Peak sun hours express a day's solar energy as an equivalent number of hours at full standard irradiance, which makes the yield calculation a simple product: kWp × peak sun hours × performance ratio. Enter those three along with module efficiency and an electricity price to get daily and annual energy, specific yield in kWh per kWp, and the roof area needed.
Calculator
Units:
kWp
Total peak power of the modules at standard test conditions
h/day
Annual daily average. Northern Europe 2.5–3, Mediterranean 4.5–5, desert 5.5–6.5
%
75 to 85% for a well-designed system
%
Affects the area required, not the energy produced
per kWh
For the annual value figure in the working
Calculation Result
Press Calculate for the daily and annual energy, the specific yield in kWh per kWp per year, and the aperture area the array occupies. The specific yield is the figure to compare against local benchmarks.
Step-by-Step Solution
Preliminary design aid. Results follow the published formulas cited
below and are intended for estimating, study and early design. Final design must be
verified by a licensed Professional Engineer against the code in force for your project.
Key Benefits
✓Uses the peak sun hours convention, which makes the arithmetic transparent
✓Separates rated power from module efficiency, which do different jobs
✓Reports specific yield, the standard way to compare systems and locations
✓Gives the aperture area, which is what a roof survey actually constrains
✓Warns when the performance ratio or yield is outside observed ranges
✓Shareable links and CSV export for feasibility records
What Is Solar Panel Output?
Photovoltaic modules are rated at standard test conditions: 1000 W/m² of irradiance at a 25 °C cell temperature. That rating is the array's peak power, kWp. Peak sun hours restate a location's daily solar energy as the number of hours it would take at that same 1000 W/m² to deliver the same total — so a site with 3.5 peak sun hours receives 3.5 kWh/m² per day. Multiplying kWp by peak sun hours therefore gives the ideal daily energy directly.
What the performance ratio absorbs
Real systems never reach the ideal, and the performance ratio is the single factor that accounts for everything in the way: cell temperature above 25 °C, inverter losses, cable losses, soiling, module mismatch, and shading. Well-designed systems achieve 75 to 85%. A figure above about 88% is not observed in practice, and below 65% points to a specific fault — significant shading, a badly matched inverter, or an orientation far from optimal.
Why efficiency does not appear in the energy calculation
A 400 W module and a 300 W module of the same physical size differ in efficiency, but a 5 kWp array is 5 kWp either way — it just takes more or fewer modules and more or less roof. Efficiency matters when roof area is the binding constraint, which on a domestic pitched roof it often is. Where space is abundant, higher efficiency buys nothing but a smaller footprint at a higher price per watt.
Formula
E_daily = P_kWp × PSH × PR
Daily energy from array rating, peak sun hours and performance ratio
Related Formulas
E_annual = E_daily × 365
Y_specific = E_annual / P_kWp
A = P_kWp × 1000 / (1000 × η)
Variable Definitions
Symbol
Variable
Unit
Description
P_kWp
Array Rating
kWp
Peak power at standard test conditions — 1000 W/m², 25 °C cell temperature.
PSH
Peak Sun Hours
h/day
Daily solar energy expressed as equivalent hours at 1000 W/m².
PR
Performance Ratio
%
Everything that reduces real output below ideal. 75 to 85% for good systems.
η
Module Efficiency
%
Determines the area a given rating occupies, not the energy it produces.
Y
Specific Yield
kWh/kWp/yr
Annual energy per kWp installed — the figure used to compare locations.
A
Aperture Area
m²
Module area required for the rating at the stated efficiency.
How to Use This Calculator
Use an annual average for peak sun hoursTake the figure for your location from an irradiance database, adjusted for the array's tilt and orientation. Northern Europe runs 2.5 to 3, the Mediterranean 4.5 to 5, and desert regions 5.5 to 6.5. A south-facing tilt near the latitude angle maximises the annual total.
Be realistic about the performance ratio80% is a reasonable default for a well-designed unshaded system. Anything above 88% should be treated as an error rather than an aspiration, and below 65% indicates a specific problem worth identifying rather than accepting.
Enter the array rating, not the module countThe rating is the total peak power of all modules. Twelve 420 W modules is 5.04 kWp regardless of their physical size, and that rating is what determines the energy.
Treat module efficiency as an area questionIt determines how much roof a given rating occupies and nothing else. Higher efficiency is worth paying for when the roof is the constraint and worth nothing when it is not.
Read the specific yield against local benchmarksSpecific yield normalises out the array size, so it compares directly against other systems in the same region. A figure well below the local norm points to shading, orientation or an equipment problem rather than to the array being too small.
Worked Examples
Example 1
A 5 kWp domestic array at a site with 3.5 peak sun hours, a performance ratio of 80%, using 21% efficient modules. Electricity is 0.28 per kWh.
Step-by-Step Solution
Daily energy: 5 × 3.5 × 0.80 = 14.00 kWh/day
Annual energy: 14.00 × 365 = 5,110 kWh/year
Specific yield: 5,110 / 5 = 1,022 kWh per kWp per year
Aperture area: 5,000 W / (1000 W/m² × 0.21) = 23.8 m²
Annual value at 0.28 per kWh: 5,110 × 0.28 = 1,431
Interpretation: 1,022 kWh/kWp is a typical northern European result. The 23.8 m² is the module area alone — the roof needs more than that for spacing, edge clearance and access.
Example 2
The same 5 kWp array built from modules of three different efficiencies, to isolate what efficiency actually changes.
Step-by-Step Solution
18% modules: daily energy 14.00 kWh, annual 5,110 kWh, specific yield 1,022 kWh/kWp
21% modules: daily energy 14.00 kWh, annual 5,110 kWh, specific yield 1,022 kWh/kWp
23% modules: daily energy 14.00 kWh, annual 5,110 kWh, specific yield 1,022 kWh/kWp
Every energy figure is identical. Only the area differs: 27.8 m², 23.8 m² and 21.7 m² respectively.
This is the point that the marketing around panel efficiency tends to obscure. A kilowatt-peak is a kilowatt-peak; efficiency describes how densely that rating is packed into the module, not how much energy it generates.
The 23% modules need 22% less roof than the 18% ones. On a large flat roof that saving is worth little. On a small pitched domestic roof where 5 kWp barely fits, it is the difference between the system being possible and not.
The corollary is worth stating plainly: if the roof has room to spare, the cheaper route to more energy is more kWp of lower-efficiency modules, not the same kWp of higher-efficiency ones.
Location Sensitivity
Energy is directly proportional to peak sun hours, so all three energy series are straight lines through the origin. The area series is flat, because module efficiency and not location determines it — which is the distinction this calculator exists to make. The marker shows your current site.
Annual Energy vs Peak Sun Hours
Recomputed live from your inputs. The marker shows your current value.
Line chart of Annual Energy against Peak Sun Hours. The same
values are listed in the data table below.
Values plotted above, sampled across the peak sun hours range.
How to Interpret Your Results
Specific yield is the figure that tells you whether the system is performing. It normalises out the array size, so it compares directly against other installations in the same region.
Specific Yield: < 700Low yield — investigate the cause
A specific yield of your result kWh/kWp/year is low for almost any location. Check for shading, a poor orientation, or a performance ratio that reflects a real fault rather than normal losses. This is usually a correctable problem, not a site limitation.
Specific Yield: 700 – 1100Typical northern European yield
A specific yield of your result kWh/kWp/year is the normal range for northern Europe and similar latitudes. Compare against local benchmark data — a figure at the bottom of this band on a good site still merits a look at shading and orientation.
Specific Yield: 1100 – 1600Good yield
A specific yield of your result kWh/kWp/year suits southern Europe, the southern United States and similar climates. At these irradiance levels cell temperature becomes a larger loss, so ventilation behind the modules matters more than it does in cooler regions.
Specific Yield: ≥ 1600Very high yield — verify the inputs
A specific yield of your result kWh/kWp/year occurs only in high-irradiance desert locations. Check the peak sun hours and performance ratio; a figure this high from a temperate site indicates an input error rather than an exceptional installation.
Aperture Area: ≥ 30Check the roof against this area
The modules occupy your result m². The roof needs more than this for row spacing, edge clearance and maintenance access — commonly 20 to 40% more on a flat roof where rows must be spaced to avoid shading each other.
Common Mistakes to Avoid
Expecting higher-efficiency modules to generate more energy
Why it matters:For a given kWp rating they do not. Efficiency determines how much area the rating occupies. A 5 kWp array yields the same 5,110 kWh a year at 18% or 23% efficiency — the only difference is 27.8 m² against 21.7 m².
✓How to avoid it:Buy efficiency when roof area is the binding constraint. Where space is available, more kWp of cheaper modules delivers more energy for the same money.
Using an optimistic performance ratio
Why it matters:Ratios above 88% are not observed in real installations. The performance ratio has to absorb cell temperature, inverter losses, soiling, mismatch and cable losses, and those do not go away.
✓How to avoid it:Use 75 to 85% for a well-designed system. Anything higher inflates the yield estimate and the payback calculation that rests on it.
Treating the annual figure as a monthly guide
Why it matters:Output is strongly seasonal. In temperate latitudes the darkest month produces around a fifth of the brightest, so an annual average says nothing about whether a self-consumption or off-grid system will carry through winter.
✓How to avoid it:Use monthly irradiance data for anything where seasonal coverage matters. Off-grid systems are sized on the worst month, not the average.
Ignoring temperature losses
Why it matters:Modules are rated at a 25 °C cell temperature, but a roof-mounted array on a hot day sits 30 °C above ambient. At 0.3 to 0.4% loss per degree, that is a 10 to 15% reduction — which is why the performance ratio is well below unity even in perfect conditions.
✓How to avoid it:The performance ratio already covers it. Do not apply a temperature derating on top, or the loss is counted twice.
Confusing kWp with kWh
Why it matters:kWp is an instantaneous power rating at standard conditions; kWh is energy over time. A 5 kWp array almost never produces 5 kW, and the annual energy depends entirely on the site's irradiance.
✓How to avoid it:Keep the two separate. Specific yield in kWh per kWp per year is the bridge between them, and it is the figure worth comparing across systems.
Sizing the array on the roof area alone
Why it matters:Aperture area is the module area, not the roof area needed. Flat roofs need row spacing so rows do not shade each other, and every roof needs edge clearance and access.
✓How to avoid it:Allow 20 to 40% more roof than the aperture area on a flat roof, and check the actual usable area after obstructions, hips and shading are taken off.
Practical Applications
▸Estimating annual generation for a proposed PV array
▸Comparing sites by their specific yield
▸Checking an installed system against expected performance
▸Sizing an array to meet a target annual consumption
▸Estimating the roof area a given rating requires
▸Screening feasibility before a detailed simulation
Industry Use Cases
Domestic installation
Roof area is usually the binding constraint on a pitched domestic roof, which is what makes high-efficiency modules worth their premium — not because they generate more per kWp, but because more kWp fits. The trade-off inverts entirely on a large flat roof.
Commercial rooftop
Flat commercial roofs need row spacing to prevent inter-row shading, so the usable area is well below the total. Tilt angle and row pitch are optimised together: a shallower tilt packs more modules but yields less per module.
Performance monitoring
Specific yield is the standard metric for comparing an installed system against its neighbours and against its own history. A gradual decline points to soiling or degradation; a sudden step points to an inverter or string fault.
Expert Tips
💡Module efficiency changes the area required, not the energy produced.
💡kWp is a power rating at test conditions; kWh is what you actually get.
💡Performance ratios above 88% are not observed — use 75 to 85%.
💡Specific yield normalises out array size, so it compares systems directly.
💡The darkest month yields around a fifth of the brightest in temperate latitudes.
💡Aperture area is not roof area — allow 20 to 40% more on a flat roof.
Advantages & Limitations
Advantages
✓Makes the peak sun hours convention explicit, so the arithmetic is checkable
✓Separates the energy calculation from the area calculation, which is where confusion lives
✓Reports specific yield, the metric used to compare real installations
✓Warns when the performance ratio or yield falls outside observed ranges
✓Fast enough to compare sites and array sizes during feasibility work
Limitations
!Uses an annual average, so it says nothing about seasonal or monthly coverage
!The performance ratio is a single lumped factor rather than itemised losses
!Takes no account of tilt, azimuth or shading beyond what the ratio absorbs
!Assumes no annual degradation, which is typically 0.5% per year
!Does not model self-consumption, export or storage
!Aperture area excludes spacing, clearance and access
!For a bankable estimate, a time-series simulation with local irradiance data is needed
The Same 5 kWp Array in Different Locations
A 5 kWp array at 80% performance ratio and 21% module efficiency. Only the peak sun hours change — and note that the area column does not move at all.
5 kWp, 80% performance ratio, 21% modules. Energy is directly proportional to peak sun hours — the best site here yields 2.4 times the worst from an identical array. The constant area column is the point: location determines energy, efficiency determines area, and neither crosses over.
Multiply the array rating in kWp by the site's peak sun hours and by the performance ratio. A 5 kWp array at 3.5 peak sun hours and 80% gives 14 kWh a day, or 5,110 kWh a year.
What are peak sun hours?
A restatement of the daily solar energy as the number of hours it would take at 1000 W/m² to deliver the same total. A site with 3.5 peak sun hours receives 3.5 kWh/m² per day.
What is a performance ratio?
The single factor accounting for everything between the ideal and the real output — cell temperature, inverter and cable losses, soiling, mismatch and shading. Well-designed systems reach 75 to 85%.
Do more efficient panels produce more energy?
Not for a given kWp. A 5 kWp array yields the same energy at 18% or 23% efficiency; the difference is 27.8 m² of roof against 21.7 m². Efficiency buys space, not electricity.
What is specific yield?
Annual energy divided by array rating, in kWh per kWp per year. It normalises out the system size, so it compares directly against other installations in the same region.
How much roof does a 5 kWp system need?
About 23.8 m² of module aperture at 21% efficiency. The roof needs more — commonly 20 to 40% more on a flat roof, where rows must be spaced so they do not shade each other.
What specific yield should I expect?
Roughly 700 to 1,100 kWh/kWp/year in northern Europe, 1,100 to 1,600 in southern Europe and the southern United States, and up to about 1,800 in high-irradiance desert regions.
Does temperature affect solar output?
Yes. Modules are rated at 25 °C cell temperature and lose 0.3 to 0.4% per degree above it. A roof array on a hot day can run 30 °C above ambient, which is a 10 to 15% loss — already included in the performance ratio.
How does output vary through the year?
Substantially. In temperate latitudes the darkest month produces around a fifth of the brightest, so an annual average is no guide to winter coverage. Off-grid systems are sized on the worst month.
What is the difference between kWp and kW?
kWp is the rated power at standard test conditions — 1000 W/m² and 25 °C. Actual output is almost always below it, because real irradiance and real temperatures rarely match those conditions.
Glossary
kWp
Peak power at standard test conditions: 1000 W/m² irradiance, 25 °C cell temperature.
Peak sun hours
Daily solar energy expressed as equivalent hours at 1000 W/m².
Performance ratio
Actual output divided by ideal output, absorbing all real-world losses.
Specific yield
Annual energy per kWp installed, the standard comparison metric.
Standard test conditions
The reference conditions at which modules are rated.
Aperture area
The module area itself, excluding spacing, clearance and access.
Module efficiency
The fraction of incident solar energy a module converts, which sets its area per watt.
Inter-row shading
Shading of one module row by another, which sets the row pitch on flat roofs.
Soiling
Loss of output from dust, dirt and bird droppings on the module surface.
Degradation
The gradual annual decline in module output, typically around 0.5% per year.
Scientific & Standards References
IEC 61724-1 — Photovoltaic system performance: Monitoring — International Electrotechnical Commission
IEC 61215 — Terrestrial photovoltaic modules: Design qualification and type approval — International Electrotechnical Commission
PVGIS — Photovoltaic Geographical Information System — European Commission Joint Research Centre
NREL — PVWatts Calculator Technical Reference — National Renewable Energy Laboratory
IEA PVPS Task 13 — Performance and Reliability of Photovoltaic Systems — International Energy Agency
Conclusion
Photovoltaic yield is one product — rating times peak sun hours times performance ratio — and the arithmetic is only as good as the middle term, which comes from irradiance data for the specific site, tilt and orientation. The performance ratio absorbs everything else, and 75 to 85% is what real systems achieve; anything above 88% should be read as an error rather than an ambition. The distinction most worth carrying away is between the two quantities that sound interchangeable. Location determines energy: the table above shows an identical 5 kWp array yielding 3,650 kWh a year at 2.5 peak sun hours and 8,760 at 6.0. Module efficiency determines area, and only area: the same array occupies 27.8 m² at 18% efficiency and 21.7 m² at 23%, producing exactly the same electricity either way. Buy efficiency when the roof is the constraint, and buy more kWp when it is not.
Enter your array size, site and performance ratio above to estimate the yield.
