--- title: "Spinning reserve versus reserve power" subject: "Power Plant Design" chapter: 2 key: "semester_8/power_plant_design:spinning-reserve-vs-reserve" source: https://www.bctengineer.com/q/semester_8/power_plant_design/spinning-reserve-vs-reserve site: BCT Engineering --- # Spinning reserve versus reserve power Power Plant Design · Chapter 2: Integrated System Planning in Design Approach - Asked in: 2079 Chaitra, 4 marks - Video: [Cold Reserve, Hot Reserve, Spinning Reserve & Firm Power](https://www.youtube.com/watch?v=k_i_XwfJ3oE) — Neso Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/spinning-reserve-vs-reserve - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-2 ## Question What is the difference between spinning reserve and reserve power? ## Short answer Spinning reserve is unused capacity of synchronised running units, available in seconds to minutes, sized at least largest running unit. Reserve power is total spare capacity above peak, including spinning, non-spinning and cold reserve, typically 15-25% of peak. Spinning reserve is part of total reserve. ## Answer **Spinning reserve** is the unused capacity of generators that are already running and synchronised to the grid, ready to pick up load within seconds to minutes. **Reserve power** (total reserve) is all the capacity available above the peak demand, including spinning, non-spinning (quick start) and cold reserve. #### Difference | Point | Spinning reserve | Reserve power (total reserve) | |---|---|---| | Meaning | Unloaded capacity of synchronised units | Total spare capacity over peak demand | | State of units | Running and connected | Running, standby or shut down | | Response time | Seconds to ~10 min | Seconds to hours/days | | Purpose | Cover sudden loss of a unit, load jumps; frequency control | Cover outages, maintenance, forecast error, low hydrology | | Size | Usually ≥ largest running unit | Usually 15–25% of peak (reserve margin) | | Cost | Higher (units run part-loaded) | Mainly capital cost of extra plants | | Planning horizon | Operational (daily) | Long-term planning | **Example:** A 1,000 MW system with 1,200 MW installed has 200 MW reserve. If units on line can deliver 1,080 MW while load is 1,000 MW, spinning reserve is 80 MW; the remaining 120 MW is non-spinning/cold reserve. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which unit state describes spinning reserve? A. Unused capacity of generators running and synchronised to the grid B. Capacity of units that are shut down and need hours to start C. Capacity of quick-start gas turbines that are off-line D. Capacity of the largest unit only when it is tripped Answer: A. Spinning reserve is the unloaded capacity of units already running and synchronised, so it can respond within seconds to minutes. (Key point: Meaning of spinning reserve) 2. Reserve power (total reserve) differs from spinning reserve mainly because it: A. includes only the running units B. includes spinning, non-spinning and cold reserve above peak demand C. is only the capacity of hydro plants D. is measured only in the daily operation Answer: B. Total reserve is all the spare capacity above peak demand, of which spinning reserve is only the running part. (Key point: Scope of reserve power) 3. Which pairing is correct? A. Spinning reserve: long-term planning; total reserve: frequency control B. Spinning reserve: operational, covers sudden unit loss and frequency control; total reserve: long-term planning for outages and low hydrology C. Both are used only for maintenance planning D. Spinning reserve covers forecast error over years; total reserve covers seconds Answer: B. Spinning reserve is an operational (daily) tool; total reserve is a planning quantity covering outages, maintenance and dry years. (Key point: Purpose and planning horizon) 4. A system has 1,200 MW installed, load 1,000 MW, and units on line can deliver 1,080 MW. What are the spinning reserve and the other (non-spinning/cold) reserve? A. 200 MW and 0 MW B. 80 MW and 120 MW C. 120 MW and 80 MW D. 80 MW and 200 MW Answer: B. Spinning = 1080 - 1000 = 80 MW; total reserve = 200 MW, so the remaining 120 MW is non-spinning/cold. (Key point: Worked example) ## Related in Power Plant Design - [Define spinning reserve](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adefine-spinning-reserve) — Power Plant Design, chapter 2 - [Define reserve power](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adefine-reserve-power) — Power Plant Design, chapter 2 - [Power system security and reserve requirement](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asecurity-reserve-requirement) — Power Plant Design, chapter 2 - [Regional interconnection and reserve bottling](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aregional-interconnection-reserve) — Power Plant Design, chapter 2 - [Reserve planning and utility interconnection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Areserve-planning-interconnection) — Power Plant Design, chapter 2 - [Thermal versus nuclear power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Athermal-vs-nuclear) — Power Plant Design, chapter 1, asked 2× ## Related in other subjects - [Reserve versus resource](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-vs-resource) — Engineering Geology II, chapter 7, asked 3× - [RMS, phase difference and power from v and i](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Av-500sin-400t-i-5sin-rms-phase-difference) — Basic Electrical Engineering, chapter 5 - [Potential difference versus emf](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Apotential-difference-vs-emf) — Basic Electrical Engineering, chapter 1 - [Aggregate reserve: sandstone N42W/34, 80 m](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areserve-sandstone-n42w-34) — Engineering Geology II, chapter 7, asked 2×