Introduction

As the solar PV industry continues to scale rapidly across Pakistan, a parallel trend has emerged: the widespread use of simplified rules of thumb in technical and commercial discussions.

Many of these rules are not entirely wrong — but they are often incomplete, misapplied, or treated as guarantees rather than approximations.This article addresses some of the most persistent foundational misconceptions in solar PV projects:

• Confusing power (kW) with energy (kWh)
• Treating averages as fixed outcomes
• Judging system performance without accounting for context and resource variability

These misunderstandings are responsible for a large share of real-world issues, including:

• Unrealistic energy expectations
• Incorrect fault diagnosis
• Mistrust between clients and EPCs
• Perceived underperformance of otherwise well-designed systems

Myth 1: “kW and kWh Are Basically the Same Thing”

The Misunderstanding

Installed system size (kW) is frequently used interchangeably with energy production (kWh). Statements such as:

“We installed a 100 kW system, so it should give 400 units per day.”

are extremely common — and fundamentally incorrect.

Reality: Power and Energy Are Different Quantities

• kW (kilowatt) represents instantaneous power capacity
• kWh (kilowatt-hour) represents energy accumulated over time

A PV system’s kW rating tells you how large the system is, not how much energy it will produce.

Energy output depends on multiple variables, including:

• Solar irradiance
• Time and seasonal variation
• System efficiency
• Thermal and electrical operating conditions

Confusing power with energy almost always leads to unrealistic expectations and misinterpretation of performance.

Myth 2: “1 kWp Always Produces 4 kWh per Day”

Why This Myth Persists

The figure “4 kWh per kWp per day” is easy to remember, appears to work on average, and is repeated so frequently that it gains perceived authority.

However, it is not a physical constant.

Where the Number Comes From

This estimate is derived from:

• Annual average solar irradiance
• Expressed as Peak Sun Hours (PSH)
• Averaged across months and weather conditions

It is a statistical approximation, not a performance guarantee.

Why It Fails in Practice

In real-world operation:

• Solar irradiance varies significantly by location
• It changes by season
• It depends on orientation, tilt, and shading

As a result:

• Some months exceed the average
• Others — especially winter months — fall well below it

Using this figure as a firm design or performance benchmark is risky, particularly for commercial or mission-critical systems.

Myth 3: “Peak Sun Hours Are Actual Operating Hours”

The Intuitive Mistake

Peak Sun Hours are often interpreted as:

“The system runs at full power for X hours per day.”

This is not what PSH represent.

Reality: PSH Are an Energy Abstraction

Peak Sun Hours compress the area under the irradiance curve into a single equivalent number. They do not imply:

• Flat irradiance
• Constant output power
• Uniform system efficiency

In reality, morning and evening sunlight contributes less usable energy due to:

• Angle-of-incidence losses
• Inverter wake-up thresholds
• Reduced low-power efficiency

Not all sunlight contributes equally to usable electrical energy.

Myth 4: “If Output Is Low, the System Underperformed”

Why This Conclusion Is Tempting

Energy (kWh) is easy to measure and compare. Performance is not.

So when energy output drops, the immediate assumption is:

“Something must be wrong with the system.”

Reality: Performance Must Be Normalized

A PV system can:

• Produce less energy
• And still perform exactly as designed

if the available sunlight was lower.

This is precisely why Performance Ratio (PR) is used.

Why Performance Ratio (PR) Matters

PR answers a more meaningful question:

How efficiently did the system convert the available solar resource into electrical energy?

• PR removes weather variability from the evaluation
• It enables fair comparison across days, months, and seasons
• It separates resource limitation from system behavior

Without PR, objective performance assessment is impossible.

Why These Myths Matter

Taken together, these misunderstandings lead to:

• Inflated energy expectations
• Incorrect fault diagnosis
• Misplaced blame on equipment or EPCs
• Poor system design and sizing decisions

Over time, they erode trust between clients, engineers, and project developers.

A Better Way to Think About PV Systems

Instead of asking:

“How many units should this system give?”

Ask:

“How much sunlight was available — and how effectively did the system convert it?”

This shift from averages to context is the foundation of sound PV engineering and realistic system evaluation.

Closing Thought

Solar PV systems do not operate on rules of thumb. They operate on physics, time, and environment.

Rules of thumb can be useful — but only when their assumptions and limitations are clearly understood.

Next in This Series

Myths Part 3: Battery Capacity Is Not Autonomy

About the Author

Abis Husein is the Founder and Chief of Technology at Sustainable Energies Enterprise, bringing over 20 years of experience across sustainable energy systems, solar PV design, and real-world project performance.

An engineer with hands-on expertise in solar PV and battery energy storage systems, he has worked extensively across system design, technical evaluation, and operational performance assessment in Pakistan’s evolving energy landscape.

Through direct project involvement, the author has observed how simplified engineering assumptions — while useful for estimation — often become misleading when reused without proper context, particularly in commercial discussions and system expectations.