Pakistan has an excellent solar resource, but saying that the country receives “a lot of sunshine” is not enough to design a photovoltaic system.

Solar engineering requires numbers.

And several apparently conflicting numbers frequently appear:

1,000 W/m²

236 W/m²

5.2 kWh/m²/day

and

5.2 Peak Sun Hours

They can all be correct.

They simply describe different things.

Understanding the difference is essential for interpreting solar potential, comparing quotations and estimating how much electricity a PV system can actually generate.

Start With Power Versus Energy

Power and energy are related, but they are not interchangeable.

Power describes the rate at which energy is being transferred at a particular instant.

Energy describes the amount transferred over a period of time.

Solar engineering makes exactly the same distinction.

Irradiance

Solar irradiance is instantaneous solar power received per unit area.

Its unit is:

W/m²

Irradiation or Insolation

Solar irradiation is solar energy received per unit area over a specified period.

Typical units include:

Wh/m²

or:

kWh/m²/day

This distinction between W and Wh is fundamental.

What Does 1,000 W/m² Mean?

The number 1,000 W/m² appears constantly in photovoltaic specifications because it is the reference irradiance used under Standard Test Conditions (STC).

PV modules are conventionally rated at:

Irradiance = 1,000 W/m²

Cell temperature = 25°C

together with a standardized reference solar spectrum.

DOE explains the use of these standardized conditions when discussing PV module performance. DOE — Optimizing Solar PV Performance

This allows manufacturers and engineers to compare modules on a common basis.

A 700 W module is therefore not promising to generate 700 W from sunrise to sunset.

It is a module whose rated power has been established under defined reference conditions.

Actual outdoor conditions continually depart from those conditions.

Then What Is 236 W/m²?

This number comes from a completely different calculation.

Solar irradiance outside Earth’s atmosphere can be approximated at around:

1,350 W/m²

For a simplified energy-balance calculation, assume approximately 30% attenuation through atmospheric and related effects.

The remaining amount is approximately:

1,350 × 0.70 = 945 W/m²

But Earth is a sphere.

The Sun’s radiation is intercepted by Earth’s projected disc:

πr²

while the complete spherical surface has an area:

4πr²

Therefore, averaging the intercepted solar energy over the whole spherical surface introduces a factor of:

1/4

giving:

1,350 × 0.70 × 1/4

≈ 236 W/m²

This does not mean that a solar module sitting in midday sunlight receives only 236 W/m².

The value is a simplified spatial-and-temporal average arising from the geometry and assumptions of the calculation.

It therefore answers a completely different question from the 1,000 W/m² STC reference.

What Does 5.2 kWh/m²/day Mean?

Now consider an actual solar site.

Suppose the location receives:

5.2 kWh/m²/day

of solar irradiation.

That tells us that each square metre receives a total of approximately 5.2 kilowatt-hours of solar energy during the day.

The irradiance was not necessarily constant.

It may have been low shortly after sunrise, risen through the morning, approached its maximum around solar noon and fallen through the afternoon.

Clouds, aerosols and atmospheric conditions can modify the curve further.

If we plot:

irradiance versus time

then the area underneath that curve represents solar irradiation.

This is the bridge between power and energy.

What Are Peak Sun Hours?

Peak Sun Hours are a convenient way of expressing that daily solar energy.

Suppose the changing irradiance throughout the day delivers:

5.2 kWh/m²

Now imagine replacing the entire irregular irradiance curve with exactly:

1 kW/m²

of constant irradiance.

How long would that reference irradiance need to continue to deliver the same energy?

The answer is:

1 kW/m² × 5.2 h = 5.2 kWh/m²

Therefore:

5.2 kWh/m²/day = 5.2 equivalent Peak Sun Hours

This does not mean Pakistan receives daylight for only 5.2 hours.

Peak Sun Hours are equivalent full-sun hours, not literal daylight hours.

That is one of the most important distinctions in practical solar calculations.

What Happens If We Average 5.2 kWh/m² Across 24 Hours?

There is another useful calculation.

Take:

5.2 kWh/m²/day

and divide it by 24 hours:

5.2 ÷ 24 = 0.2167 kW/m²

or:

216.7 W/m²

approximately:

217–220 W/m²

This means that 5.2 kWh/m²/day is mathematically equivalent to a continuous 24-hour average of approximately 217 W/m².

It does not mean the Sun actually supplies 217 W/m² throughout the night.

It is simply an energy-equivalent average.

This makes our earlier numbers interesting:

Simplified global average ≈ 236 W/m²

5.2-PSH site’s 24-hour equivalent ≈ 217 W/m²

The numbers are relatively close, but they should not be equated.

They originate from different calculations.

Is 5.2 Peak Sun Hours the Figure for All of Pakistan?

No.

It is a useful illustrative value, but Pakistan is geographically large and solar resources vary by location, season, orientation and weather.

The World Bank and Solargis have mapped Pakistan’s solar resource in detail. Their work shows meaningful geographical variation in both solar irradiation and modeled photovoltaic output. World Bank — Pakistan Solar Resource Report

The World Bank’s Global Solar Atlas exists precisely because solar-resource assessment needs geographical data rather than a single number assigned to an entire country. World Bank — Global Solar Atlas Dataset

Therefore:

Pakistan ≠ universally 5.2 PSH every day.

A professional energy-yield calculation should use location-specific resource data.

What Does 5.2 PSH Mean for a 10 kW Solar System?

Now the concept becomes immediately practical.

Suppose a site has an illustrative daily solar resource of:

5.2 PSH

and a PV array has a rated capacity of:

10 kWp

A first-order calculation gives:

10 kW × 5.2 h

= 52 kWh/day

This provides a useful pre-loss energy basis.

It does not mean the customer is guaranteed exactly 52 kWh at the meter every day.

Real systems experience temperature effects, inverter conversion losses, cable losses, mismatch, soiling, shading, degradation, clipping and other operating losses.

The calculation is therefore the beginning of an energy-yield assessment—not the end.

Why Not 10 kW × 24 Hours?

A 10 kW generator running continuously at rated output for 24 hours would generate:

10 kW × 24 h = 240 kWh

But a solar PV array cannot operate at rated solar input throughout a 24-hour period.

There is no solar irradiance at night.

During daylight, irradiance changes continuously.

Using our illustrative 5.2 PSH resource:

10 × 5.2 = 52 kWh/day

before system losses.

The difference between 240 kWh and 52 kWh is not an indication that the solar system is inefficient.

The 240 kWh calculation assumes 24 continuous hours at rated output, which is not the available solar resource.

GHI, DNI and GTI Also Mean Different Things

Solar-resource assessments introduce another set of important terms.

GHI — Global Horizontal Irradiation

Solar irradiation falling on a horizontal surface, incorporating direct and diffuse components.

DNI — Direct Normal Irradiation

Direct solar radiation received by a surface maintained perpendicular to the Sun’s rays.

GTI — Global Tilted Irradiation

Solar irradiation incident on an inclined plane.

The World Bank’s solar-resource datasets distinguish these quantities because they are not interchangeable. World Bank — Solar Resource Data

For a tilted rooftop solar array, GTI can therefore be more directly relevant to the energy arriving on the module plane than a horizontal irradiance figure alone.

Orientation matters.

Tilt matters.

Shading matters.

Geography matters.

Four Solar Numbers That Should Never Be Confused

We can now put the four numbers together.

And:

5.2 kWh/m²/day ÷ 24 h ≈ 217 W/m²

is the corresponding 24-hour equivalent average power density.

There is no contradiction among these figures.

Each describes solar energy from a different perspective.

What Should a Solar Customer Ask?

Rather than asking:

“How many hours of sunlight do we have?”

a more useful engineering question is:

“How much solar irradiation reaches the proposed array plane at this location over the year?”

Then we can ask:

“How efficiently can the proposed PV system convert that resource into usable electrical energy?”

That requires considering location, module orientation, tilt, temperature, shading, equipment efficiency and system losses.

The World Bank has characterized Pakistan as having substantial photovoltaic potential, but the resource varies geographically. World Bank — Solar PV Power Potential by Country

That is why credible solar design should move beyond generic claims such as “Pakistan gets eight hours of sunshine.”

Daylight hours are not Peak Sun Hours.

Irradiance is not irradiation.

Installed kW is not daily kWh.

And understanding those distinctions is where serious photovoltaic engineering begins.