Solar & battery
Solar Peak Sun Hours Calculator
Works peak sun hours, which is a solar resource figure rather than hours of daylight.
Result
Peak sun hours are not daylight hours. They are the day's total solar energy expressed as hours at 1,000 W per square metre, so a long overcast day can be worth fewer peak sun hours than a short bright one. Use a figure for your own location and month rather than a national average. Seasonal factors here are broad and swing far more at high latitudes: a northern site can see winter production at a third of summer rather than 60%. Use measured data for your own location and tilt.
Get a Free Business AuditHow the calculation works
Peak sun hours is the day's total solar energy on the array plane expressed as the equivalent number of hours at full test irradiance. It is not daylight hours: a long summer day with weak morning and evening sun yields fewer peak sun hours than the daylight length suggests, which is what makes the figure useful for production arithmetic.
What moves the number
- Latitude and season.
- Array tilt and azimuth, which change the plane of incidence.
- Local climate, particularly cloud cover.
- Shading, which removes hours selectively.
Where this one goes wrong
Use a figure for the array's actual plane rather than a horizontal one, because tilt and orientation change the resource substantially, and published tools let you specify both. A regional average also hides microclimate: coastal fog, valley cloud and local terrain shading can move the real figure well away from the published regional value, which is why on-site assessment matters for anything beyond a first estimate.
Before you quote
- Use a figure for the array plane, not horizontal.
- Check local microclimate against the regional average.
- Assess shading on site rather than from imagery alone.
Where the rules come from
Questions
About this calculator
Is peak sun hours the same as hours of daylight?
No. It is the day's solar energy expressed as equivalent hours at full test irradiance, so a long day with weak morning and evening sun yields fewer peak sun hours than its daylight length. That is exactly what makes it useful for production arithmetic.
Does tilt change peak sun hours?
Yes, because the figure depends on the plane the energy falls on. A horizontal figure and a tilted-array figure differ meaningfully, and published tools let you specify tilt and azimuth, which is worth doing rather than using a generic number.
How reliable are regional averages?
Reasonable as a starting point and capable of being well off locally, because coastal fog, valley cloud and terrain shading all vary within a region. On-site shading assessment is what separates an estimate from a design.
The formula
peak sun hours = daily irradiance in kWh per square metre x seasonal factor
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How to use this calculator
- Daily irradiance (kWh per square metre). Enter your own figure; the page starts at 4.5.
- Season adjustment. Enter your own figure; the page starts at Annual average.
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- System derate (%). Enter your own figure; the page starts at 80%.
The result updates as you type, so you can try a figure, see what it does to peak sun hours, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Daily irradiance (kWh per square metre) | 4.5 |
| Season adjustment | Annual average |
| System size (kW DC) | 7.2 |
| System derate | 80% |
What comes out
| Result | Value |
|---|---|
| Peak sun hours main result | 4.5 |
| Production at that figure | 25.9 |
| Per month | 778 |
| If it held all year | 9,461 |
4.5 peak sun hours a day, which for a 7.2 kW system is 25.9 kWh daily and about 778 kWh a month.
How sensitive the answer is
Holding everything else steady and changing only daily irradiance (kwh per square metre), this is what happens to peak sun hours. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Daily irradiance (kWh per square metre) | Peak sun hours |
|---|---|
| 2.25 | 2.3 |
| 3.38 | 3.4 |
| 4.5 example above | 4.5 |
| 6.75 | 6.8 |
| 9 | 9.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Daily irradiance (kWh per square metre)
- A number, between 0 and 12, starting at 4.5.
- Season adjustment
- Choose from Annual average, Summer (about 130%), Winter (about 60%), Spring or autumn (about 100%).
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
What the results tell you
| Result | What it is |
|---|---|
| Peak sun hours | The headline result. A decimal figure. |
| Production at that figure | A decimal figure. |
| Per month | A number. |
| If it held all year | A number. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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