Japan Capacity Market Deep Dive: System Design, Auction Mechanism & Contract Terms

Introduction: Why Does Japan Need a Capacity Market?

In the wake of electricity market liberalization, Japan's power system faces a fundamental paradox. While spot market price signals efficiently balance short-term supply and demand, they fail to provide sufficient incentives for generators to make long-term investments in new capacity or maintain aging facilities. As renewable energy penetration deepens and conventional thermal plants exit the market due to declining profitability, maintaining system reserve capacity — or supply adequacy — has become a critical challenge.

The capacity market (容量市場) is the institutional solution designed to address this "Missing Money" problem. Through capacity auctions held four years in advance, the Organization for Cross-regional Coordination of Transmission Operators (OCCTO) pays capacity providers a "Capacity Assurance Contract Amount" as compensation for maintaining available capacity during a specified future delivery year, thereby ensuring the long-term supply adequacy of Japan's national power system.

Market Architecture: Who Are the Participants?

OCCTO serves as the Market Operator at the core of the capacity market. Capacity providers — the supply side of the market — include owners and operators of conventional thermal power, nuclear, hydro, pumped storage, renewable energy (solar, wind), and demand response (DR) resources. Under Article 1 of the Contract Terms, capacity providers must be electricity retailers or entities recognized as prospective retailers during the delivery year, ensuring that participants have genuine power supply capabilities rather than being purely financial speculators.

Power Source Type	Classification	Key Characteristics
Thermal (Gas, Coal, Oil)	Stable Power Source	Dispatchable, high availability
Nuclear	Stable Power Source	High baseload, low marginal cost
Run-of-River Hydro	Stable Power Source	Subject to hydrological conditions
Pumped Storage	Stable Power (with regulation)	Storage-capable, frequency regulation
Solar PV	Variable Power Source	Output dependent on irradiance
Wind	Variable Power Source (standalone)	Output dependent on wind speed
Demand Response	Dispatchable Power Source	Demand-side resource

Auction Mechanism: Procuring Capacity Four Years Ahead

The core trading mechanism of the capacity market is the Main Auction (メインオークション), conducted four years before the delivery year. OCCTO sets a Required Supply Capacity target based on future demand forecasts and solicits bids from capacity providers. The auction uses a uniform-price clearing mechanism, where all successful bidders receive the same market clearing price.

Article 6 of the Contract Terms establishes supplementary auction types to address post-Main Auction supply-demand changes. The Procurement Auction (調達オークション) is held one year before the delivery year when the Main Auction has procured insufficient capacity. Conversely, the Release Auction (リリースオークション) allows contracted providers to return excess capacity to the market when the Main Auction has over-procured. A Special Auction (特別オークション) may also be held when OCCTO identifies emerging supply security concerns.

Contract Amount Calculation: The Core Financial Formula

The Capacity Assurance Contract Amount is the central financial mechanism of the capacity market. Article 7 of the Contract Terms specifies the calculation formula:

Unit Contract Price: Weighted Average Formula

When both the Main Auction and Procurement Auction have successful bidders, the Unit Contract Price is calculated as:

Worked Example: Standard Gas-Fired Power Plant

Using a typical 100 MW natural gas combined-cycle power plant that wins the Main Auction as an example:

Parameter	Value	Notes
Clearing Price (P_M)	¥14,000/kW·year	Market clearing price (assumed)
Cleared Capacity (Q_M)	100,000 kW	100 MW
Contract Capacity	100,000 kW	Assumed equal to cleared capacity

Annual Contract Amount Calculation:

• Unit Contract Price: ¥14,000/kW·year
• Capacity Assurance Contract Amount: ¥14,000 × 100,000 kW = ¥1,400,000,000/year (assuming no economic penalties)
• Monthly Payment: ¥1,400,000,000 ÷ 12 = ¥116,666,666/month

Inefficient Coal Plant: 20% Reduction Calculation

From the FY2025 delivery year, coal plants with a design efficiency (Higher Heating Value basis) below 42% face a 20% reduction. Using the same 100 MW plant as an aging coal facility:

Capacity Contribution Fee: How Retailers Bear the Cost

The total capacity market cost is ultimately borne by electricity retailers as a "capacity contribution fee" (容量拠出金), calculated as:

As a numerical example: assume a retailer's annual supply volume is 5 TWh, total market supply volume is 800 TWh, and the total Capacity Assurance Contract Amount is ¥600,000,000,000:

• Allocation ratio: 5 TWh ÷ 800 TWh = 0.625%
• Capacity Contribution Fee: ¥600,000,000,000 × 0.625% = ¥3,750,000,000/year
• Monthly allocation: ¥312,500,000/month

Monthly Payment Mechanism

The Capacity Assurance Contract Amount is not paid in a lump sum. It is distributed across 12 monthly payments from September of the delivery year through August of the following year. Each monthly payment equals the annual contract amount divided by 12 (with the final month receiving the annual total minus the sum of the prior 11 monthly payments). OCCTO processes payments or billing by the last business day of each month (or the preceding business day if that date falls on a bank holiday).

Performance Obligations: The Responsibilities of Capacity Providers

The core logic of the capacity market is "Pay for Availability." In exchange for receiving the contract amount, capacity providers assume stringent performance obligations spanning the entire contract lifecycle from execution through the delivery year.

Under Article 9, OCCTO conducts a supply-demand balance assessment on the day before each delivery day, distinguishing between "normal conditions" and "low regional reserve margin conditions" in 30-minute intervals and publishing the results publicly. This mechanism serves as the critical interface between the capacity market and the spot market. Article 10 requires providers of stable power sources and standalone variable power sources to submit Capacity Suspension Plans by OCCTO-specified deadlines, with a tiered submission timeline that balances system predictability with operational flexibility for generators.

Market Exit: 13 Triggering Conditions

Article 12 enumerates 13 conditions that trigger full or partial market exit for a contracted power source. These range from retirement or decommissioning decisions and failed source substitutions to non-submission of required documents and post-contract discovery of FIT/FIP status. Two conditions deserve particular attention: first, if an existing thermal power source wins a bid in the Long-term Decarbonization Power Source Auction, its corresponding capacity exits the capacity market, preventing double subsidization. Second, dispatchable power sources that fail effectiveness tests (実効性テスト) face partial or full market exit, ensuring that procured capacity is genuinely available when needed.

Penalty Structure: A Three-Tier Enforcement System

The capacity market establishes a comprehensive three-tier penalty system targeting non-compliance at different stages of the contract lifecycle, creating a robust compliance incentive structure.

Penalty Type	Article	Trigger Condition
Market Exit Economic Penalty	Article 13	Market exit event occurs for contracted source
Pre-Delivery Period Economic Penalty	Article 16	Failure to meet requirements before delivery period
During-Delivery Period Economic Penalty	Article 19	Failure to meet availability requirements during delivery

Article 20 establishes a cap on during-delivery period penalties, preventing a single incident from causing disproportionate financial impact. Article 22 governs the monthly settlement of contract amounts, ensuring timely deduction of penalties from monthly payments and proper fund transfers between OCCTO and capacity providers.

Practical Implications for Market Participants

For electricity retailers, capacity market costs are ultimately passed through to end consumers as "capacity contribution fees" (容量拠出金), allocated proportionally based on each retailer's supply capacity share. Understanding the capacity market's mechanics enables retailers to more accurately forecast their cost structures and design more competitive retail tariffs. For generators, the capacity market provides stable capacity fee income that reduces the financial risk of long-term investment decisions, though the stringent performance obligations and penalty mechanisms require robust asset management and compliance systems. The 20% reduction for inefficient coal plants will accelerate retirement decisions for aging coal units. For renewable energy developers, the capacity market offers an additional revenue stream, but the recognized contract capacity for variable sources and effectiveness test requirements demand careful attention during project planning.

Conclusion: The Road Ahead for Japan's Capacity Market

Since its launch in 2020, Japan's capacity market has undergone continuous institutional refinement, establishing a relatively mature operational framework. The February 2026 revision — particularly the 20% reduction for inefficient coal power — clearly signals the policy direction: ensuring supply adequacy while driving the decarbonization of the generation mix. Looking ahead, how to provide appropriate market access mechanisms for new resource types — such as large-scale battery storage and hydrogen power generation — while maintaining system security will be the central challenge for the next phase of Japan's capacity market reform.