A practical guide to electrical units used in EV charging, solar PV, and energy storage.

1. Why Is “k” Lowercase While “W” Is Uppercase?

W, V, and A: Units Named After Scientists

In the International System of Units (SI), some unit symbols use uppercase letters to honor scientists who made important contributions to science.

· V — Volt: Named after Italian physicist Alessandro Volta, whose invention of the voltaic pile was an important milestone in the development of early batteries.

· W — Watt: Named after Scottish inventor James Watt, whose improvements to the steam engine played a major role in the Industrial Revolution.

· A — Ampere: Named after French physicist André-Marie Ampère, known for his important contributions to electromagnetism.

The unit names are derived from these scientists’ surnames, and their symbols are written with capital letters: V, W, and A.

In EV charging and electrical systems, these units are commonly used to express voltage (V), power (W), and electric current (A), respectively.

“k” Is a Prefix Meaning “Thousand”

Unlike W, V, and A, “k” is not a unit symbol. It is an SI prefix meaning one thousand, or 10³. For example:

· 1 kW = 1,000 W

· 1 kV = 1,000 V

· 1 kWh = 1,000 Wh

Because “k” represents a numerical multiplier rather than a unit named after a person, it is written in lowercase.

SI prefixes are case-sensitive, and different capitalization can indicate vastly different magnitudes:

For example, mΩ means milliohm, while MΩ means megaohm—a difference of nine orders of magnitude.

The correct forms are kW, kWh, MW, and MWh—not KW, KWH, Mw, or MWH.

2. Common Units in EV Charging, Solar PV, and Energy Storage

Knowing how to capitalize unit symbols is only the first step. It is equally important to distinguish power, energy, battery charge capacity, and photovoltaic peak power. These parameters frequently appear in charger specifications, energy storage proposals, and solar PV project documents, but they describe different things.

Power: W, kW, and MW

Power describes the rate at which energy is transferred, converted, produced, or consumed. In simple terms, it indicates how quickly energy is being used or delivered.

Common units include W (watt), kW (kilowatt), MW (megawatt), and GW (gigawatt).

Conversions: 1 kW = 1,000 W; 1 MW = 1,000 kW.

In EV charging, a charger's rated power is usually expressed in kW. For example, a 120 kW DC fast charger has a rated power of 120 kW. The actual charging power may vary depending on the vehicle's battery state, charging curve, temperature, and operating conditions.

In solar PV and energy storage projects, kW or MW is also used to describe the rated power of inverters and power conversion systems (PCS), as well as the power capacity of a plant.

Energy: Wh, kWh, and MWh

Energy refers to the total amount of energy produced, stored, transferred, or consumed over a period of time.

Common units include Wh (watt-hour), kWh (kilowatt-hour), and MWh (megawatt-hour).

Conversions: 1 kWh = 1,000 Wh; 1 MWh = 1,000 kWh.

One kWh is equivalent to one unit of electricity commonly used for billing in many markets.

Energy is related to power and time:

Energy (kWh) = Power (kW) × Time (h)

For example, a 10 kW device operating at a constant 10 kW for two hours uses or delivers 20 kWh of energy.

In energy storage systems, kWh or MWh is commonly used to describe rated or usable energy capacity. In EV charging, kWh can describe the energy delivered to a vehicle or consumed during a charging session.

Battery Charge Capacity: Ah

Ah stands for ampere-hour and is a common unit for expressing a battery’s electric charge capacity. It is the product of current and time. Under ideal conditions, a 100 Ah battery discharging at a constant 10 A would theoretically provide current for 10 hours.

However, Ah alone does not fully describe how much energy a battery stores, because energy also depends on voltage.

Energy (Wh) ≈ Voltage (V) × Capacity (Ah)

For example, a battery with a nominal voltage of 48 V and a capacity of 100 Ah has a nominal energy of approximately 4,800 Wh, or 4.8 kWh. Actual usable energy depends on factors such as battery chemistry, depth of discharge, temperature, conversion efficiency, and system design.

Photovoltaic Peak Power: kWp

kWp stands for kilowatt-peak. It is commonly used to express the peak rated power of a solar PV module or system under Standard Test Conditions (STC).

STC typically includes:

· Irradiance: 1,000 W/m²

· Cell temperature: 25°C

· A defined standard solar spectrum

For example, a 100 kWp solar PV system has a rated peak power of 100 kW under STC. This does not mean the system will continuously output 100 kW in real-world operation. Actual output depends on solar irradiance, module temperature, shading, installation angle, inverter efficiency, and other system losses.

3. Understanding kW and kWh Through an Example

Suppose a commercial energy storage system has a rated power of 100 kW and a rated energy capacity of 261 kWh. What do these two parameters mean?

This example illustrates the key distinction: kW describes power capability, while kWh describes the amount of energy. Both are needed to understand the performance of EV charging equipment and energy storage systems.

4. Quick Reference: Correct Unit Notation

Use the correct unit symbols in technical documents, product pages, and business communications across the EV charging, solar PV, and energy storage industries.

Remember: “k” is lowercase, while unit symbols named after people—such as W, V, and A—are uppercase. Use kW for power and kWh for energy.

These may seem like small writing conventions, but they are essential for clear technical communication. Whether selecting EV charging equipment or designing a solar-plus-storage system, understanding units and specifications helps ensure that technical requirements and project proposals are communicated accurately.

About FDEnergy

Future Digital Energy (FDEnergy) specializes in EV charging equipment and energy solutions, offering AC chargers, DC fast chargers, and commercial and industrial energy storage solutions for customers worldwide.

Website: https://www.futdigit.com/

Email: info@futdigit.com