How much electricity will my PV system actually generate? This question is at the heart of every economic feasibility calculation. The answer is more complex than a glance at the module datasheet – because the power rating in Watt peak (Wp) only applies under ideal laboratory conditions at 1,000 W/m² irradiance and 25°C cell temperature.

In real-world operation, numerous factors influence yield: the location with its specific solar irradiance, module orientation and tilt, potential shading, temperature effects and the efficiency of installed components. This article explains these relationships, presents the calculation formulas and shows which factors have the greatest impact on electricity output.

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The Basic Formula for Yield Calculation

The annual yield of a PV system results from incident solar energy multiplied by a chain of efficiency factors and correction coefficients:

E = G × A × η_Module × η_Inv × η_Cable × k_Temp × k_Orientation × k_Shading

Variable	Meaning	Typical Range
E	Annual yield	700–1,000 kWh/kWp
G	Global irradiance	800–1,100 kWh/(m²·a)
A	Active module area	m²
ηModule	Module efficiency	18–23%
ηInv	Inverter efficiency	95–98%
ηCable	Cable efficiency	98–99%
kTemp	Temperature factor	0.92–0.97
kOrientation	Orientation factor	0.70–1.00
kShading	Shading factor	0.80–1.00

The formula illustrates: each factor multiplies with the others. A 5% loss at one point and 10% at another doesn't result in 15% total loss, but 0.95 × 0.90 = 0.855, i.e. 14.5%. With many small losses, this effect adds up considerably.

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Physical Principles

Global irradiance G comprises three components:

G = G_direct + G_diffuse + G_reflected

Direct radiation travels in a straight line from the sun to the earth's surface. It dominates under clear skies and delivers the highest intensity. Diffuse radiation results from scattering by air molecules, aerosols and clouds. In the UK, it accounts for approximately 55–60% of the annual total and comes uniformly from the entire sky hemisphere. Reflected radiation (albedo) is reflected from the surroundings and contributes to yield particularly with snow or light-coloured surfaces.

Regional Differences

In the United Kingdom, annual global irradiance varies between 800 kWh/m² in northern Scotland and 1,100 kWh/m² in Cornwall and the Channel Islands. This 30% difference directly proportionally affects PV yield.

Region	Global Irradiance	Specific Yield*
Scotland	800–900 kWh/m²	700–800 kWh/kWp
Northern England	850–950 kWh/m²	750–850 kWh/kWp
Midlands	900–1,000 kWh/m²	800–900 kWh/kWp
Southern England	1,000–1,050 kWh/m²	880–950 kWh/kWp
Cornwall, Channel Islands	1,050–1,100 kWh/m²	920–1,000 kWh/kWp

*With optimal south orientation and 35° tilt

Data Sources for Calculation

Our solar calculator uses irradiance data from the European Commission's PVGis database. This is based on satellite measurements and meteorological models spanning more than 10 years. For every location in Europe, PVGis provides mean global irradiance – differentiated by direct and diffuse components as well as by month.

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Angle of Incidence

The radiation striking a module depends on the angle between sunrays and module surface. When the sun is perpendicular to the module surface, maximum energy is received. At shallow angles, the same radiant power spreads over a larger area – intensity per square metre decreases proportionally to the cosine of the angle of incidence.

I_Module = I_Sun × cos(θ)

where θ is the angle between sunrays and module normal.

Optimal Orientation in the UK

For maximum annual yield in the UK, south orientation with 35 to 40° tilt is optimal. This geometry represents a compromise between summer sun (high position, long days) and winter sun (low position, short days). The slightly steeper tilt compared to continental Europe compensates for the UK's more northerly latitude.

Deviation from optimum can be quantified using the orientation factor k_Orientation:

Orientation	Tilt 10°	Tilt 30°	Tilt 45°	Tilt 60°
South (0°)	0.92	0.99	1.00	0.93
SE/SW (±45°)	0.92	0.94	0.92	0.84
East/West (±90°)	0.89	0.84	0.77	0.67
North (180°)	0.84	0.64	0.54	0.44

East-West Orientation: A Special Case

East-west systems on flat roofs generate 10 to 15% less annual yield than south-facing systems, but offer advantages:

Power generation is distributed more evenly throughout the day. While a south-facing system produces a high peak around midday, east-west systems deliver more in morning and evening hours. This increases self-consumption with typical load profiles and reduces grid stress and feed-in peaks. Additionally, more modules fit on a flat roof as the lower tilt requires less spacing to avoid mutual shading.

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Why Shading Is Critical

Shading is the most frequently underestimated yield killer. Unlike many other losses, partial shading doesn't affect yield proportionally – it can drastically reduce the yield of an entire module string.

The reason lies in series connection: solar modules are connected into strings through which the same current flows. A partially shaded module limits the current of the entire string. Shading of 10% of module area can lead to yield losses of 30 to 50% if no countermeasures are taken.

Types of Shading

Horizon shading from terrain, hills or distant buildings affects all modules equally and is predictable. It primarily reduces yield during morning and evening hours when the sun is low.

Near shading from chimneys, dormers, antennas or trees creates sharp shadows affecting only parts of the system. These local shadows move with the sun's position and can affect different modules at different times.

Self-shading occurs with rack-mounted systems on flat roofs when row spacing is chosen too small. Especially in winter with low sun angles, front module rows shade those behind.

Bypass Diodes and Their Limitations

Modern modules contain bypass diodes that electrically bridge shaded cell areas. A typical 60-cell module has three bypass diodes, each capable of bypassing 20 cells. When one cell is shaded, one-third of the module is bypassed – current flows around the shaded area, but that module third's output is lost.

The bypass diode prevents hotspots and module damage but can only limit, not prevent, yield loss.

Shading Analysis

Precise shading analysis is essential for yield calculation. Our solar calculator enables input of shading obstacles and calculates their impact on annual yield – time-resolved for each month and time of day.

For on-site analysis, sun path diagrams or digital tools that capture the horizon and compare it with the sun's path are suitable.

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Physical Background

Solar cells are based on semiconductors whose electrical properties are temperature-dependent. As temperature rises, thermal movement of charge carriers in the crystal lattice increases. This leads to more recombination of electrons and holes before they can flow away as current.

The effect shows primarily in reduced open-circuit voltage U_OC. The temperature coefficient for voltage in crystalline silicon cells is typically –0.3%/K. Short-circuit current I_SC rises slightly with temperature (+0.05%/K), but the voltage effect dominates.

Temperature Coefficients

The power temperature coefficient γ (gamma) indicates how much module output decreases per Kelvin temperature rise above 25°C:

Module Technology	Temperature Coefficient γ
Monocrystalline PERC	–0.35 to –0.40%/K
Polycrystalline	–0.40 to –0.45%/K
TOPCon	–0.30 to –0.35%/K
Heterojunction (HJT)	–0.25 to –0.30%/K
Thin-film CdTe	–0.20 to –0.25%/K

Heterojunction and thin-film modules have lower temperature losses and are therefore particularly suitable for hot locations or systems with restricted rear ventilation.

Module Temperature in Operation

Cell temperature during operation is significantly above ambient temperature. A dark module absorbs about 80% of incident radiation and converts only 20% to electricity – the rest becomes heat.

Module temperature can be approximately calculated:

T_Module = T_Ambient + k × G

where k is a factor depending on mounting situation:

• Freestanding, well-ventilated: k ≈ 0.025 K·m²/W
• Roof-mounted with ventilation: k ≈ 0.030 K·m²/W
• Roof-integrated without air gap: k ≈ 0.050 K·m²/W

At 1,000 W/m² irradiance and 25°C ambient temperature, a well-ventilated roof-mounted module reaches about 55°C, an integrated module up to 75°C.

Temperature Losses Over the Year

The temperature factor k_Temp describes mean yield reduction over the year:

k_Temp = 1 + γ × (T_Module,mean – 25°C)

In the UK, the mean weighted module temperature (weighted by irradiance) lies between 35 and 42°C. With a temperature coefficient of –0.38%/K:

k_Temp = 1 + (–0.0038) × (38 – 25) = 1 – 0.049 = 0.951

Temperature losses in the UK therefore amount to approximately 5 to 6% annually – lower than in continental Europe due to the temperate maritime climate.

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Factor 5: Inverter and Power Electronics

Inverter Functions

The inverter is the central link between PV generator and electricity grid. Its main tasks:

1. DC/AC conversion: Converting module direct current to grid-compliant alternating current (230 V, 50 Hz)
2. MPP tracking: Continuously finding the optimal operating point of modules
3. Grid monitoring: Maintaining voltage and frequency limits, disconnection during grid faults
4. Monitoring: Recording and transmitting yield data

Inverter Efficiency

Inverter efficiency η_Inv is not constant but load-dependent. In partial load range (below 20% of rated power), efficiency drops because the electronics' self-consumption remains constant while throughput power decreases.

Load Point	Typical Efficiency
5% load	88–92%
10% load	93–95%
20% load	95–97%
50% load	96–98%
100% load	95–97%

The European efficiency η_EU is a weighted average considering the typical load profile of a PV system:

η_EU = 0.03×η_5% + 0.06×η_10% + 0.13×η_20% + 0.10×η_30% + 0.48×η_50% + 0.20×η_100%

High-quality inverters achieve European efficiencies of 97 to 98%, budget devices range from 94 to 96%.

MPPT: Maximum Power Point Tracking

Solar modules have a non-linear current-voltage characteristic. The Maximum Power Point (MPP) is the operating point of maximum output – and this shifts constantly with irradiance and temperature. The inverter's MPPT algorithm continuously seeks this optimal operating point.

MPPT tracking quality varies between manufacturers and directly affects yield. High-quality inverters achieve MPPT efficiencies above 99.5%. Differences become particularly apparent with partial shading: simple algorithms can get stuck in local power maxima, while modern devices scan the entire voltage range to find the global maximum.

For systems with multiple roof surfaces or partial shading, inverters with multiple independent MPPT inputs are recommended. Module optimisers or microinverters offer even finer per-module control but cause additional conversion losses of 1 to 2%.

Other System Losses

Loss Source	Typical Value	Cause
DC-side cabling	0.5–2%	Ohmic losses, plug connections
AC-side cabling	0.2–0.5%	Line losses to meter
Mismatch	0.5–2%	Different module outputs in string
Soiling	1–3%	Dust, pollen, bird droppings, leaves
Reflection	1–3%	Losses at shallow angles of incidence
Snow cover	0–2%	Regional and tilt-dependent
Downtime	0.5–1%	Maintenance, faults, grid outages

The system efficiency or Performance Ratio (PR) summarises all losses between module output and grid feed-in. Well-planned systems achieve PR values of 80 to 88%.

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Simplified Yield Calculation

For a rough estimate, the formula using specific yield suffices:

E = P × Y_f

Variable	Meaning	Unit
E	Annual yield	kWh/a
P	System capacity	kWp
Yf	Specific yield (Final Yield)	kWh/kWp

The specific yield Y_f combines all influencing factors in one figure. It corresponds to full-load hours – the time the system would hypothetically need to run at rated power to generate the annual yield.

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Example Calculation: 8 kWp System in Southern England

Input data:

• Location: London (51.5° N, 0.1° W)
• System capacity: 8 kWp (20 modules at 400 Wp)
• Module area: 34.4 m² (20 × 1.72 m²)
• Module efficiency: 23.3%
• Orientation: South-southwest (azimuth 200°), 35° tilt
• Shading: Chimney, 2% annual loss
• Module type: Monocrystalline TOPCon (γ = –0.32%/K)
• Inverter: η_EU = 97.5%, 2 MPPT

Step 1: Global irradiance on tilted surface Horizontal global irradiance at location: 1,000 kWh/m²·a Orientation factor for SSW/35°: 0.99 Irradiance on module surface: 1,000 × 0.99 = 990 kWh/m²·a

Step 2: Temperature losses Mean weighted module temperature: 38°C Temperature loss: –0.32%/K × (38 – 25) K = –4.2% Temperature factor: k_Temp = 0.958

Step 3: System losses

• Inverter: η_Inv = 0.975
• DC cabling: η_Cable = 0.985
• Mismatch: 0.99
• Shading: k_Shading = 0.98
• Soiling: 0.98
• Reflection: 0.98

System factor: 0.975 × 0.985 × 0.99 × 0.98 × 0.98 × 0.98 = 0.891

Step 4: Annual yield Theoretical yield: 990 kWh/m² × 34.4 m² × 0.233 = 7,930 kWh Actual yield: 7,930 × 0.958 × 0.891 = 6,770 kWh/a Specific yield: 6,770 ÷ 8 = 846 kWh/kWp

Performance Ratio: 6,770 ÷ (990 × 34.4 × 0.233) = 85.4%

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Yield Throughout the Year

A PV system's power generation follows the sun's position and is distributed unevenly throughout the year:

Month	Share of Annual Yield	Typical Yield (8 kWp)
January	2–3%	150–200 kWh
February	4–5%	280–350 kWh
March	7–8%	500–570 kWh
April	10–11%	700–780 kWh
May	12–13%	850–920 kWh
June	12–13%	850–920 kWh
July	12–13%	850–920 kWh
August	11–12%	780–850 kWh
September	8–9%	570–640 kWh
October	5–6%	350–420 kWh
November	3–4%	210–280 kWh
December	2–3%	140–200 kWh

Approximately 70% of annual yield occurs between April and September. May, June and July typically deliver similar yields – long days combined with moderate temperatures create optimal conditions.

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Monitoring and Validating Yield

After commissioning, actual yield should be regularly compared with the forecast. Deviations under 10% are normal and due to weather variations. Larger deviations indicate problems:

• Systematic underperformance: Soiling, faulty system design, inverter malfunction
• Sudden yield drop: Defective module, cable break, inverter failure
• Seasonal deviation: New shading source (grown tree, new building)

Modern inverters offer monitoring via app or web portal. Recording of string currents enables localisation of problems down to module level.

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Why Excel Cannot Replace a Proper Solar Calculator

Many system planners attempt to calculate PV yield using self-built Excel spreadsheets. For initial orientation this may suffice – but for a reliable forecast it falls far short.

The fundamental problem: A realistic yield calculation requires hour-by-hour simulations across an entire year. Only this approach correctly captures the interactions between sun position, temperature, cloud cover and shading. That's 8,760 hours, each with different irradiance values, sun positions and module temperatures.

Shading analysis illustrates the complexity: The shadow of a chimney moves across the module surface throughout the day – and behaves completely differently in July compared to December. An Excel sheet with monthly flat-rate values cannot capture this temporal dynamics. Professional tools calculate for every hour of the year which modules are shaded to what extent and how this affects string yield through series connection.

Add to this the non-linear effects of module physics: The bypass diode switching during partial shading, the inverter's search for the global MPP when multiple local maxima exist, the temperature-dependent shift of the operating point. These relationships require time-resolved simulation, not static multiplication chains.

Irradiance data is also critical. Professional tools like our solar calculator access validated databases such as PVGis, which provide direct and diffuse irradiance separately for each location – including typical monthly values and variability. An Excel spreadsheet at best works with regional averages that ignore local peculiarities (fog-prone areas, elevated locations, albedo from water surfaces).

The crucial point: Errors don't add up – they multiply. An error of 5% in shading, 3% in temperature and 2% in efficiency doesn't result in 10% total error, but depending on the sign can lead to deviations of 15% or more. For a 7 kWp system, this means a forecast error of over 700 kWh per year – or several hundred pounds difference in the economic calculation.

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Conclusion

A reliable yield forecast is the foundation of any economic PV planning. It determines payback period, optimal storage size and expected electricity cost savings. Plan conservatively with realistic system losses – a pleasant surprise is better than disappointment.

Further information on system planning can be found in the article Planning a Solar System: Step by Step. The physical principles of power generation are explained in the article From Photon to Volt: How Solar Cells Work.

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Calculate Your PV Yield Now

With our Solar Calculator, you can calculate the expected electricity yield for your location – including shading analysis, self-consumption optimisation and economic feasibility calculation.

→ To the Solar Calculator

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Sources

• IEC 61724-1: Photovoltaic System Performance Monitoring
• IEC 61853: Photovoltaic Module Performance Testing
• PVGis – Photovoltaic Geographical Information System (European Commission)
• MCS: Microgeneration Certification Scheme Guide to PV System Installation
• Fraunhofer ISE: Photovoltaics Report 2025