Solar Output Calculator by Location: Annual kWh

Last updated: November 2026

SurgePV publishes this post and sells solar design software.

A homeowner in Phoenix and a homeowner in Seattle install identical 6 kWp systems. PVGIS puts the Phoenix system near 10,900 kWh a year and the Seattle system near 7,000 kWh. Same panels. Same inverter. Same price. The only difference is where the roof sits.

That 57 percent gap is why every project starts with one question: how much will this system actually produce? Not the nameplate rating and not the optimistic quote. The real number, built from local sunshine, roof geometry and losses.

A solar panel output calculator answers that question. It turns system size, location, orientation and losses into an annual kWh estimate. Installers use it for proposals. Homeowners use it to check quotes. Lenders use it to underwrite loans. This guide explains how the math works, which inputs matter most, and gives location tables built from the European Commission’s PVGIS tool.

In this guide:

• How solar panel output calculators work, with the math
• Solar irradiance by location, and what drives it
• Panel efficiency, degradation and temperature effects
• Shading, orientation and tilt, with PVGIS numbers for each
• System losses, and why performance ratio is not a constant
• Annual kWh for 31 cities across the US, Europe and Asia-Pacific
• How to run a professional output estimate, and common mistakes

How Solar Panel Output Calculators Work

An output calculator multiplies system size by the sunshine the panels receive and by a performance ratio. The whole job is estimating the specific yield, which is annual kWh per kWp.

Annual energy (kWh) = system size (kWp) x specific yield (kWh/kWp/year)

Specific yield is not one number. It combines location, geometry and a cascade of losses.

The Basic Formula

At its simplest:

Annual kWh = kWp x annual irradiation on the panel plane (kWh/m²) x performance ratio

Where:

• kWp is the system capacity under standard test conditions (STC).
• Irradiation on the panel plane is the solar energy hitting the tilted panels over a year. Tools call it plane-of-array (POA) irradiation. Because STC irradiance is 1 kW/m², kWh/m² can be used as equivalent full-sun hours.
• Performance ratio (PR) is the share of that ideal energy that reaches the meter.

Worked example for a 6 kWp system in Phoenix. PVGIS gives 2,458 kWh/m² on the panel plane. With an implied PR of 0.74:

6 x 2,458 x 0.74 = about 10,900 kWh per year

The PR in this example is not assumed. It is the ratio of PVGIS’s specific yield (1,824 kWh/kWp) to its POA irradiation (2,458 kWh/m²). That ratio already includes the 14 percent system loss plus heat and angle-of-incidence effects. This is the back-of-envelope method. Professional tools go deeper.

The Hour-by-Hour Simulation Method

Tools such as PVWatts, PVsyst and professional solar design software simulate each hour of a typical year:

1. Import weather data. A typical meteorological year (TMY) file holds hourly irradiance, temperature and wind speed for the site.
2. Calculate plane-of-array irradiance. For each hour, split sunlight into direct, diffuse and reflected parts and project them onto the tilted panel surface.
3. Apply a temperature model. Cell temperature rises above ambient with irradiance and falls with wind and ventilation.
4. Apply the module model. Convert irradiance and cell temperature to DC power using the module’s temperature coefficient and low-light behavior.
5. Apply the inverter model. Convert DC to AC with the efficiency curve, and clip power when DC exceeds the inverter rating.
6. Apply system losses. Wiring, soiling, mismatch, availability and degradation.
7. Sum the hourly AC output into daily, monthly and annual totals.

This captures interactions that the simple formula misses. Heat losses peak in summer, when irradiance is also high. Shading losses vary by season and time of day. Inverter clipping only happens at peak hours. NREL’s PVWatts calculator is a free tool built on the same approach.

Standard Test Conditions vs. Real World

Panel wattage ratings are measured at STC: 1,000 W/m² irradiance, 25°C cell temperature and an air mass of 1.5. Real roofs rarely sit at those conditions. On a hot afternoon a cell can run far above 25°C and give up part of its rated power. For example, with an assumed coefficient of -0.38 percent per °C and a 65°C cell, a 400 W panel loses about 15 percent and delivers roughly 340 W. The coefficient in your module’s datasheet is the number to use.

Output calculators bridge the gap by modeling how a panel behaves across the full range of operating conditions.

Solar Irradiance by Location: The Primary Driver

Solar irradiance is the solar power striking a surface. Irradiation is the energy over time, in kWh/m² per year. It is the most important input to any output calculator, and it varies a lot by place.

What Drives Irradiance Differences

Latitude. Locations nearer the equator get more direct sunlight across the year and less seasonal swing. At high latitudes the winter sun is low and short-lived, so the same energy spreads over a larger area.

Cloud cover. Marine climates such as Seattle and London have persistent cloud. Desert climates such as Phoenix and Dubai have mostly clear skies. The PVGIS runs below show the effect in numbers.

Altitude. Thinner air at altitude lets more clear-sky light through. Denver, at roughly 1,600 m, has a yield close to Phoenix in the runs below even though it is far to the north.

Season. In the PVGIS monthly data, a 6 kWp system in Berlin makes about 208 kWh in January and 753 kWh in July, a ratio above 3.5. Singapore’s monthly output stays within a narrow band all year.

From Horizontal to Tilted Surface

Calculators do not use horizontal irradiance directly. They calculate plane-of-array (POA) irradiance, the energy on the tilted panels. POA depends on:

• Direct normal irradiance (DNI): beam radiation from the sun’s disk
• Diffuse horizontal irradiance (DHI): scattered radiation from the sky
• Ground-reflected irradiance: reflection from the surface below, called albedo
• Tilt angle: the panel’s angle from horizontal
• Azimuth angle: the compass direction the panel faces

A well-tilted, equator-facing array collects more than a horizontal one, especially in winter when the sun is low. The tables below use POA irradiation for that reason.

Panel Efficiency, Degradation and Temperature

Not all panels turn the same sunlight into the same energy. Efficiency, degradation rate and temperature coefficient shape long-term output, and the datasheet states each one.

What to Read on the Datasheet

Three module datasheet values feed an output calculator:

• Efficiency and rated power set how much energy the module makes from a given roof area.
• Temperature coefficient of power says how much output falls per °C above 25°C. A smaller number means better hot-weather output. See temperature coefficient for the definition.
• Power warranty and degradation rate set the output you can expect in later years. See the degradation rate glossary entry.

Use the manufacturer’s datasheet for the exact module in the quote. Do not carry numbers over from a different model.

Degradation: The Compounding Effect

Panels lose output over time. The table shows compound annual degradation. It is arithmetic on assumed rates, not a warranty promise.

Some manufacturers warrant a larger drop in year one and a smaller one afterward, and some warranties are linear. Read the power warranty in the datasheet and model it as written. A 25-year financial model that uses year-one production for every year overstates returns.

Bifacial Gain

Bifacial panels also collect light reflected from the ground or roof behind them. The gain depends on ground albedo, mounting height and row spacing, and it is usually larger for elevated ground-mount arrays over light surfaces than for standard rooftops. For a deeper treatment, see our bifacial solar panel design guide.

Temperature: Why Hot Climates Lose Production

Solar cells convert sunlight less efficiently as they heat up. The temperature coefficient of power says how much.

Power loss = (cell temperature - 25°C) x temperature coefficient

For an assumed coefficient of -0.38 percent per °C and a 70°C cell on a hot day: (70 - 25) x 0.38% = 17.1% power loss. At a 30°C cell, the same panel loses 1.9%. The coefficient is an illustration. Use your module’s datasheet.

PVGIS shows the effect on annual yield. Phoenix has 86 percent more annual irradiation on the panel plane than Berlin (2,458 against 1,322 kWh/m²), but only 72 percent more specific yield (1,824 against 1,059 kWh/kWp). Heat takes a share of the advantage. Singapore has 37 percent more irradiation than Berlin and 30 percent more yield.

Roof-mounted systems with a small air gap run hotter than ground-mounted ones, because the roof traps heat. A larger ventilation gap and a lower temperature coefficient both help in hot climates.

Shading and Orientation: The Silent Killers

Shading and poor orientation are common causes of underperforming systems, and both are often underestimated in quick calculations.

Shading Loss Mechanics

Shading is not proportional. In a standard string, panels are wired in series, so current through the string is limited by the weakest panel. A small shadow on one module can cut the output of the whole string. That is why shade is hard to guess and easy to model.

Ways to reduce the damage include bypass diodes in the modules, module-level power electronics such as power optimizers and microinverters, and separate strings or MPPT inputs for shaded and unshaded roof areas. Which one fits depends on the roof, the budget and the shade pattern.

Quantifying Shading Loss

Shading analysis needs a 3D model of the obstructions: trees, neighboring buildings, chimneys and HVAC equipment. The analysis yields a shading factor for each hour of the year. Different obstructions behave differently:

A professional shading analysis models each obstruction’s shadow path across the roof through the year. The useful output is a monthly shading profile, because winter losses are usually much larger than summer losses.

Orientation and Azimuth Effects

In the northern hemisphere, south-facing panels produce the most annual energy. PVGIS shows how fast output falls as the roof turns away. The table is for Denver, Colorado, at a 35 degree tilt.

Source: PVGIS 5.3 PVcalc, Denver (39.74 N, 104.99 W), 14 percent system loss, 35 degree tilt, November 2026.

East-west split arrays are common on gabled roofs and low-slope commercial roofs. At a 10 degree tilt in Denver, PVGIS gives about 1,433 kWh/kWp facing east and 1,486 facing west, an average about 10 percent below a south-facing array at the same tilt. At steeper tilts the penalty grows. East-west layouts can still make sense when morning and evening demand matters more than the annual total.

Tilt Angle: Finding the Optimal Geometry

Tilt and azimuth decide how well the panel surface lines up with the sun’s path. Calculators apply correction factors or simulate every hour.

Optimal Fixed Tilt by Location

PVGIS can optimize the fixed tilt for annual energy. The results show that the best angle tracks latitude at low latitudes and falls below latitude at higher ones.

Source: PVGIS 5.3 PVcalc with the optimal slope option and a south-facing azimuth, November 2026. Results can shift with the radiation database.

The old rule of thumb, tilt equals latitude, is a reasonable start near the tropics and for mid-latitudes. At high latitudes it over-tilts for annual energy. The curve is also flat near the optimum, so a few degrees either way costs little.

Tilt Loss for Common Roof Pitches

Most roofs are not at the optimal tilt. The table converts common roof pitches to angles and shows the PVGIS specific yield for a south-facing array in Denver.

Source: PVGIS 5.3 PVcalc, Denver, south-facing, 14 percent system loss, November 2026. Loss is measured against the best result in the table.

Standard roof pitches at mid-latitudes sit close to the optimum. Flat roofs need tilted racking, which adds cost and row spacing but recovers production.

Seasonal Production by Tilt

A steeper tilt raises winter output and lowers summer output. That matters for off-grid systems that must survive winter. Grid-tied systems usually do best with annual optimization. The table shows Denver at three tilts.

Source: PVGIS 5.3 PVcalc, Denver, south-facing, November 2026.

System Losses: Where the Energy Goes

The performance ratio bundles all the losses into one number. Knowing each category helps you diagnose underperformance.

What PVGIS Treats as System Loss

The PVGIS user manual describes the 14 percent default as covering “losses in cables, power inverters, dirt (sometimes snow) on the modules,” and ageing, and notes it can be lowered for very efficient equipment. Temperature and irradiance effects are modeled separately. The manual does not break the 14 percent into components, so treat it as a planning default, not a measurement.

Loss Categories to Check in Any Calculator

Inverter Losses and Clipping

Inverters are not perfectly efficient, and efficiency varies with load. Inverter clipping occurs when DC panel output exceeds the inverter’s maximum AC output. It happens in peak-sun hours. Sizing the DC array above the inverter rating is common practice, because the extra panels add energy in weaker light and lose only a little to clipping. The right ratio depends on the site and the equipment, so model it instead of copying a rule. See solar inverter sizing for background.

Soiling

Dust, pollen, bird droppings and pollution reduce the light reaching the cells. Soiling varies widely by climate. Dry, dusty and agricultural areas need more cleaning than areas with frequent rain, and steeper panels shed dirt better than flat ones. Use a site-specific soiling assumption and a cleaning plan, not a generic default.

Mismatch and Degradation Variation

No two panels are identical. In a string, the lowest-current panel limits the rest, and panels degrade at slightly different rates over time. Module-level power electronics reduce string mismatch losses.

Location-Based Output Tables

These tables show PVGIS estimates for a 6 kWp system. Implied PR is specific yield divided by POA irradiation. All rows use the same method, so the cities compare fairly.

United States

Source: PVGIS 5.3 PVcalc, tilt 0.76 x latitude + 3.1 degrees, south-facing, 14 percent system loss, queried November 2026. Radiation database: PVGIS-ERA5 for all US rows.

Europe

Source: PVGIS 5.3 PVcalc, same method as above. Radiation database: PVGIS-SARAH3 for all European rows.

Asia-Pacific, Middle East and Africa

Source: PVGIS 5.3 PVcalc, same method as above. Southern hemisphere rows face north. Databases: PVGIS-ERA5 and PVGIS-SARAH3 as PVGIS selects them for each location.

Monthly Production Profile Example

A 6 kWp system in Denver, Colorado, using the PVGIS monthly output for a tilt of 33 degrees facing south:

Denver’s seasonal swing is small, because winter skies are clear and the tilt favors the low winter sun. A cloudier site swings more. In the same PVGIS runs, a 6 kWp system in Seattle makes about 256 kWh in December and 848 kWh in July, and Berlin goes from about 186 kWh in December to 768 kWh in June. That swing is critical for off-grid battery sizing, and it explains why a system that looks fine in summer can underperform in winter.

How to Use a Professional Output Calculator

Simple online calculators give a ballpark. A professional workflow goes further.

Step 1: Define the Site

Input the exact location, not the nearest large city. A suburb a few dozen kilometers from the city center can have different irradiance because of altitude, coast or haze. Use latitude and longitude.

Step 2: Define the Array Geometry

Measure or model:

• Usable roof area: subtract setbacks, walkways and vents
• Tilt angle: measure it or derive it from the roof pitch
• Azimuth: measure with a compass app, corrected for magnetic declination
• Obstructions: photograph and measure trees, nearby buildings and chimneys

Step 3: Select Equipment

• Module model: the exact panel sets temperature coefficient, low-light behavior and degradation
• Inverter model: efficiency curve and clipping behavior differ by model
• Mounting system: ventilation affects cell temperature

Step 4: Run the Simulation

A solar design platform can produce, depending on the tool:

• Annual, monthly and hourly production
• A loss breakdown by category
• The performance ratio and the specific yield
• A multi-year production forecast with degradation

SurgePV supports energy-yield modeling and shading analysis. Results depend on source data, assumptions, equipment models and review, as with any tool.

Step 5: Validate Against Reality

For an existing system, compare the calculator output with actual production from the monitoring system. Gaps reveal missed shade, heavier soiling or equipment problems. A one-year comparison is the best way to correct your assumptions.

SurgePV’s losses panel lists the loss categories that feed the output estimate. Adjust each parameter to match site conditions.

Common Mistakes in Output Estimation

Even experienced designers make these errors.

Mistake 1: Using Nameplate Rating as Expected Output

A 6 kWp system does not make 6 kWh every hour of the day. It reaches rated power only at STC. Annual production is size times specific yield, and the yields in this guide run from roughly 1,000 to 1,800 kWh/kWp depending on location, not size times hours of daylight.

Mistake 2: Using One Rule of Thumb Everywhere

A flat “1,500 kWh per kWp” rule is wrong in both directions. Compared with PVGIS, it overstates Seattle (1,165) by about 29 percent and understates Phoenix (1,824) by about 18 percent.

Mistake 3: Ignoring Temperature in Hot Climates

Phoenix, Dubai and Delhi have high irradiation but lower implied PR than cooler sites. Use the module’s datasheet coefficient and a realistic mounting type. A textbook PR copied from a temperate climate will overstate hot-site output.

Mistake 4: Underestimating Soiling

“We get rain, so soiling is minimal” is not always true. Pollen, agricultural dust and urban pollution can leave films that light rain does not remove. Use a site-specific assumption and a cleaning plan.

Mistake 5: Assuming South-Facing When the Roof Faces East-West

A homeowner reads “south is best” and assumes full output. Their roof faces southeast and southwest. In the Denver table, a 45 degree turn costs about 10 percent, which can move payback by months.

Mistake 6: Using the Annual Average for Off-Grid Sizing

Off-grid systems must survive the worst month, not the average month. Use the lowest-production month as the design constraint.

Mistake 7: Forgetting Degradation in Long-Term Financial Models

A 25-year model that uses year-one production for every year overstates returns. Apply the module’s power warranty or a documented annual rate. A generation and financial tool can carry production through the cash-flow model.

What Most Guides Miss About Output Calculators

Most articles stop at the basic formula and a list of loss factors. Three nuances matter more.

Nuance 1: The Performance Ratio Is Not a Constant

Rules of thumb often quote one PR for every site. The PVGIS runs show otherwise. With the same 14 percent system loss, the implied PR is 0.74 in Phoenix, 0.77 in Singapore, 0.79 in Seattle, 0.80 in Berlin and 0.81 in London. Heat is the main difference. PR also falls with age, soiling and poor maintenance. Using one PR for every location and every year is a modeling error.

Nuance 2: Hourly Profiles Matter More Than Annual Totals

Two systems with the same annual kWh can have different value. A west-facing system produces more in the afternoon, which can matter under time-of-use tariffs. An east-facing system produces more in the morning. For self-consumption, the hourly profile decides how much solar is used on site and how much is exported. Annual kWh alone is not enough for an accurate ROI calculation.

Nuance 3: Satellite Irradiance Data Has Known Limits

Free calculators rely on satellite or reanalysis data such as PVGIS and the NSRDB. These datasets suit most locations, but local effects can be missed. Coastal fog, steep terrain that shades valleys and dense urban areas can all differ from the gridded data. For high-stakes projects, consider ground-station data or on-site measurement, and say which dataset was used.

Regional Comparison: Where Solar Output Varies Most

The PVGIS tables above group naturally into three bands.

Position in the table depends on climate, tilt and the dataset. It says nothing about whether solar is a good investment at a particular site, which depends on electricity prices, incentives and installed cost.

The Tradeoff: Accuracy vs. Speed

Output tools range from quick estimates to engineering-grade simulations. More inputs and more careful data take more time and give a more defensible number.

For residential installers, the practical sweet spot is a design tool that handles roof geometry and shading without days of manual work. See how PVsyst alternatives compare if you are choosing between tools.

Conclusion

Solar panel output calculators are physics models. They convert sunlight data, panel specifications and system geometry into energy estimates. Quality depends on the inputs and the model.

Three principles for accurate output estimation:

1. Use location-specific weather data, not rules of thumb. In the PVGIS runs, a 6 kWp system in Phoenix makes 57 percent more than the same system in Seattle. A flat national average misleads both installers and customers.
2. Model losses from their causes, not from a single performance ratio. Temperature, soiling, shading and inverter behavior interact. Hourly simulation with explicit loss categories captures that.
3. Validate against metered production. Every installed system is a calibration opportunity. Compare the prediction with monitored output and fix the assumption that was wrong.

For homeowners checking quotes, ask the installer for the specific yield (kWh/kWp/year), the performance ratio and the weather dataset. Compare the yield with the tables above. For installers, book a SurgePV demo to see energy-yield modeling and shading analysis in one workflow.

Tools and Further Reading

• Solar design software
• Customer-ready solar proposals
• Production estimate assumptions to standardize
• Solar glossary

Frequently Asked Questions

How does a solar panel output calculator work?

A solar panel output calculator multiplies system size in kWp by the annual irradiation on the tilted panels and by a performance ratio that captures temperature, inverter, wiring, soiling and other losses. For Phoenix, PVGIS gives about 2,458 kWh/m² on the panel plane and an implied performance ratio of 0.74, so a 6 kWp system produces about 10,900 kWh a year.

How much do solar panels produce per month?

Monthly output follows the seasons and the climate. PVGIS gives a 6 kWp system in Berlin about 208 kWh in January and 753 kWh in July, while the same system in Phoenix makes about 822 kWh in January and 897 kWh in July. Calculators use monthly or hourly weather data instead of dividing the annual total by 12.

What factors reduce solar panel output?

The main factors are heat, shading, soiling, inverter and wiring losses, module mismatch, orientation and tilt, and long-term degradation. PVGIS applies a default 14 percent system loss for cables, inverters, dirt and ageing, and models temperature and irradiance effects separately. Shading and orientation depend on your roof and are the easiest to check.

How accurate are solar panel output calculators?

Accuracy depends on the weather dataset, the loss assumptions and how well the tilt, azimuth and shading are entered. No single accuracy figure applies to every tool. Check any estimate against the first year of metered production and update the assumptions that were off.

What is a good performance ratio for solar?

There is no universal good value, because performance ratio depends on climate, mounting and maintenance. In the PVGIS runs used in this guide, the implied ratio is 0.74 in Phoenix and 0.81 in London at the same 14 percent system loss. Judge a real system by its measured ratio over a full year.

What is the difference between kWp and kWh in solar?

kWp is the rated peak power of a system under standard test conditions of 1,000 W/m² irradiance and 25°C cell temperature. kWh is the energy produced over time. A 6 kWp system produces about 10,900 kWh a year in Phoenix but about 7,000 kWh in Seattle, and the ratio of annual kWh to kWp is the specific yield.

What is the best tilt angle for solar panels?

For a fixed south-facing array in the northern hemisphere, PVGIS optimal tilt is close to latitude at low latitudes and lower than latitude at high ones. For example, it gives about 34 degrees in Phoenix, 40 in Denver and 40 in Berlin. Use PVGIS or your design software to find the best angle for your site.