Tag Archive for: STC

kWp vs kWh — these two units appear on every solar quote and datasheet. Yet they are often confused. Confusing them leads to undersized systems, missed savings, and wrong payback estimates.

This guide explains exactly what kWp and kWh mean in solar. You will learn how they differ, how to convert one to the other, and how both affect your system design.

If you need a quick refresher on kW vs kWh first, see our guide on kWh vs kW explained. Otherwise, read on for the full kWp vs kWh breakdown.

kWp vs kWh: What Does kWp (Kilowatt-Peak) Mean?

kWp stands for kilowatt-peak. It is the rated maximum power output of a solar panel or array. This rating is measured under controlled laboratory conditions called Standard Test Conditions (STC). Therefore, kWp tells you the best-case output — not real-world output.

STC are used by every solar module manufacturer. They create a level playing field so buyers can compare panels from different brands on equal terms.

kWp STC Conditions — What the Rating Is Based On

• Solar irradiance: 1,000 W/m² — equivalent to full midday sun at sea level
• Cell temperature: 25 °C — cooler than most real rooftop conditions
• Air mass: AM 1.5 — a standard mid-latitude atmospheric path

Under these conditions, a 400 Wp panel produces exactly 400 W. Ten such panels form a 4 kWp array. However, these conditions rarely exist on a real rooftop.

kWp vs kWh: NOCT Gives a More Realistic kWp Figure

NOCT (Normal Operating Cell Temperature) tests panels at 800 W/m² irradiance, 45 °C cell temperature, and 1 m/s wind — conditions much closer to a real rooftop. Consequently, NOCT power ratings run 10–15% lower than STC kWp figures. When comparing panels, always check both ratings on the datasheet.

The IEC 61215 standard governs how manufacturers measure both STC and NOCT performance, making these ratings internationally comparable.

What is the difference between specific yield (kWh/kWp) and panel efficiency?

While both terms appear frequently on datasheets, they measure entirely different variables. Panel efficiency represents how effectively a solar cell converts sunlight into electricity within a fixed square meter of physical space—essentially telling you how compact the technology is.

On the other hand, specific yield (kWh/kWp) measures how much total energy (kWh) your entire system delivers over a year for every kilowatt of capacity (kWp) installed. While panel efficiency is fixed by the manufacturer, specific yield is heavily dependent on your geographic location, tilt angle, and climate.

kWp vs kWh: What Does kWh (Kilowatt-Hour) Mean in Solar?

kWh stands for kilowatt-hour. It measures the actual energy your solar system generates over time. While kWp is the rated capacity, kWh is the real-world output.

Think of it this way: kWp is the engine size of a car. kWh is the distance it actually travels. A powerful engine is useless if it only runs for one hour a day.

How to Calculate kWh Output from a kWp Solar System

The formula below converts kWp into expected annual kWh generation:

How do I calculate how many solar panels I need based on my kWh usage?

If you are trying to size an array to match your electricity bill, you can reverse-engineer our calculation formula. First, look at your annual energy bill to find your total consumption in kWh. Next, divide that number by your local annual specific yield (for instance, 1,500 kWh/kWp).

The resulting number gives you your required system size in kWp. To find the physical number of panels needed, simply divide that total kWp by the individual wattage of your preferred panel (e.g., dividing a 5 kWp requirement by a 400 Wp or 0.4 kWp panel yields exactly 13 panels).

Peak Sun Hours (PSH) measure how many hours per day a location receives the equivalent of 1,000 W/m² irradiance. For example, Dubai averages 6.1 PSH/day. London averages 2.8 PSH/day. Therefore, the same kWp system produces far more kWh in Dubai than in London.

You can look up PSH for any location using NREL’s PVWatts Calculator, which is a free and reliable tool from the US Department of Energy.

System Efficiency accounts for inverter losses, wiring resistance, soiling, and temperature derating. A well-designed system typically runs at 78–85% overall efficiency. However, shading or poor installation can push this below 70%.

kWp vs kWh Worked Example: Same System, Two Locations

The result is striking: the same 10 kWp system generates over twice as many kWh in Phoenix as in London. As a result, quoting kWp without specifying location is meaningless for project economics.

kWp vs kWh: A Direct Side-by-Side Comparison

The table below shows the core differences between kWp and kWh in solar:

5 Factors That Affect How Much kWh Your kWp System Delivers

Several real-world factors determine how many kWh a given kWp system produces. Understanding these is essential for accurate yield forecasting.

1. Location and Solar Irradiance Affect kWh Output Most

Solar irradiance varies enormously by region. The Middle East, Australia, and the US Southwest receive 1,800–2,500 kWh/m² annually. Northern Europe receives 900–1,200 kWh/m². Consequently, a solar project in Dubai generates two to three times more kWh per kWp than the same system in Scotland.

For detailed peak sun hours data by country, see our guide on peak sun hours by location. Furthermore, the Global Solar Atlas provides free, downloadable irradiance maps for any location worldwide.

2. Panel Orientation and Tilt Angle Change kWh Yield

South-facing panels at a tilt angle matching the site latitude produce the highest annual kWh. East or west-facing installations lose 15–20% of yield compared to south-facing. In addition, north-facing installations at high latitudes can lose 30–40% of potential kWh output.

3. Shading and Soiling Reduce kWh Production

Partial shading cuts kWh output significantly. In conventional string-wired systems, one shaded panel reduces output across the whole string.

Soiling — dust, pollen, bird droppings — causes a further 2–6% loss in temperate climates. However, in dry desert regions, soiling losses can reach 15–25% without regular panel cleaning.

4. Temperature Coefficient Lowers kWh in Hot Climates

Solar panels lose power as cell temperature rises above 25 °C. A typical monocrystalline silicon panel loses approximately 0.35% of its kWp output for every degree above 25 °C.

At 60 °C cell temperature — common on hot rooftops — that is a 12% reduction from the STC kWp rating. As a result, hot climates produce fewer kWh per kWp than cool climates, despite having more sunlight.

5. Inverter and System Losses Reduce Final kWh

The inverter converts DC solar power to AC. It operates at 94–98% efficiency. Additional losses come from wiring resistance, transformer losses, and module mismatch.

Combined, these losses typically reduce kWh output by 15–25% from the theoretical kWp-based maximum. Therefore, always factor in a realistic loss value — not the best-case figure — when modelling project yield.

kWp vs kWh in Solar System Sizing: Three Real Examples

These examples show how kWp and kWh interact in real projects at different scales.

Residential kWp vs kWh Example: 5 kWp System in New Delhi

• Location: New Delhi (5.4 peak sun hours/day)
• System size: 5 kWp — approximately 12–13 panels at 400 Wp each
• System efficiency: 80%
• Annual output: 5 × 5.4 × 365 × 0.80 = 7,884 kWh/year
• Monthly average: approximately 657 kWh/month
• Typical household consumption: 300–500 kWh/month — system covers 130–220% of demand

Result: The 5 kWp system comfortably covers an average household’s electricity needs. Furthermore, it generates surplus kWh for export or battery storage on most days.

Why doesn’t my 5 kWp system show 5 kW on my inverter app?

A common point of confusion for homeowners post-installation is opening their monitoring app on a sunny day and seeing an instantaneous output of only 3.5 kW to 4 kW. This is completely normal.

Remember that your 5 kWp rating is calculated under perfect laboratory conditions ($25^\circ\text{C}$). In the real world, rooftop heat (which degrades panel efficiency), inverter conversion losses, and slight angle misalignments naturally reduce your real-time performance. This is precisely why we design systems based on cumulative kWh energy yield over time rather than looking solely at the peak kW capacity.

Commercial kWp vs kWh Example: 200 kWp System in Dubai

• Location: Dubai (6.1 peak sun hours/day)
• System size: 200 kWp
• System efficiency: 78% — lower due to desert soiling losses
• Annual output: 200 × 6.1 × 365 × 0.78 = 347,334 kWh/year (347 MWh/year)
• Specific yield: 1,737 kWh/kWp/year
• Estimated saving: At AED 0.30/kWh — approximately AED 104,200/year (USD 28,300)

Result: The 200 kWp system delivers strong kWh yield. However, soiling management is essential to maintain this specific yield over time.

Utility-Scale kWp vs kWh Example: 50 MWp Farm in Spain

• Location: Spain (5.2 peak sun hours/day)
• System size: 50,000 kWp (50 MWp)
• System efficiency: 82% — bifacial panels with single-axis trackers
• Annual output: 50,000 × 5.2 × 365 × 0.82 = 77.7 GWh/year
• Specific yield: 1,555 kWh/kWp/year — enhanced by tracking
• Equivalent households: approximately 22,000 Spanish homes at 3,500 kWh/year each

Result: Single-axis trackers boost kWh yield by 20–30% over fixed-tilt systems. As a result, they significantly improve the kWh economics of large solar farms.

kWp vs kWh When Solar Is Paired with Battery Storage

When solar is paired with a Battery Energy Storage System (BESS), both kWp and kWh take on new roles. Correctly matching them is the foundation of a good solar-plus-storage design.

kWp Controls How Fast the Battery Charges

The kWp rating sets the maximum power available to charge the battery at any moment. For example, a 100 kWp array with 80% system efficiency delivers roughly 80 kW to the battery in peak conditions.

Consequently, a 200 kWh battery paired with this array takes a minimum of 2.5 hours to charge from empty. This determines whether the battery completes a full cycle before sunset.

kWh Controls How Long the Battery Can Supply Load

The battery’s kWh capacity sets dispatch duration — how many hours it can supply load after solar drops. A 200 kWh BESS at 50 kW discharge sustains load for four hours after sunset. Therefore, matching solar kWp with the right battery kWh is critical. See our guide on BESS C-Rate Explained for more on this relationship.

kWp vs kWh Mismatch: What Happens When Solar Is Oversized

In systems with limited grid export, too much solar kWp relative to battery kWh causes curtailment — wasted solar energy.

For example, a 50 kWp array at 80% efficiency producing 40 kW fills a 50 kWh battery in just 1.25 hours. After that, excess kWh is wasted. Our guide on choosing solar panels and batteries for a 100 kWh load shows how to avoid this in a full worked example.

For a broader view of how storage losses affect kWh throughput, see our article on energy storage losses in BESS.

4 Common kWp vs kWh Mistakes in Solar Quotes

These are the most frequent errors buyers make when reading and comparing solar proposals.

Mistake 1: Comparing kWp Without Factoring in Location

Two quotes showing ’10 kWp’ are not equal if the systems are in different locations. Always request an annual kWh yield estimate alongside the kWp figure.

Reputable suppliers use tools such as PVWatts or PVGIS from the EU Joint Research Centre to produce site-specific yield reports. Insist on seeing these before signing.

Mistake 2: Accepting kWh Estimates With Unrealistic Losses

Some suppliers inflate kWh projections by assuming only 5–10% system losses instead of the more realistic 15–25%.

Always ask which loss factors are included: temperature derating, soiling, inverter efficiency, wiring resistance, shading, and module mismatch. A credible yield report lists each factor explicitly.

Mistake 3: Sizing Battery Storage from kWp Instead of kWh

Sizing a battery based on peak kWp — rather than actual daily kWh generation — leads to oversized and overpriced storage. The battery must match the actual kWh generated each day, not the theoretical maximum.

Furthermore, use hourly generation profiles rather than peak values when sizing storage. This avoids undersizing the battery for mornings and evenings when kWp output is low.

Mistake 4: Ignoring kWp Degradation and Its Effect on kWh

Solar panels degrade annually — typically 0.5–0.8% per year for monocrystalline silicon. Consequently, a panel with 0.7%/year degradation retains about 82.5% of its kWp rating after 25 years.

This means fewer kWh per year as the system ages. Financial models must incorporate this degradation into their annual kWh projections. Ignoring it overstates long-term savings.

kWp vs kWh Quick Reference Summary

Frequently Asked Questions (FAQs)

Conclusion: kWp vs kWh — Use Both for Better Solar Decisions

kWp and kWh answer two completely different questions. kWp tells you what the system is rated to produce under ideal lab conditions. kWh tells you what it actually delivers at your location, accounting for losses, temperature, and seasonal irradiance.

For any solar investment, both metrics are essential. kWp helps you compare panels and size the system. kWh helps you calculate real energy savings and payback period. Therefore, never evaluate a solar quote on kWp alone.

At Sunlith Energy, every solar proposal includes a site-specific kWh yield model using validated irradiance data — so you see what the system will actually deliver. Contact our team to request a free yield assessment for your project.