Week 07 — Storage Is Two Problems Wearing One Name
Power and energy as separate specifications, and which one a multi-day event actually tests.
Week 07 · duration · 7 September 2026
Power capacity and energy capacity solve different problems, and confusing them is how a system is under-built.
- Property
- duration
- Stance
- Complicates it
- How
- An illustrative 1 GW / 1 GWh battery brought to a multi-day event.
- Studio activity
- Manipulate power, energy, state of charge and round-trip efficiency against a fixed deficit.
“How much storage do we need?” is two questions in a trench coat. How fast can it deliver, and how long can it keep doing that. The first is power. The second is energy. A specification that answers one and assumes the other is how a system arrives at an event under-built.
How this complicates the thesis#
Storage is the intuitive answer to a drought, and this week makes the answer conditional. A battery sized for the daily peak — the problem a daily cycle poses — can be irrelevant to a multi-day event, because the binding constraint has changed from instantaneous delivery to total stored energy. The event does not just make storage more necessary; it changes which number matters.
In the session#
We start from a deliberately absurd illustrative configuration — nominal power of 1 GW paired with 1 GWh of energy, an hour of duration — and ask what it does for an event lasting days. Then you manipulate power, energy, starting state of charge and round-trip efficiency against a fixed deficit and watch which lever actually moves the outcome.
The slides are at the week 7 deck.
Its relationship to the canonical event#
The event is defined by a span, not a depth, and that is what makes duration the binding constraint. An event of the same depth lasting a fifth as long would be a storage problem a daily-cycle battery is already sized for.
What next week needs#
When storage runs out, something has to burn. Week 8 is about paying for a machine you hope never runs.
The storage sizer
Set power and energy independently. Watch which one binds.
Hourly deficit and what the store covers — % of installed capacity
Demand exceeds combined wind and solar output in every hour of this 72-hour window, peaking at a deficit of 39% of capacity. Covering the whole window would take 2386 capacity-percent-hours of stored energy.
Energy left in the store — capacity-percent-hours
Stored energy remaining, hour by hour, on the same time axis as the panel above. The two are kept on separate axes because a rate and a quantity are different measurements.
- Survived
- 5 h
- First shortfall
- h5
- Unserved energy
- 1019 %·h
- Binding constraint
- power
At 20% discharge power the store cannot meet the hourly deficit at all, so it fails at hour 5 with 1019 %·h unserved — and it still has energy left. This run is power-limited: adding stored energy will not move the outcome.
Demand is a flat 40% of capacity and the window is 72 hours around the reference event — both synthetic teaching parameters declared in one module Synthetic trace. No megawatt figure is claimed anywhere.
The spec
What to bring. Written so you can tell whether you have met it without asking.
- you can say what your portfolio's shortfall looks like hour by hour
- you can distinguish a megawatt from a megawatt-hour without hesitating