How to Calculate Solar Panel Output

Keep these concepts separate:

1. Panel rating (Wp) — power under standard test conditions.
2. System size (kWp) — the sum of the panel ratings.
3. Solar resource — how much sunlight reaches the panels at your site, for their orientation and tilt, over a year.
4. Losses — temperature, shading, soiling, cables, inverter and ageing.
5. Energy output (kWh) — what the system actually delivers.

The rest of this guide follows that chain. The Solar hub collects the related tools and guides.

A kilowatt (kW) is a rate of energy flow — power. A kilowatt-hour (kWh) is an amount of energy: 1 kW sustained for 1 hour. Your electricity bill is in kWh; a solar system's size is in kW.

The nameplate capacity (kWp, "kilowatt-peak") is the DC power the panels would produce at standard test conditions. On a real roof the instantaneous output moves continuously with the sun, clouds and temperature, and is often below the nameplate figure. Annual energy is the sum of that varying power over every hour of the year, which is why two identical 4 kWp systems in different places — or on different roofs of the same house — produce different kWh.

Two more capacity numbers can appear on a quote:

• Inverter AC rating (kW) — the maximum AC power the inverter will deliver. It is often lower than the panels' kWp. When the panels could produce more than the inverter's limit, the excess is "clipped"; the PVWatts and PVGIS tools and installer software can model this.
• Export limit — some grid connections cap how much power may be exported at any moment. This is a local grid rule, not a property of the panels.

Panels are rated in watts-peak (Wp) at standard test conditions (STC): 1,000 W/m² of irradiance, a cell temperature of 25 °C and the AM1.5 reference solar spectrum defined in IEC 60904-3. Real conditions are rarely exactly that, so the rating is a reference for comparing panels, not an output prediction. Read the rating, the power tolerance, the temperature coefficient and the degradation warranty from the panel's datasheet.

System size (kWp) = Number of panels × Panel rating (Wp) ÷ 1,000

Illustrative: 12 panels × 400 Wp = 4,800 Wp = 4.8 kWp.

Many datasheets also give performance at NMOT or NOCT — a lower irradiance and a higher, more realistic cell temperature. Those figures are closer to normal operation than STC but are still test conditions, not a site estimate.

Two related terms are easy to confuse:

• Irradiance (W/m²) — the instantaneous power of sunlight on a surface.
• Irradiation (kWh/m²) — irradiance accumulated over time, for example per day or per year.

Weather data is usually published as irradiation on a horizontal surface. Panels are tilted and face a particular direction, so what matters is irradiation on the plane of the array. Converting from horizontal to plane-of-array depends on the sun's path at your latitude and the split between direct and diffuse light — which is what the official tools do for you:

• PVGIS — European Commission Joint Research Centre; covers Europe, Africa, much of Asia and the Americas using satellite-based solar radiation data.
• PVWatts — US National Laboratory of the Rockies (formerly NREL); estimates the output of grid-connected PV systems for locations worldwide using typical-year weather data.

Both return a modelled annual and monthly output for the location, orientation, tilt and losses you enter. They model a typical year; any individual year can be sunnier or duller.

Orientation (azimuth) is the compass direction the panels face. Tilt is their angle from horizontal. Panels collect most energy over a year when they face the equator (south in the northern hemisphere, north in the southern) at a tilt related to latitude, but the best combination and the penalty for deviating from it are site-specific — they depend on latitude and on the local mix of direct and diffuse light.

Rather than applying a general percentage, test your own roof:

1. Enter your location in PVGIS or PVWatts.
2. Run it with your actual azimuth and tilt.
3. Run it again with the tool's optimised or equator-facing setting.
4. The difference between the two annual results is the orientation and tilt effect for your site.

East–west layouts spread production more evenly across the day than a single equator-facing plane. Whether that matters depends on when you use electricity — an economic question covered in the solar payback guide.

Shading from chimneys, trees, neighbouring buildings or the horizon reduces output, and its effect can be larger than the shaded area suggests. Cells in a panel are wired in series, so a shaded cell limits the current of the cells around it; bypass diodes in the panel limit the damage by skipping the affected section. Panels are themselves usually wired in series strings, so shading on one panel can reduce the whole string unless module-level electronics (power optimisers or microinverters) are used.

How to account for it:

• PVGIS uses a terrain horizon by default and lets you upload your own horizon profile.
• PVWatts lets you include shading in its system-loss input.
• An installer should provide a shading analysis for your roof — ask what tool and data they used, and how many hours or which months are affected.

Shading that only occurs in winter, when the sun is low, matters less for annual kWh than shading in summer, but it can matter more if winter output is your concern.

Crystalline silicon panels produce less power as cell temperature rises above 25 °C. On a sunny day cells run well above air temperature, so the loss is real even in a mild climate; cold, bright days can produce more power than hot ones at the same irradiance. The panel datasheet gives the temperature coefficient of power (Pmax) in %/°C.

Between the panels and your meter, more energy is lost:

• Inverter conversion — DC to AC. The inverter datasheet gives its efficiency; a weighted figure (European or CEC efficiency) reflects part-load operation better than the peak figure.
• Cables and connections — resistance losses, smaller with shorter runs and thicker cable.
• Soiling — dust, pollen, bird droppings and snow.
• Mismatch — small differences between panels in the same string.
• Clipping — energy lost when the panels could produce more than the inverter's AC limit.

PVWatts and PVGIS each ask for a combined system-loss percentage. PVGIS offers a default of 14% and PVWatts a default of about 14% (as shown in their input forms in October 2026). That default is a modelling starting point, not a measurement of your equipment; an installer using your actual inverter, cable lengths and site conditions may use a different figure.

Where a source quotes a performance ratio (PR), it bundles temperature and system losses into one factor: the ratio of actual output to what the same array would produce at STC efficiency under the same irradiation.

The most direct way to estimate annual energy is to multiply system size by the specific yield — kWh produced per kWp per year at your site. Specific yield already contains the solar resource, orientation, tilt, shading and losses, so it is the one number that summarises all of the sections above. Take it from a PVGIS or PVWatts run for your roof, or from an installer's quote (ask how it was calculated). Do not use a national or "typical" figure.

The table holds the system size fixed at 4.8 kWp and varies only the specific yield, to show how strongly the site drives output. The yields are illustrative inputs chosen to span a wide range; they are not values for any named country or city.

Some sources give the solar resource as peak sun hours (PSH): the daily plane-of-array irradiation expressed as hours at 1,000 W/m² (so 3 PSH = 3 kWh/m² per day). Because STC also uses 1,000 W/m², each kWp produces about PSH kWh per day before losses; a performance ratio then accounts for losses.

Output is not spread evenly through the year. Summer months produce more than winter months, and the gap grows with distance from the equator because winter days are shorter and the sun is lower. Cloud patterns also vary by season and region.

This matters whenever the question is about a particular period rather than the year:

• Winter coverage — how much of your winter electricity solar can supply.
• Matching a load — for example heat pump use (mostly winter) versus air conditioning (mostly summer).
• Checking performance — a low-output December is expected; compare each month with its own modelled figure, not with one twelfth of the annual total.

Both PVGIS and PVWatts report modelled output month by month. Use those monthly figures for any seasonal question.

Panel output declines slowly with age. The figure to use for your own panels is the rate guaranteed in the manufacturer's performance warranty. For context, a review by NREL researchers of about 2,000 published degradation measurements reported a median rate of about 0.5% per year, with wide variation between technologies, climates and studies (Jordan & Kurtz, Photovoltaic Degradation Rates — An Analytical Review, 2012). That is a summary of field studies, not a promise for any product.

To size a system for a target annual output, reverse the specific-yield formula:

System size (kWp) = Target annual output (kWh) ÷ Specific yield (kWh/kWp/year) Number of panels = System size × 1,000 ÷ Panel rating (Wp), rounded up

Illustrative: 4,000 kWh ÷ 950 = 4.21 kWp; 4,210.5 ÷ 400 = 10.5 → 11 panels = 4.4 kWp, producing about 4.4 × 950 = 4,180 kWh/year.

Roof area, layout, shading, fire-access rules and any grid connection limits constrain system size in practice. Producing as many kWh per year as you consume does not mean all of it is used at home — generation peaks around midday and in summer.

• Quote check — divide the quoted annual kWh by the quoted kWp to get the installer's assumed specific yield, then compare it with a PVGIS or PVWatts run for the same location, orientation, tilt and loss setting. A large gap is a reason to ask how the figure was calculated.
• Installed-system check — read a full year of generation from the inverter or generation meter and divide by kWp. Compare month by month with the modelled figures, remembering that real weather differs from a typical year.
• Sudden drops — a fall in output compared with the same month last year can indicate soiling, new shading, a failed string or an inverter fault.

Output in kWh is the input to any savings estimate, but its value depends on how much you use yourself, your tariff and any export payment. The solar payback guide covers that economic calculation, and the Solar Payback Calculator takes your annual output (or system size × specific yield) as its starting input. The methodology page documents the calculator formulas and data sources.