Predict which parts of the system are hardest to change, and why.
A grid gets to 70% clean electricity fairly quickly and then slows sharply. What is the most likely reason?
Yes.The first tranche displaces fuel whenever the sun or wind shows up. The last tranche has to cover a still, cold, dark evening — a completely different engineering problem.
Not quite.It often does, and the slowdown appears even where political support is strong, because the remaining problem is harder.
Not quite.It has kept improving. What changed is which problem the extra capacity has to solve.
Adding generation is easy. Covering the hours when the weather does not cooperate is the part that costs, and it is a different job.
Slide the share of a year's electricity that must come from weather-dependent sources.
20%50%80%98%
Extra backup and storage needed5
Generation built20
20%Existing plants absorb the variation easily. Almost nothing extra is needed.
Extra backup and storage needed25
Generation built50
50%Some surplus is wasted, and plants must run more flexibly. Costs start appearing beside the turbines.
Extra backup and storage needed70
Generation built80
80%Large surpluses, and long still periods that must be covered by something dispatchable.
Extra backup and storage needed140
Generation built98
98%The last few per cent need capacity that stands idle nearly all year, for the worst week.
The last few per cent need more supporting capacity than the whole first half did. Why?
Yes.A system sized for the worst still week has to hold capacity that earns almost nothing the rest of the year. That is why the final stretch is argued about so fiercely — it is a different product with a different bill.
Not quite.A turbine works the same at 98% as at 20%. What changed is what the system needs around it.
Not quite.Demand is held constant here. The cost comes from covering the gaps.
Move the control to see what changes.
Which parts of energy use are relatively easy to switch, and which are genuinely hard?
Lighting and household appliances.
Cars for daily driving.
Cement and steel production.
Long-haul aviation.
Home heating in a mild climate.
Yes.The hard column shares one feature: it needs very high temperatures or very dense fuel, and electricity is poor at both today. That is a physics-and-chemistry problem, not a willingness problem.
Put in order why the last stretch of an energy transition costs the most.
Tap them in order — first to last.
Easy hours covered→Hard hours left→Idle capacity needed→Paid for by few hours
The last tenth is a different product from the first.Yes.It is the same shape as the peaking plant from lesson five, applied to a whole system. Rarely-used capacity is expensive per unit and cheap per disaster avoided.
A backup plant costs £40m a year to keep available and runs 200 hours a year at 500 MW. What does its availability cost per MWh generated, in pounds?
£/MWh
Yes.£40m ÷ (200 × 500) MWh. Enormous per unit, and the point of it was never the units — it was that the lights stayed on in the worst week.
What is the fair summary of where the difficulty actually lies?
Yes.It explains why cheap solar and a hard transition are both true at once. The remaining problems are timing, storage, and heat — and each is a distinct engineering job rather than a matter of enthusiasm.
Not quite.That has fallen dramatically, and the difficulty stayed. Which is the clearest sign it was never the binding constraint.
Not quite.Demand reduction is powerful — you did that arithmetic in lesson one. It is not what makes the last tenth hard.
Lesson complete
The last tenth is not more of the same — it is cover for the worst week.