Renewable Energy · Basic level · 3 min read ·
What is the capacity factor?
The capacity factor is the energy a system actually produces in a year, divided by what it would produce running at full capacity for all 8,760 hours of the year. For solar it is low by nature: there is no sun at night and clouds let less light through. A value near 15% is normal.
In 30 seconds
- It compares the year's real energy with the theoretical maximum at full capacity.
- For a solar system it usually sits between 13% and 20%, depending on the site and design.
- A low value doesn't mean the system works poorly: night counts too.
Why should you care?
Installed capacity says how much a system could produce at its best moment. The capacity factor says how much it produces in practice.
- You turn a project's kWp into real energy over the year.
- You compare projects of different sizes with a single measure.
- You understand why annual production is far below capacity × hours in a year.
Think of it as a taxi that doesn't drive 24 hours
A taxi could cover a certain distance if it drove at top speed all day. In practice it rests, stops at lights and crawls in traffic. The capacity factor is the share of that maximum it actually covers.
How does the capacity factor work?
- 1
Work out the theoretical maximum
Multiply installed capacity by the 8,760 hours in a year.
- 2
Take the year's real energy
What the system produced, or is projected to produce, over 12 months.
- 3
Divide and express as a percentage
Real energy ÷ theoretical maximum, times 100.
- 4
Read it as equivalent hours
15% is like running at full capacity about 3.6 hours a day: 4.5 peak sun hours × 0.8 system performance.
The formula
Capacity factor = Real annual energy ÷ (Capacity × 8,760 h)
- Real annual energy
- = kWh produced or projected in a year
- Capacity
- = the system's installed capacity, in kWp
- 8,760 h
- = hours in a year (365 × 24)
An example with numbers
A 10 kWp system in an area with 4.5 peak sun hours a day and a performance ratio (PR) of 0.8, as in the irradiance lesson. Illustrative figures, unrelated to any project.
| Energy per day (10 × 4.5 × 0.8) | 36 kWh |
|---|---|
| Real energy per year (36 × 365) | 13,140 kWh |
| Theoretical maximum (10 × 8,760) | 87,600 kWh |
| Capacity factor | 15% |
13,140 ÷ 87,600 = 0.15. It is as if the system ran flat out for 3.6 hours a day and stayed off the rest of the time.
Where do you see it at Circular Urban?
Each solar project sheet shows the capacity factor next to estimated annual production. It is a non-binding reference projection: real production depends on each year's weather, panel degradation and the project's actual performance.
- Each solar project sheet, in Energy Production: Capacity Factor.
- In the same section: Annual Production.
What changes in your finances once you get it
You run your own numbers
With capacity and capacity factor you estimate annual energy: kWp × 8,760 × factor.
You read capacity realistically
You know 10 kWp doesn't mean 87,600 kWh a year, only a fraction of it.
You compare by the same rule
Before comparing two capacity factors, you check that both use the same reference capacity.
Common mistakes
- Myth
A 15% capacity factor means the system works poorly.
- In reality
It is normal for solar: there is no production at night, and morning and afternoon light is weaker.
- Myth
A 15% capacity factor means the system only produces during 15% of the hours.
- In reality
It produces throughout daylight, at varying power. The 15% is an equivalence: as if it ran flat out for 3.6 hours.
- Myth
Capacity factor and panel efficiency are the same thing.
- In reality
Efficiency measures how much light a panel turns into electricity. Capacity factor measures how much of the annual maximum the system produces.
Test yourself
A 100 kWp system produces 140,160 kWh in a year. What is its capacity factor?
Keep learning
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.
