cstjnodLaboratory notebook

Technical memorandumSpecial edition

File
TM-26-1111
From
Charlie O'Donoghue
Date
Reading
5 min
Subject
Energy · Systems thinking

Flexibility is the grid's fastest loop

Electricity flexibility explained through system dynamics, from Jay Forrester's delays at MIT to a grid that has to balance every second, and the GB programmes that now pay homes to help.

AI-forward: Claude drafted this from my notes and checked every source against its claim. The argument, the cuts and the edit are mine.

Every second, Britain’s generators and batteries put exactly as much electricity into the grid as the country takes out. Any gap shows in the frequency, one number every machine on the grid shares. Flexibility is whatever holds it still.

A servo engineer looks at a factory

Jay Forrester built servomechanisms at MIT.MIT News (2016), Professor Emeritus Jay Forrester, digital computing and system dynamics pioneer, dies at 98; MIT Lincoln Laboratory, SAGE: Semi-Automatic Ground Environment air defense system, which credits a 1953 memo by George Valley and Forrester and calls Whirlwind the precursor of SAGE’s computer. In 1956 he took feedback thinking to what became its Sloan School of Management.MIT News (2016), the same obituary: Sloan in 1956, Industrial Dynamics 1961, Urban Dynamics 1969, World Dynamics 1971, The Limits to Growth 1972.

His first case was a General Electric appliance plant stuck in boom and bust. His model showed the swings came from its own management, not the market.MIT Sloan (2016), Jay W. Forrester dies at 98: the GE plant, and the Refrigerator Game that became the Beer Game. In his Beer Game, a Sloan orientation ritual for decades, sensible orders placed through a delay make stock swing wildly.

System dynamics at MIT Great Britain's grid time, not to scale 1956 Forrester turns to management after servos, Whirlwind and SAGE 1961 Industrial Dynamics delays make supply chains swing 1972 The Limits to Growth his students model the world 2000 Business Dynamics Sterman's textbook 2022 Demand Flexibility Service homes paid to use less 2024 NESO, a public operator from 1 October 2025 Half-hourly settlement meter migrations began in October 2026 Paid to use more, too the service now goes both ways
From a factory model to the grid. Not to scale.

Stocks, flows and the two kinds of loop are in a systems dynamics MBA, from a bathtub. The rule to carry: a balancing loop with a delay overshoots, and the longer the delay, the bigger the swing.

The grid is a bath

The grid’s stock is energy spinning in the rotors of big generators, all in step. Generation is the tap, demand the plughole. If the tap wins, the machines speed up and frequency rises. If demand wins, they slow and it falls. NESO, the grid operator, calls the stored spin inertia. It slows sudden change while the control room acts.NESO, What is inertia? Frequency is the waterline, and NESO keeps it within 1% of 50 Hz.NESO, Frequency response services: 50 Hz within a 1% range, and each service’s response time.

Balancing loop B1 holds it there: the control room watches frequency and forecasts and tells generators to turn up or down. Its problem is delay. Big power stations turn slowly, and NESO’s main tool, the Balancing Mechanism, trades in half-hour periods.NESO, What is the Balancing Mechanism?

Great Britain's grid Spinning energy frequency, 50 Hz generation demand frequency plants are slow B1 control room, minutes frequency B2 fast flexibility, seconds dE/dt = generation − demand energy information delay
The grid as a bath. B1 is the slow loop through power stations; B2 is flexibility, a second balancing loop on the other valve with almost no delay.

So the grid needs a faster loop, B2: kit that reads frequency itself and reacts within a second. That is flexibility at its fastest. The government’s roadmap works on longer timescales. It defines clean flexibility as “the ability to shift in time or location the demand or supply of electricity, over hours, days or seasons, while reducing emissions”.DESNZ, Ofgem and NESO (July 2025), Clean Flexibility Roadmap. In practice: turning demand up or down, shifting it in time, or storing energy.

Wind and sun move the tap

Coal and gas worked the old tap on command and brought inertia as a by-product. Wind and sun set their own and bring no inertia, so the level moves faster.

More wind and solar means bigger swings, so flexibility is worth more. Years later, once built, that flexibility lets the grid take more wind and solar: reinforcing loop R1. But it also cuts balancing costs, and with them what each unit earns: balancing loop B3.

Wind and solar share Swings in supply Value of flexibility Flexible capacity batteries, EVs, homes Balancing cost + + years + + − + R1 B3 + same direction − opposite direction delay R reinforcing B balancing
Why flexibility grows with wind and solar (R1), and why each unit of it earns less as more arrives (B3). My argument, not a model anyone has run.

The Clean Power 2030 Action Plan sees batteries grow from 4.5 GW in late 2024 to 23 to 27 GW by 2030, and home and business flexibility from 2.5 GW in 2023 to 10 to 12 GW.DESNZ (December 2024), Clean Power 2030 Action Plan, Table 1. “Consumer-led flexibility” is the plan’s term. By the end of 2025 the UK already had 7.5 GW of grid-scale batteries.DESNZ (July 2026), Clean flexibility roadmap: July 2026 update. The update’s figure is for the UK; the plan’s is for Great Britain. Counting interconnectors, long-duration storage and low-carbon dispatchable power too, the Clean Flexibility Roadmap puts the total at 51 to 66 GW.

Who runs the loops now

Seconds: frequency response. Dynamic Containment, Dynamic Moderation and Dynamic Regulation pay batteries and others to respond to frequency unprompted, within a second (ten for Regulation). This is B2.

Minutes to hours: the Balancing Mechanism. Generators and large users post bids and offers, for each half-hour period, and the control room accepts them minute by minute.The names run backwards to most people. A bid is a price to use more or generate less; an offer is a price to use less or generate more.The names run backwards to most people. A bid is a price to use more or generate less; an offer is a price to use less or generate more.

Hours, from homes: the Demand Flexibility Service. Since winter 2022/23 it has paid households to use less at the evening peak, and since 27 November 2024 year-round.NESO, Demand Flexibility Service (DFS). Since April 2026 it also pays them to use more when low-carbon power is plentiful. About 2.46 million homes and businesses have joined.NESO (March 2026), NESO announces shakeup to new look Demand Flexibility Service, approved by Ofgem on 25 March 2026. You join through a supplier or aggregator, who sets the reward, and need a smart meter.NESO, Demand Flexibility Service explained.

Streets: local flexibility. Britain’s electricity networks secured a record 9 GW of flexibility in 2024.Energy Networks Association figures, reported by Transport+Energy (July 2025), ENA reveals flexibility figures.

The meter: half-hourly settlement. None of this pays unless someone sees when you used electricity. Every meter must move to market-wide half-hourly settlement by 7 May 2027. Over 25 million, more than 70%, had moved by late October 2026.MHHS Programme, Key programme milestones (M15, 7 May 2027); MHHS Programme (October 2026), Reflecting on one year of MHHS migrations.

What it means at home

An EV or home battery is a stock that fills when you charge and drains when you drive or discharge. You rarely mind when, so long as it’s full by morning. That slack is flexibility.

In EVs are home energy’s gateway drug I argued the charger is where that slack becomes sellable. The loops add three.

Shorten the delay. B2 works because it has almost none. Winners respond in time and prove it with half-hourly data.

Expect your own balancing loop. B3 hits anyone selling flexibility: the more arrives, the less each unit earns. Build for narrowing spreads.

Don’t make the person the controller. Manual changes work on the odd cold evening. Loops that scale let the person set the goal, ready by seven, and the device balance.

The grid’s answer to Forrester’s delays is a fast, small balancing loop in every home that can store energy.

Notes

  1. MIT News (2016), Professor Emeritus Jay Forrester, digital computing and system dynamics pioneer, dies at 98; MIT Lincoln Laboratory, SAGE: Semi-Automatic Ground Environment air defense system, which credits a 1953 memo by George Valley and Forrester and calls Whirlwind the precursor of SAGE’s computer.

  2. MIT News (2016), the same obituary: Sloan in 1956, Industrial Dynamics 1961, Urban Dynamics 1969, World Dynamics 1971, The Limits to Growth 1972.

  3. MIT Sloan (2016), Jay W. Forrester dies at 98: the GE plant, and the Refrigerator Game that became the Beer Game.

  4. NESO, What is inertia?

  5. NESO, Frequency response services: 50 Hz within a 1% range, and each service’s response time.

  6. NESO, What is the Balancing Mechanism?

  7. DESNZ, Ofgem and NESO (July 2025), Clean Flexibility Roadmap.

  8. DESNZ (December 2024), Clean Power 2030 Action Plan, Table 1. “Consumer-led flexibility” is the plan’s term.

  9. DESNZ (July 2026), Clean flexibility roadmap: July 2026 update. The update’s figure is for the UK; the plan’s is for Great Britain.

  10. NESO, Demand Flexibility Service (DFS).

  11. NESO (March 2026), NESO announces shakeup to new look Demand Flexibility Service, approved by Ofgem on 25 March 2026.

  12. NESO, Demand Flexibility Service explained.

  13. Energy Networks Association figures, reported by Transport+Energy (July 2025), ENA reveals flexibility figures.

  14. MHHS Programme, Key programme milestones (M15, 7 May 2027); MHHS Programme (October 2026), Reflecting on one year of MHHS migrations.