Lesson 04 · 3 min · 5 things to do
Storage is a time machine that leaks
Predict what storage can and cannot solve.
A battery is charged with 100 units of electricity. Later it gives back about 88. Where did the rest go?
- Yes.Every round trip through storage loses something. Batteries are among the best at about 85–90%; pumped hydro is around 80%; converting to hydrogen and back is far worse.
- Not quite.Self-discharge is real and slow. The bigger loss happens in the conversion itself, at both ends.
- Not quite.It was stored and returned, minus the losses of moving energy in and out of a chemical form.
Storage moves energy through time, and charges a fee in energy for the journey. Nothing stores electricity for free.
Storage is measured in two numbers, and confusing them is the most common mistake in energy debate. Slide between them.
PowerEnergyBoth togetherHow fast it can deliver2 GWEvening peak demand46 GWPowerHow fast it can deliver — the size of the tap. Measured in gigawatts.
How much it holds4 GWhUsed in one evening180 GWhEnergyHow much it holds — the size of the tank. Measured in gigawatt-hours.
Hours it can run at full output2 hoursHours in a still winter week168 hoursBoth together2 GW for 2 hours. It can shave a peak; it cannot carry a still, cold week.
A country announces "5 GW of new storage". What has it not told you?
- Yes.Power without duration is half a specification. Almost all storage built so far is measured in hours, which makes it excellent for evenings and irrelevant to a windless fortnight.
- Not quite.Worth knowing, and it cannot be judged at all without the duration.
- Not quite.Useful for judging losses and lifetime. Duration is the number that decides what problem it can solve.
Move the control to see what changes.
Which of these problems can a few hours of battery storage solve?
Solar generation stopping at sunset.
A sudden generator failure at 3pm.
Two weeks of still, overcast winter weather.
A summer surplus you would like to use in January.
The evening demand peak.
Yes.Batteries are superb within a day and useless across a season. The seasonal problem needs something else entirely — interconnection, dispatchable plant, or a fuel you can keep in a tank.A store returns 80% of what goes in. To deliver 100 units to customers, how many units must be put in?
unitsYes.100 ÷ 0.8. Every unit that travels through storage requires a quarter more to be generated — which is why the cheapest storage is always not needing it.Why is hydrogen discussed for seasonal storage despite poor round-trip efficiency?
- Yes.The trade is efficiency for duration. Losing over half the energy is a terrible deal within a day and a serious option if the alternative is having nothing in February.
- Not quite.It is much less efficient — often under half, once you count making it, storing it and burning it.
- Not quite.It is not, at present. The argument is about what a system needs for a rare, long, still period.
Lesson complete
Storage is measured in power AND duration, and the duration is what tells you what it can solve.
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