Capacity factor in renewables

The Capacity Factor is one of the most widely used indicators in our industry. It indicates the level of utilisation of a generation plant and is a simple and quick way to get an idea of the “efficiency” of a wind or solar project. It is therefore worth looking into this concept in more detail. Today we take a look at the world of renewable capacity factors.

 

What is the Capacity Factor or CF?

 

It is defined as the percentage of energy generated over the theoretical maximum that a plant could generate in a period of time. Its formula is very simple:

 

 

It is a way of expressing the percentage of time a plant is operating at maximum equivalent power in a year and is an indicator used in all power generation technologies.

 

In conventional generation sources, this factor only depends on the degree of utilisation and availability as the fuel is always available. For example, in nuclear, which does not usually shut down voluntarily, its CF is always very high and the difference up to 100% usually indicates scheduled and unscheduled outages.

 

But in renewables the CF depends on more factors as the “fuel” is intermittent. Let’s look at the main factors affecting the Capacity Factor:

 

• Resource: either wind or irradiance is the main factor that will influence the CF. Sites with more resource will achieve higher CFs by producing more hours.

 

• Efficiency: in wind power, the ratio between rotor diameter and nominal power (power density in W/m2) is key to achieving a higher CF. With very large rotors for medium power, it is possible to capture a lot of wind and work close to or at rated power for many hours, increasing the CF.

 

• Availability: the hours when the plant is unavailable for maintenance (planned or unplanned) will reduce the CF

 

• Electrical losses: if the electricity generated is metered at the connection or evacuation point of the plant, the electrical losses of the cables will lower the CF.

 

• Curtailment: reduce the CF by stopping production.

 

 

Is the CF a good project comparator?

 

Actually, the CF is not the most suitable for comparing generation plants because, as we have seen before, there are many factors that influence it, so drawing conclusions is very risky. But because it is so simple to calculate and understand, it has become one of the main indicators for measuring the performance of a plant. In addition to the factors that influence it, its calculation is not standardised and it is difficult to know if we are comparing apples with apples:

 

• Measurement point of the energy generated: as mentioned above, it is not the same to measure the generation at the output of the wind turbine or the solar inverter as it is at the point of interconnection (POI).

 

• MWdc vs MWac: In solar PV it is also unclear whether MWdc (panels) or MWac (inverter) is used as the nominal power of the plant. Since the DC/AC ratio can be very relevant (between 1.2 and 1.5), the CF can vary a lot. Ideally, MWac should be used so that the theoretical maximum is achievable.

 

• Over-installation: in wind power, something similar to the above can happen when more MWs are installed than the evacuation capacity. In this way, the CF is increased as long as the nominal capacity is taken as the evacuation capacity. If the CF calculation takes the generation at the connection point but divides it by the installed capacity, we will be reducing the CF.

 

These examples show that it is not so easy to compare CF without knowing in detail how it has been calculated.

 

Let’s forget all of the above and compare!

 

Leaving aside everything explained above, let’s use the CF for the most fun part: comparing

 

1. CF by generation sources

 

 

This comparison is a classic, although mixing conventional and intermittent renewables does not make much sense. And it will make even less sense to compare “efficiencies” as operators increasingly use revenue optimisation strategies that involve plant shutdowns (including nuclear), so that the CF will be very distorted.

 

Obviously, the CF of renewables is limited by the resource. Solar will never exceed 50% per year due to the hours of sunshine, while conventional renewables, not having this constraint, could in theory approach 100% (as is the case with nuclear).

 

2. Wind CF by region

 

If we focus on wind, we can see trends in CF by region:

 

 

Comparing the historical wind CFs of China and the USA, we see a big difference in favour of the USA. The main explanation is due to the resource, as China has many low wind areas. But the big drop in China between 2004 and 2008 is very striking and coincides with the big expansion of local manufacturers.

 

 

As for Europe, we see that the onshore CFs are similar to those in China because also in Europe there are many low wind areas and there are very good resource areas with older turbines with small rotor. As older sites are repowered, the CF will improve. As an example, in the Tahivilla repowering being carried out by Acciona, it is estimated that the CF will improve by more than 15 points.

 

 

Comparing by country with IEA data, it is curious that the average offshore CF in Europe is similar to the onshore CF in the USA. It can also be seen that the UK has the highest CF in Europe thanks to its large resource, while in Germany, onshore farms hardly reach 20%.

 

 

As for Spain, its onshore wind CF is in the European average, close to 25% but not reaching it.

 

 

3. CF by operators

 

The results presentations of large operators are a valuable source of information on CFs as it is a ratio that is highly valued by analysts for its simplicity and ease of understanding.

 

• ACCIONA

 

ACCIONA has a presence in 25 countries with 13.5 GW in operation, mostly wind. It also has a reputation for having one of the best-maintained and most optimised fleets, with its CF being highly correlated to the resource.

 

 

The CF in Costa Rica, which exceeds 50%, is particularly noteworthy. This is the Chiripa wind farm, which with 33 AW77/1500 turbines installed in 2014, exceeds 5,000 equivalent hours, considerably more than any other offshore wind farm. It will undoubtedly be one of the onshore farms with the highest CF in the world.

 

At the other extreme, we have Italy, with CFs that do not even reach 20%. These are old farms in low-wind areas.

 

Also striking is the low CF in the USA, well below the country’s averages. Acciona has a total of 1.1 GW installed there, so the low CF of the entire fleet is very striking.

 

 

In terms of Solar PV, Chile and South Africa achieve higher CFs than some wind averages, clearly showing the large solar resource that exists in these countries.

 

 

• RWE

 

Let us now turn to another wind power giant, Germany’s RWE. Moreover, this operator has both onshore and offshore fleets.

 

 

In onshore, the USA is confirmed as a high CF area together with the Netherlands. At the other extreme is Germany, with very low levels. It should be remembered that in Germany a lot of capacity has been installed in low wind areas.

 

As for offshore, the figure for CF in Germany in 2023 is very striking. RWE blames it on grid curtailments, but it seems to me to be too much of a drop to be caused only by curtailment, as RWE has its offshore capacity distributed in 5 different windfarms in this country.

 

The truth is that with the exception of Sweden, which has only one 48 MW windfarm, offshore CFs are in line with WindEurope data, with values of around 35%.

 

 

• BROOKFIELD

 

Another giant in the exploitation of renewable assets is Brookfield which has 11 GW wind and 7 GW solar in operation worldwide. Unfortunately it does not report data by country but by region:

 

 

The CF of onshore wind in Brazil is spectacular, approaching 50% by 2023. There are 809 MW in operation in the Guanambí and Natal areas.

 

Europe’s CF in 2023 is also surprising, with 38.4%. This is 1.4 GW of wind power and the explanation for such a high figure may come from the fact that the Canadian company concentrates a large part of its fleet in the UK and Ireland, areas of high wind. Even so, the increase in 2023 vs 2022 is very striking.

 

As for the solar CF, it is also very high and can only be explained by the fact that the company selects very well the countries where it develops solar, avoiding low irradiance markets such as Northern Europe.

 

Overall, it is clear that Brookfield is trying to maximise the CF of its fleet as in all areas it is in the upper end of the value range.

 

 

• ORSTED

 

We finish our review of some of the world’s most important operators with the Danish offshore giant: Orsted. With 9 GW in operation and another 7 GW under construction, it is the most experienced offshore wind operator in the world.

 

 

The first thing that is striking is that the CF levels of onshore and offshore are very similar. There are even years in which onshore is above offshore. This is very surprising because offshore projects are more expensive and have more risk but also report more revenue via higher production but if the production level is similar to onshore, then the business case does not make much sense.

 

As in the case of Brookfield, Orsted concentrates its onshore wind farms in areas with very high winds: Ireland, UK, northern Germany and USA, which would explain the very high CF values. The offshore values are not bad considering that the projects are mostly in Europe and are above average.

 

In terms of solar, very similar values to Brookfield, with projects concentrated in areas of high irradiance.

 

 

• TETRASPAR and HYWIND

 

The Tetraspar floating wind prototype developed by Stiesdal announced a CF from installation of 54% and 63% in 2024 alone. This is a Siemens Gamesa 3.6 MW turbine installed in 2021.

 

 

Equinor’s HyWind Scotland (30 MW) also recently reported 54% since commissioning, peaking in 2020 at 57%.

 

 

These data from floating prototypes demonstrate the very high CFs that can be achieved in floating offshore wind.

 

 

• BONUS TRACK: Offshore capacity factors per project

 

And as a bonus track, a page that is a marvel as it shows all the CFs of the offshore parks in the UK, Denmark, Germany and Belgium. It is EnergyNumbers and the post is written by a person named Andrew. The pity is that the data is from May 2022 and I have not found an updated version. Andrew, if you are reading this article, congratulations on your site and please update the data for your followers!

 

 

And we stop now with the capacity factors because it is a never-ending and engaging world. Although we are aware that it is not the best ratio to compare projects, we love to do it and try to draw conclusions (most of them incomplete) but as trend indicators it is a very good indicator.