The units "kW" and "kWh" are frequently encountered in the realm of electricity and energy. Despite their initial similarity, these units possess starkly divergent meanings, a distinction that is of paramount importance when assessing the intricacies of electric power structures and energy policy.

The expression "equivalent to xx nuclear reactors" is frequently employed in relation to these two units, particularly in the context of renewable energy, though its brevity often leads to misunderstandings. This article aims to elucidate the distinction between kW and kWh, and to address some of the issues associated with the expression "equivalent to xx nuclear reactors”.

Difference between kW and kWh

First, it is important to understand the distinction between kW (kilowatt) and kWh (kilowatt-hour). In essence, kW signifies the magnitude of "installed capacity" or "power consumption," whereas kWh denotes the "amount of energy”. To illustrate this concept, consider the example of a water supply. In this example, kW can be likened to the force of water coming out of a faucet, while kWh is analogous to the amount of water collected from that faucet (Figure 1). If the faucet is turned on at full blast and water flows continuously at a high rate, more water (kWh) will accumulate. Conversely, if the faucet is opened only a little, the amount of water accumulated will be less, but the time required to accumulate water will be the same. In summary, if a power generation facility continues to output 1 kW for 1 hour, 1 kWh of electricity will be supplied.

Figure 1: Difference between kW and kWh

(Source) Dai-ichi Life Research Institute.

Figure 1: Difference between kW and kWh

(Source) Dai-ichi Life Research Institute.

The practice of characterizing renewable energy facilities, such as large-scale solar power plants, as being equivalent to XX nuclear reactors, is frequently employed to provide a sense of scale. However, this comparison is not without concern. There is a possibility of confusion between the installed capacity (kW) and the actual amount of electricity generated (kWh).

Facility utilization rate

The facility utilization rate indicates the percentage of actual power generation out of the amount of power generated (facility capacity) when the power generation facility was in full operation.

Figure 2: Approximate facility utilization rate per power source

(Note) The facility utilization rate is calculated based on the actual results for the 30 years prior to the Great East Japan Earthquake.

(Source) Compiled by Dai-ichi Life Research Institute from Document 1, 6th Power Generation Cost Verification Working Group, General Resources and Energy Investigation Committee, Ministry of Economy, Trade and Industry.

Figure 2: Approximate facility utilization rate per power source

(Note) The facility utilization rate is calculated based on the actual results for the 30 years prior to the Great East Japan Earthquake.

(Source) Compiled by Dai-ichi Life Research Institute from Document 1, 6th Power Generation Cost Verification Working Group, General Resources and Energy Investigation Committee, Ministry of Economy, Trade and Industry.

The facility utilization rate of nuclear power plants is high, averaging around 70% for the 30 years prior to the Great East Japan Earthquake. (Note that the average facility utilization rate of nuclear power plants in Japan in FY2023 will remain below 30% due to the fact that 21 out of 33 nuclear power plants, excluding those under construction or planned, are not in operation.) In comparison, the facility utilization rate for nuclear power plants in the United States, South Korea, Finland, and other countries is approximately 90%. Conversely, the equipment utilization rate for solar power generation hovers around 15-20%, and wind power generation stands at around 30%. This disparity can be attributed to the intermittency nature of renewable energy sources, which is a key factor in determining the actual amount of power generated.

To provide a concrete example, the amount of electricity generated per day for a power plant with the same installed capacity of 1 million kW (equivalent to one nuclear power plant) would be as follows.

• Nuclear power generation (facility utilization rate 70%):

1 million kW × 24 hours × 70% = 16.8 million kWh

• Solar power generation (facility utilization rate 15%):

1 million kW × 24 hours × 15% = 3.6 million kWh

Despite identical "installed capacity," there is a considerable disparity in the actual power generation. The attribute term "equivalent to xx nuclear reactors" disregards the facility utilization rate and does not accurately reflect the true electricity generation amount or its attributes, potentially leading to an overestimation of renewable energy potential.

Electricity's unique characteristic of not being able to hold inventory

Electricity has the characteristic of not being able to hold "inventory." This is due to the physical nature of electricity. Electricity flows out through the power grid the moment it is generated and is consumed wherever it is needed. In other words, the electricity produced must be consumed immediately, and it is basically impossible to store the unused portion.

For example, a bakery can prepare for a high volume of customers by increasing its production of bread and ensuring it is available on the shelves. This establishes a system of inventory management for standard products. However, electricity operates differently due to its real-time supply and demand dynamics. Since electricity produced by power plants is delivered to consumers instantaneously via power lines, it is challenging to "prepare and reserve a large amount."

In order to store electricity generated by renewable energy, large-scale storage batteries need to be prepared. The cost of such batteries is approximately 76,000 yen/kWh in FY2023. To illustrate, if a facility with the capacity to store the amount of electricity generated by one hour of solar power (150,000 kWh) is installed, along with a capacity of 1 million kW and a utilization rate of 15%, a simple calculation indicates the cost would be 11.4 billion yen.

Consequently, power generation facilities are required to make precise adjustments, such as ramping up electricity production during periods of high demand and reducing output during times of low demand.

Power generation methods and facility utilization rate

Given that electricity cannot be held in inventory, which power generation methods are being used to meet electricity demand immediately?

Solar and wind power generation are subject to natural conditions, so it is standard practice to generate all the electricity that can be produced and leave the adjustment to other power sources. Despite this, the facility utilization rate remains low.

Coal-fired power plants are frequently operated continuously due to their low fuel costs. Nuclear power also typically operates without interruption, with the exception of annual periodic inspections, leading to the enhanced facility utilization previously referenced. While nuclear power generation in Japan requires shutdowns for maintenance approximately three months per year, the impact is mitigated by conducting maintenance during periods of low demand. These are designated as baseload power sources.

The Organization for Cross-regional Coordination of Transmission Operators (OCCTO) has presented a plan for the electricity supply in FY2024 that includes LNG. According to this plan, the facility utilization rate will fall to 34% by 2033. However, the output of this facility can be easily adjusted because it operates on a jet engine-like principle. As a result, it plays an important role as a regulating power source to support the natural variability of renewable energy.

The facility utilization rate is contingent upon its application; therefore, larger is not necessarily better. While humans can to some extent decide when to start and stop power generation in other power generation facilities, with renewable energy, the decision is left up to nature. As a result, ingenuity is required, such as discarding power when there is too much and using other power generation methods to compensate when there is not enough.

The rate directly impacts the amount of electricity generated

A fundamental understanding of the distinction between kW and kWh is crucial for comprehending power generation facilities and methodologies. While the term "equivalent to xx nuclear reactors" is expedient, it fails to account for variations in facility utilization rates, potentially leading to an overestimation of renewable energy.

While the amount of electricity generated remains uncontrollable, expanding the use of renewable energy is imperative for addressing climate change, ensuring the sustainability of fossil fuel resources, and enhancing energy security. Recognizing the unique properties of renewable energy and implementing a systematic approach to its effective operation could be pivotal in formulating future energy policies.

Energy issues are extremely complex, and there is no single correct answer. In order to overcome the conflict between renewable energy and nuclear power and to discuss the best power generation method for the right person, it is important for each of us to acquire the correct knowledge and to incorporate energy considerations into our daily lives.

Original in Japanese:
https://www.dlri.co.jp/report/dlri/417687.html