Watt-peak (Wp) is the rated power of a solar module under standardised laboratory test conditions – not the power it delivers in everyday use on your roof. The sum of all modules gives the system size in kilowatt-peak (kWp). The actual annual yield in kilowatt-hours (kWh) additionally depends on location, orientation and weather.
Key points in brief
- Wp measures output under standard test conditions (STC) – a laboratory comparison value, not an everyday promise.
- kWp describes the system's size, kWh the result over a year – the most common mix-up of all.
- On the Swiss Plateau, 1 kWp delivers roughly 900 to 1,050 kWh per year, depending on location and orientation.
- Quotes are sensibly compared per kWp, not per module or per piece.
What does watt-peak actually mean?
Watt-peak is a unit of measurement for a solar module's rated power. It's determined under so-called standard test conditions (STC): irradiance of 1,000 watts per square metre, a cell temperature of 25 degrees Celsius and a defined light spectrum. That's a laboratory value that lets modules from different manufacturers be compared fairly – comparable to a car's standard fuel consumption, which doesn't exactly match reality on the motorway either.
A 430 Wp module therefore delivers 430 watts under exactly these laboratory conditions. On the roof, the angle of irradiance, temperature and weather almost always deviate from these conditions. That's normal and no cause for concern – the figure on the data sheet remains the most important yardstick for comparing modules and offers at all.
What's the difference between kWp and kWh?
kWp describes the size of the system, kWh the result over a certain period – that's the core difference most homeowners initially mix up. An 8 kWp system simply consists of enough modules for their added-up rated power to come to 8 kilowatt-peak.
The car analogy helps here: kWp is like a vehicle's horsepower – a fixed technical quantity describing what is maximally possible. kWh, by contrast, are like the kilometres driven: the result that actually accumulates over time and depends on driving style, route and weather. A system with more kWp does not automatically mean more yield, it merely has the greater potential for it.
In practice that means: if two neighbours both own a 6 kWp system, their annual yields in kWh can still turn out different – depending on roof orientation, shading and tilt. The kWp figure alone therefore says nothing about the effective yield, it only describes the installed size.
How much yield does one kWp deliver on the Swiss Plateau?
As a rough rule of thumb, 1 kWp on the Swiss Plateau delivers about 900 to 1,050 kWh per year, depending on location, roof orientation, tilt angle and shading. That's an empirical value from many completed systems and not a guarantee for a specific project.
A south orientation with optimal tilt tends to sit in the upper part of this range, an east-west orientation rather below. Anyone who wants to know the exact expected yield for their roof needs an individual analysis based on location, shading and roof geometry – blanket values from the internet don't replace that. The relationships in detail: Yield per kWp in Switzerland.
| Term | What it measures | Typical use |
|---|---|---|
| Wp | rated power of a single module under laboratory conditions (STC) | comparing individual module types |
| kWp | summed rated power of the entire system | describing system size, basis for quotes |
| kWh | energy actually generated over a period | annual yield, electricity bill, self-consumption |
| kWh per kWp | yield relative to system size | comparing the yield quality of different locations |
Classification from practice, as of: July 2026 — values apply to the Swiss Plateau and vary by canton, altitude and weather year.
Why are quotes compared per kWp rather than per module?
Because the number of modules alone says little – only the price per kWp makes quotes truly comparable. Two systems with the same number of modules can have completely different total outputs if the module types differ in strength.
We advise consistently dividing the total price by the kWp figure for every quote and comparing that value between providers. That makes differences in module quality, inverter capacity and installation effort more transparent than a pure unit-price comparison. The guideline values: Cost per kWp.
In our consultations in the Zurich region, the question "Why does my app never show the full kWp figure?" comes up particularly often. Usually a glance at the current irradiance and temperature is enough to explain the deviation comprehensibly – as a rule there is no defect.
Frequently asked questions
Why does my system never reach the kWp figure on the data sheet?
The kWp figure only applies under laboratory test conditions with exactly 1,000 W/m² irradiance and 25 °C cell temperature. In everyday use, sun angle, temperature and weather almost always deviate from that. That's normal and no sign of a defect in your system.
How many modules make 1 kWp?
Current modules in house-roof format typically deliver roughly 430 to 500 Wp. So about two to three modules are enough to arrive at 1 kWp arithmetically. The exact number depends on the chosen module type and its data sheet.
Is more Wp per module automatically better?
Not automatically. A module with a higher Wp figure uses the available roof area more efficiently, but can also be more expensive. With limited area, the stronger variant often pays off; with plenty of space, the cheaper option sometimes works out just as well.
What's the difference between kW and kWp?
kW is a general unit of power, kWp specifically the rated power of a solar system measured under test conditions. In photovoltaics kWp is practically always used, because it makes clear that it's the standardised comparison value.
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Sources: EnergieSchweiz, Swissolar, manufacturer documentation.
Last updated: 9 July 2026 · Author: ecoEn editorial team
