Renewable Energy · Basic level · 3 min read ·
What is solar radiation or irradiance?
Irradiance is the solar power reaching a surface at a given moment, measured in W/m². Added up over a day it is called irradiation and is expressed in kWh/m² per day, or in peak sun hours. It is the starting point for estimating how much energy a solar system will produce.
In 30 seconds
- Irradiance measures the solar power reaching a surface, in W/m².
- Added up over a day it is called irradiation, in kWh/m² or peak sun hours (PSH).
- Projections use historical series, not a single day's reading.
Why should you care?
Two identical systems produce differently depending on where they are. Irradiance explains that difference.
- You estimate how much energy panels would produce where you are.
- You understand why a solar project's location matters so much.
- You read a project's generation projection with judgment.
Think of it as rain and a tank
Think of the panels as a tank that collects rain. The tank's size is the installed capacity; the rain that falls is the irradiance. A large tank in a dry place collects less than the same tank where it rains a lot.
How does irradiance work?
- 1
The sun delivers power
At any moment, irradiance says how many watts reach each square meter.
- 2
It adds up over the day
A full day's energy is called irradiation, in kWh/m²; 1 PSH equals 1 kWh/m².
- 3
History is what counts
Projects are sized with many years of historical data, such as the IDEAM and UPME solar atlas or satellite datasets.
- 4
It becomes production
Installed capacity × peak sun hours × system performance gives the expected energy.
The formula
Daily energy (kWh) ≈ kWp × PSH × PR
- kWp
- = the system's installed capacity
- PSH
- = the site's peak sun hours, as a daily average
- PR
- = overall system performance (losses included), around 0.8
An example with numbers
A 10 kWp system in an area with 4.5 peak sun hours per day.
| Installed capacity | 10 kWp |
|---|---|
| Peak sun hours | 4.5 per day |
| System performance (PR) | 0.8 |
| Estimated energy per day | 36 kWh |
| Estimated energy per month | ≈ 1,095 kWh |
The same system in an area with 5.5 PSH would produce around 1,338 kWh a month.
Try it with your numbers
- Estimated energy per day
- 36 kWh
- Estimated energy per month
- 1,095 kWh
- Estimated energy per year
- 13,140 kWh
Estimate using a system performance ratio (PR) of 0.8. Actual generation depends on the site and the equipment.
Where do you see it at Circular Urban?
Each solar project's generation projection starts from the historical irradiance of its location. The project sheet shows it translated into estimated annual production.
- Each solar project's sheet, under Energy Production.
What changes in your finances once you get it
You estimate before a quote
With your capacity and your area's peak sun hours you can work out approximate production.
You compare locations
You understand why the same system yields differently on the coast than in the mountains.
You read generation in context
A cloudy month lowers production; the projection rests on an average of many years.
Common mistakes
- Myth
More heat means more production.
- In reality
What counts is the light reaching the panels. Excess heat even lowers their efficiency slightly.
- Myth
On a cloudy day panels produce nothing.
- In reality
They produce less, not zero. The annual projection already accounts for cloudy and rainy days.
- Myth
Today's irradiance reading is enough to project.
- In reality
Projections use many years of historical data, because a single day doesn't represent the site.
Test yourself
A 5 kWp system sits in an area with 5 peak sun hours and a PR of 0.8. Roughly how much energy does it produce per day?
Keep learning
Helps to know first
This content is educational. Every participation carries risks inherent to the project, including the possibility of partial or total loss, and results are subject to each project's actual performance.
