Solar Panel Output Calculator

How much an array you already have, or have been quoted, will actually produce. This runs the opposite way to the system size calculator: start with panels and get kilowatt-hours. Cell temperature is treated as its own derate rather than folded into one blended loss figure, because it is the loss that varies most between a January morning and a July afternoon.

Reverse the compass points in the southern hemisphere.

Off-grid systems live or die on the winter figure.

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How It Works

The formula

Array kW = Panel W × Panel count ÷ 1000
Temp derate = 1 + coefficient × (Cell °C − 25)
Daily kWh = Array kW × Sun hours × (1 − loss) × Temp derate × Tilt factor

The system loss figure here defaults to 0.14, and deliberately excludes temperature. That is the important difference from the 0.23 blended figure the system size calculator uses. Applying 0.23 here as well as a separate temperature derate would count the same loss twice and understate output by around 8%. The 0.14 covers inverter conversion, DC and AC wiring, soiling, shading, module mismatch and connection losses; temperature is handled on its own line where it belongs.

Worked example

Ten 400 W panels at 4.5 peak sun hours, 14% system loss, a -0.35%/°C coefficient, a 45°C cell temperature and an unshaded south-facing optimum tilt:

  • Array = 10 × 400 ÷ 1000 = 4 kW
  • Temperature derate = 1 + (-0.0035 × 20) = 0.93, so 7% is lost to heat alone
  • Combined factor = 0.86 × 0.93 × 1.00 = 0.80
  • Daily output = 4 × 4.5 × 0.80 = 14.4 kWh
  • Monthly = 438 kWh, annual = 5,255 kWh
  • Specific yield = 5,255 ÷ 4 = 1,314 kWh per installed kW per year
  • Each panel contributes about 1.44 kWh a day

The specific yield figure is the one to compare against other quotes and other sites, because it is independent of array size. Anything from roughly 900 kWh/kW (northern Europe, imperfect orientation) to 1,600 kWh/kW (US Southwest, ideal tilt) is normal; a quote claiming much more than that is either assuming a better site than yours or has not applied its losses.

Winter is a different calculation, not a smaller one

Two things change together in December and they pull in opposite directions. Peak sun hours collapse, often to under half the annual average, while cell temperature falls, which makes the panels slightly more efficient. In the example above the winter day produces 7.24 kWh against 14.4 kWh on the annual average, which is 50% rather than the 44% the sun hours alone would suggest: the cold recovers about six points of it. That still leaves an off-grid system needing to survive on half its average output for months, which is why off-grid arrays are sized on December and grid-tied ones are sized on the annual total.

Tilt and Orientation Factors

Output relative to a due-south array at the optimum tilt for its latitude, which is roughly latitude minus 10 degrees for annual yield. Reverse the compass points in the southern hemisphere; the tilt angles are unchanged. These are annual figures, and the seasonal picture differs: a steeper array loses annually but gains in winter, which is often the right trade for an off-grid system.

Orientation and tilt Factor Notes
South, optimum tilt1.00The reference case
South-east or south-west0.96Barely worth worrying about
South, steep (60 degrees)0.93Loses annually, gains in winter, sheds snow
South, flat (0 degrees)0.88Good in summer, poor in winter, collects dirt
East or west0.83An east and west split gives a flatter daily curve
Vertical south-facing wall0.70Surprisingly good in winter at high latitudes
North-east or north-west0.68Only worth it if the roof space is free
North, optimum tilt0.58A last resort in the northern hemisphere

An east-west split deserves more attention than its 0.83 factor suggests. It produces less in total but spreads the output across the morning and the evening rather than concentrating it at noon, which raises the share you consume yourself rather than export. On a modern export tariff that shift is often worth more than the 17% of production it costs, which the solar payback calculator will show directly if you compare the two cases.

Frequently Asked Questions

How much energy does one solar panel produce a day?

Multiply the panel's rating in kilowatts by your peak sun hours and by a combined loss factor of about 0.8. A 400 W panel at 4.5 peak sun hours produces roughly 0.4 times 4.5 times 0.8, which is 1.44 kWh a day, or about 525 kWh a year. That figure moves by a factor of two between a poor northern site with bad orientation and a good southern one, so it is worth using your own sun hours rather than a national average.

Why does a 400 W panel never produce 400 W?

The rating is measured at Standard Test Conditions: 1000 W per square metre of irradiance, a 25°C cell temperature and a specified light spectrum. In use, the cells sit 20 to 35 degrees above the surrounding air because they are absorbing sunlight, so a panel on a hot roof runs at 45 to 65°C and loses 7 to 14% to temperature alone before wiring, inverter and soiling losses are counted. Seeing 300 W to 330 W from a 400 W panel at noon in summer is completely normal.

What cell temperature should I enter?

For an annual average on a roof-mounted array, 45°C is a reasonable figure in a temperate climate and 50 to 55°C in a hot one. Ground-mounted and pole-mounted arrays run cooler because air circulates behind them, so 40°C is fairer. For a specific hot afternoon, take the air temperature and add 25 to 30 degrees. For the winter case, use something close to the ambient temperature, since there is far less sunlight to heat the cells.

Why is the loss figure here 14% when other calculators use 23%?

Because this calculator applies temperature separately. The PVWatts default of around 23% is a single blended figure that already contains a typical temperature loss, which is convenient when temperature is not modelled and misleading when it is. Using 23% here alongside an explicit temperature derate would deduct the same loss twice and understate output by roughly 8%. If you would rather use one blended figure, set the temperature coefficient to 0 and put 0.23 in the loss field.

What is specific yield and what is a good number?

Specific yield is annual production divided by installed capacity, in kWh per kW per year, and it lets you compare sites and quotes without array size getting in the way. Typical values run from about 900 in northern Europe with imperfect orientation, through 1,100 to 1,300 across most of the temperate world, to 1,500 or 1,600 in the US Southwest and inland Australia. If a quote implies much above the range for your latitude, ask what loss assumptions produced it.

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