Capacity Tags and Transmission Tags: How Wholesale Markets Allocate Grid Access and Costs

In wholesale electricity markets (PJM, ISO-NE, NYISO), power flows across transmission grids from generators to loads. Capacity tags and transmission tags are mechanisms allocating network usage rights and associated costs. A generator in Pennsylvania must "tag" transmission rights to sell electricity to a consumer in New Jersey—this tag represents a claim on transmission capacity for that delivery path. Similarly, capacity tags represent claims on generator capacity sold in wholesale auctions. These abstract wholesale concepts directly affect your retail electricity rates. In deregulated markets, retailers pass tag costs to customers. Understanding tags explains why your rate varies by time, location, and season. This guide covers tag mechanics, how they drive wholesale costs, and real impact on your bill.

Capacity Tags: Wholesale Generator Allocations

Capacity tags represent megawatts (MW) of generating capacity allocated in wholesale auctions (covered in previous article on capacity market auctions). When a generator wins a capacity auction at clearing price $50/kW-year, it receives a tag certifying it can provide that MW during the delivery year. The tag is essentially a property right—the generator can sell that capacity to retailers or fail to deliver and pay penalties.

Key Takeaway: Capacity tags = guaranteed generation rights sold in wholesale markets. Each tag represents 1 MW capacity for 1 year at specific location (generator location matters because transmission constraints). Retail rates include cost of capacity tags suppliers procure (~$25-70/kW-year pass-through = 2-4¢/kWh for typical customer).

Real example: Retail supplier serving 100,000 customers (100 MW average load + 20 MW reserve = 120 MW needed). Supplier buys 120 capacity tags at $50/tag = $6M/year cost. Spread across 100,000 customers = $60/customer capacity charge/year = $5/month for typical customer. This $5/month is passed through your bill as "capacity charge."

Transmission Tags: Grid Access and Congestion

Transmission tags allocate access to physical transmission lines moving electricity. When you buy electricity from a generator 200 miles away, that power must flow through transmission lines—those lines have capacity limits. A transmission tag secures your share of that capacity. Tags have direction and location: a tag for "Pennsylvania to New Jersey" differs from "Ohio to Pennsylvania"—they're specific delivery paths.

Congestion pricing example: During peak demand, transmission from low-cost generators in Ohio to expensive NYISO zone is congested. Transmission tag price for Ohio→NY path spikes to $0.05/kWh (normally $0.01/kWh). Retailers buying that transmission path must pay congestion premium. This gets passed to NY customers as "locational marginal price (LMP) premium."

Most retail customers don't see transmission tags explicitly—they're embedded in wholesale electricity price (which includes tag cost). But commercial customers buying directly from wholesale markets must account for tag costs explicitly.

How Tags Affect Your Retail Rate

Typical bill breakdown (customer in PJM at 12¢/kWh effective rate): Energy 4¢ + Capacity 2¢ + Transmission 1.5¢ + Ancillary 0.5¢ + Retail margin 1.5¢ + Taxes/fees 2.5¢ = 12¢/kWh. Capacity and transmission tags account for ~3.5¢/kWh (29% of total bill).

Congestion and Tag Price Variations

Transmission congestion (when demand exceeds line capacity) causes tag prices to spike. During extreme weather (winter cold snap or summer heat wave), demand concentrates in one region, causing congestion on transmission paths feeding that region. Tag prices for that path increase 10-100x normal levels during peak hours. Wholesale price spikes occur partly due to energy scarcity, partly due to transmission tag scarcity.

Real 2024 example (Northeast winter): Jan 23, 2024, extreme cold. NYISO wholesale price spiked to $500+/MWh (normal $30-60/MWh). Portion was energy scarcity (high heating demand), portion was transmission congestion (all available generation far from NYC demand center). Transmission tags for paths feeding NYC spiked in value, forcing retailers to pay premiums. These costs passed to NY customers as higher-than-usual bills that month.

Difference Between Capacity Tags and Transmission Tags

Capacity tags: Certify generator can produce up to X MW during delivery year. Sold in annual auctions. Price relatively stable (cleared at auction annually = $25-70/kW typical). Allocated to all customers equally (retail suppliers buy enough tags to cover customer demand + reserve margin).

Transmission tags: Certify right to use transmission path between two locations. Sold continuously in wholesale markets (day-ahead, real-time). Price highly variable based on congestion (can be $0.01/kWh in uncongested periods, $0.50/kWh during peak congestion). Allocated based on actual delivery paths (customers buying from distant generators pay more for transmission than customers buying from local generators).

Practical Implications for Customers

Why rates vary by location: ZIP codes in congested areas (urban centers, far from generation) have higher rates due to transmission tag costs. Rural areas near generators have lower rates (shorter transmission paths = cheaper tags).

Why rates vary by time: Peak periods (3-8 PM summer, 6-9 AM winter) have congestion, causing transmission tag spikes. Off-peak periods (10 PM-6 AM) have cheap transmission tags. Time-of-use rates reflect this tag cost variation.

Why rates change seasonally: Capacity tags (fixed annually) stay stable. Transmission tags fluctuate based on seasonal demand patterns (heating in winter, cooling in summer). This seasonal variation shows up in energy/transmission components.

Next Steps

Step 1: Review your bill rate breakdown. If deregulated market, request itemization showing: energy, capacity, transmission, ancillary charges. Calculate ¢/kWh for each. Transmission and capacity tags typically 30-40% of total.

Step 2: Understand your delivery location's transmission constraints. Check your utility's transmission map. Are you far from major generation hubs (higher transmission tags) or close (lower tags)? This affects rate optimization.

Step 3: Consider time-of-use rates if available. TOU rates discount off-peak hours (low transmission congestion, cheap tags). If you can shift usage to off-peak, TOU saves 10-15% vs. standard rates.

Step 4: Monitor wholesale market reports during extreme weather. Widespread outages or extreme weather cause transmission tag spikes. Some suppliers offer "extreme event" protections (rate caps during spikes). Negotiate this if you have commercial contract.

Related articles: Capacity Auctions, Reading Your Bill, Time-of-Use Rates