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Solar

Peak sun hours, and what they actually tell you

A peak sun hour is not an hour of daylight. It is a unit of energy pretending to be a unit of time.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

Rooftop solar panels catching strong low-angle evening sunlight, with long shadows across the roof tiles

Quick answer

A peak sun hour is not an hour of daylight. It is a quantity of energy — one kilowatt-hour of solar irradiation per square metre — expressed as if it had arrived at the standard test intensity of 1,000 W/m².

Its usefulness is that it makes the yield calculation trivial: system size in kWp × peak sun hours × system efficiency = kWh produced. Its danger is that people read it as clock time and conclude that a location with 16 hours of summer daylight must have 16 peak sun hours.

How to read the numbersSourced valueYour inputCalculated result
1 peak sun hour
1 kWh/m²
Sourced value
System
6 kWp
Your input
At 3.0 PSH, 85% losses
15.3 kWh/day
Calculated result

What the unit actually is

Solar panels are rated in kilowatts-peak (kWp) under standard test conditions: 1,000 W/m² of irradiance, 25 °C cell temperature, and a defined light spectrum. That 1,000 W/m² reference is the key to the whole unit.

Sum the irradiation falling on a square metre across a whole day and you get a figure in kWh/m². Divide by the 1 kW/m² reference and the units cancel into hours. A day totalling 4.5 kWh/m² is a day of 4.5 peak sun hours, whether that arrived as nine hours at half intensity or four and a half at full intensity.

  1. peak sun hours = daily irradiation (kWh/m²) ÷ 1 kW/m²
  2. daily output (kWh) = system size (kWp) × PSH × performance ratio
  3. annual output (kWh) = system size (kWp) × annual PSH × performance ratio
  4. kWh per kWp per year = annual PSH × performance ratio

The performance ratio bundles temperature, inverter, wiring, soiling and mismatch losses. 0.80–0.85 is typical for a well-installed unshaded system.

Typical values by region

The annual yield column is the number worth remembering: it already includes typical system losses and is what installers and modelling tools quote as kWh per kWp per year.

Typical annual average peak sun hours and specific yield by region
RegionAnnual average PSH per dayTypical kWh per kWp per year
Northern Europe (Oslo, Edinburgh)2.2 – 2.8750 – 950
Central Europe (Berlin, Paris)2.6 – 3.2900 – 1,100
Southern Europe (Madrid, Rome)4.0 – 4.81,350 – 1,650
US Northeast (Boston, New York)3.5 – 4.21,150 – 1,400
US Southwest (Phoenix, Las Vegas)5.5 – 6.51,700 – 2,000

Worked example: a 6 kWp roof in central Europe

  1. System size6 kWpYour input
  2. Annual average PSH3.0 per daySourced value
  3. Performance ratio0.85Site default
  4. Daily average output6 × 3.0 × 0.85 = 15.3 kWhCalculated result
  5. Annual output15.3 × 365.24 = 5,588 kWhCalculated result
  6. Specific yield5,588 ÷ 6 = 931 kWh/kWpCalculated result
Modelled annual production≈ 5,600 kWh

Peak sun hours depend on the plane, not the place

Published PSH figures are usually for a horizontal surface or for an optimally tilted one, and the two differ materially. A tilted, south-facing plane in the northern hemisphere collects more than horizontal; an east–west split array collects less at midday but spreads production across the day, which often suits self-consumption better even at a lower annual total.

If you are comparing quotes, check that everyone is quoting yield for the same plane. A modelling tool that takes tilt and azimuth as inputs, such as PVGIS or PVWatts, removes the ambiguity.

Model your own array

Enter system size, tilt, azimuth and losses to get modelled annual and monthly production, with the modelling range shown rather than a single false-precision number.

Open the solar output calculator

Assumptions and limitations

  • Regional PSH ranges are broad typical values covering a wide area. Local climate, elevation and coastal effects move them, and only a location-specific dataset is authoritative.
  • A performance ratio of 0.85 assumes no shading. Partial shading of even one module string can cost far more than every other loss combined.
  • Peak sun hours describe the resource, not your consumption. Production that arrives when you are out and cannot be stored is worth the export price, not the retail price.

Sources

  1. PVGIS — Photovoltaic Geographical Information System — European Commission Joint Research Centre

    Supports: Location-specific irradiation and modelled PV yield in kWh/kWp per year.

  2. PVWatts Calculator and National Solar Radiation Database — U.S. National Renewable Energy Laboratory

    Supports: Peak sun hours as daily irradiation in kWh/m², and modelled system output.

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