Reference
Plain-language reference for common electrical units.
What this page helps with
Electrical calculations fail quickly when units are mixed. Power, energy, current, capacity and apparent power are related, but they are not interchangeable, and using the wrong unit is one of the most common reasons a quick estimate becomes misleading.
This page helps when you want a short reminder of what V, A, W, Wh, kWh, VA, VAr, Ah and Hz represent before using the cost, runtime, inverter or cable tools.
Formula or method
The page is structured as a practical reference: start with the visible table, diagram and labels, then compare them with the exact connector, device, enclosure or symbol set in front of you.
Use the listed sources to confirm revision, naming and application context. Similar labels can hide different electrical limits, pin functions or test conditions, so the page should be treated as a guided reference rather than a universal standard text.
How to use
- Identify whether the quantity is instantaneous or accumulated. Watts and amperes describe a present state, while watt-hours and ampere-hours describe stored or consumed quantity over time.
- Check whether the system is AC or DC. In AC work, apparent power and power factor matter, while simple DC relationships are more direct.
- Keep prefixes consistent. A value in W is not the same scale as kW, and a value in Wh is not the same thing as kWh even though the symbols look similar.
Practical examples
Example 2: A versus Ah in battery work
A 10 A load tells you current at this moment. A 100 Ah battery label describes stored charge capacity over time. You still need voltage and usable depth of discharge to estimate runtime meaningfully.
Common mistakes
- Confusing power with energy, for example treating 2 kW as if it were 2 kWh.
- Confusing current with battery capacity, for example comparing A and Ah directly.
- Ignoring AC assumptions when using VA, VAr and power factor.
Limitations
- This page is a quick reference, not a full metrology or standards guide.
- It does not replace manufacturer documentation for battery, inverter or motor naming conventions.
- Any result still depends on the formula and assumptions of the calculator you open next.
FAQ
Why is kWh used for billing instead of kW?
Because utilities usually bill energy over time, not only the instantaneous size of the load.
Can Ah alone tell me battery runtime?
No. Runtime depends on Ah, voltage, usable depth of discharge, efficiency and the actual load.
When do VA and VAr matter?
They matter most in AC systems where apparent power, reactive power and power factor affect sizing and current.
Related tools
Last reviewed: 2026-06-05
Factual reference
Electrical units
| Quantity | Symbol | Relationship |
|---|---|---|
| Voltage | V | Potential difference |
| Current | A | Charge flow |
| Resistance | ohm | V / A |
| Power | W | V x A for DC; depends on PF for AC |
| Apparent power | VA | RMS voltage x RMS current |
| Reactive power | VAr | Power triangle reactive component |
| Energy | Wh | W x hours |
| Frequency | Hz | Cycles per second |
| Capacitance | F | Coulomb per volt |
| Illuminance | lx | Lumens per square metre |
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.