Availability asks whether the energy resource is present. Reliability asks whether the power source delivers the needed power at the time it is needed. Keeping the two apart lets you write accurate comparisons of energy sources.
This lesson follows reading supplied generation data and is part of power and energy resources.
What does each word mean in an exam answer?
| Idea | Question it answers | Examples of what limits it |
|---|---|---|
| Availability | Is the resource there? | Sunshine hours, wind speed, rainfall into a reservoir, amount of fuel left |
| Reliability | Can it meet demand when wanted? | Output changing hour to hour, time to start up, faults, maintenance |
| Renewable or not | Is the resource replaced naturally? | A separate classification; it does not decide reliability |
A resource can be available but not reliable as a source: wind exists, but the wind farm’s output follows the wind, not the evening demand. A source can also be controllable but not always available: a reservoir lets a hydroelectric station respond quickly, but a long dry spell lowers how much water is stored.
Worked example
A wind farm has an installed capacity of 10 MW. Over a six-hour evening, the grid needs 6 MW steadily from it.
The hourly average outputs are 8, 6, 2, 1, 4 and 9 MW. (Invented example data.)
Energy supplied: 8 + 6 + 2 + 1 + 4 + 9 = 30, and each value lasts 1 h, so 30 MWh.
Energy demanded: 6 MW × 6 h = 36 MWh.
Average output: 30 ÷ 6 = 5 MW, which is 50% of the 10 MW capacity.
Hours that met the demand (6 MW or more): 8, 6 and 9, so 3 of 6 hours.
Shortfall in the other hours: (6 − 2) + (6 − 1) + (6 − 4) = 4 + 5 + 2 = 11 MWh. The surplus in the other hours was (8 − 6) + (9 − 6) = 5 MWh, and 5 − 11 = −6, matching 30 − 36 = −6.
Conclusion: The wind was available throughout, but the farm was only reliable for half the hours of this demand. A written answer might say: “Wind is available in all six hours, but output varied between 1 MW and 9 MW and met the 6 MW demand in only 3 hours.”
The mistake to watch for
A common slip is to merge the two ideas.
Mistaken answer: “Wind is renewable so it is a reliable source.”
This answers “is it replaced naturally?” and treats it as if it answered “does it supply on demand?”.
The correction is to state each separately: “Wind is renewable, and it is available, but the output varies with wind speed, so it does not always meet demand without storage or another source alongside.” The data in the question, not the category, should decide your reliability statement.
Check yourself
1. A fuel-burning station has a full fuel store but is shut for repairs for a week. Is the problem availability of the resource or reliability of the source?
Show answer
The fuel is available, so the problem is reliability: the source cannot deliver power during the shutdown.
2. A solar farm gives at least 2 MW in 4 of the 12 daylight hours. What fraction of daylight hours is that, and is this reliability or availability data?
Show answer
4 ÷ 12 = 1/3 of the hours. It describes how well output reaches a chosen level, so it is reliability data for that 2 MW demand.
3. Explain in one sentence why a dry season affects a hydroelectric station’s availability.
Show answer
Less rain means less water stored in the reservoir, so less of the resource is available to drive the turbines.
Where this leads next
Next, explain a trade-off using stated assumptions to turn these comparisons into balanced written answers. The practice set includes a scenario on this.
If you know the terms but blur them in written answers, a teacher can mark your wording live in online one-to-one Physics tuition.