What is Wp (watt-peak)? How to convert to kWh
4 min read below · SolarFast knowledge base
Wp (watt-peak) is the peak output of a solar panel under standard test conditions. How to convert Wp to kWh per year and what to check in a quote.
Watt-peak (Wp) is the maximum power of a solar panel under standard test conditions (STC). It tells you how strong a module is, not how many kilowatt-hours (kWh) you generate per year.
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What does watt-peak mean?
Watt-peak is the power a solar panel delivers under standard test conditions (STC): 1,000 watts per square metre of light and a cell temperature of 25 degrees. Manufacturers measure every type under the same conditions so you can compare panels fairly. A 450 Wp module is stronger than a 400 Wp one.
On your roof a panel rarely reaches that peak value. Shade, angle, temperature and cloud cover determine the real result. Wp is a comparison measure, not a promise about yearly output. How much to expect per roof orientation is shown by Milieu Centraal with fixed assumptions per direction and tilt.
From watt-peak to kilowatt-hours
Wp is about power, kilowatt-hours (kWh) about energy generated over time. In the Netherlands a system produces roughly 0.85 to 0.9 kWh per Wp per year. That is a rule of thumb for a pitched south-facing roof; your actual yield depends on orientation, shade and weather.
| Power per panel | Yield per year (indicative) |
|---|---|
| 400 Wp | approx. 340 to 360 kWh |
| 435 Wp | approx. 370 to 390 kWh |
| 450 Wp | approx. 380 to 405 kWh |
Rule of thumb 0.85 to 0.9 kWh per Wp, south-facing; yield per panel is indicative. Common panels now sit around 435 Wp. More examples: solar panel yield.
Ten 400 Wp panels make 4,000 Wp together, or 4 kWp. Multiply that total wattage by 0.85 to 0.9 for a first estimate of your yearly output in kWh.
How much watt-peak per solar panel?
Common monocrystalline panels in 2026 often sit between 400 and 450 Wp. Higher watt-peak per unit means you need fewer panels for the same total wattage on your roof. That helps when roof space is limited.
More Wp per module does not automatically mean a better deal. Also look at price per watt-peak, warranty, inverter and how the panel fits your roof. In a quote we always compare the full system, not just the number on the panel.
Why it matters in your quote
On a small roof, high power per panel helps: you need fewer units for the same kWp. On a large roof, price per watt-peak often weighs more than the highest Wp figure. It is about the combination of power, yield and what you use yourself.
We pick the wattage that fits your roof, use and meter cabinet. Want to know how much watt-peak you need? Start with your yearly use in kWh on your annual statement and read more on our page about yield and return, or request a quote via solar panels.
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Solar panel annual yield: kWh and benefits
Annual solar panel yield: count on 0.85 to 0.9 kWh per watt-peak. How to calculate it, what it saves on your bill, and the financial benefits.
How many solar panels do I need? The calculation
Work out in three steps how many solar panels you need: yearly use, watt-peak and what fits on your roof. With worked examples from real installations.
What is a kWh (kilowatt-hour)?
A kilowatt-hour (kWh) is the energy a 1,000-watt appliance uses in one hour. How to convert watts to kWh and read your use on your annual statement.
Frequently asked questions
Is more watt-peak always better?
Not necessarily. On a small roof, high power per unit helps. On a large roof, price per watt-peak often matters more. It is about the combination of power, yield and price.
What is the difference between Wp and kWh?
Watt-peak is power (how strong a panel is), kilowatt-hour is energy over time (how much you generate). The rule of thumb of 0.85 to 0.9 kWh per Wp converts Wp into kWh per year.
What is the difference between Wp and kWp?
Wp is the power of one panel. kWp (kilowatt-peak) is the total of all your panels together. Ten 400 Wp panels are 4 kWp.
How much watt-peak do I need?
That depends on your yearly use in kWh. Check your annual statement for how much electricity you use and divide by 0.85 to 0.9 for a first estimate. Our solar yield page has concrete calculation examples.
Does my panel really reach its watt-peak?
Rarely, because STC test conditions are ideal. Wp is a comparison measure between panels. You estimate yearly output with the rule of thumb, not the peak value on the label.
We apply this every day
The same knowledge you're reading here, we put to work for households across the Netherlands.