Generation data gives power at different times. Energy comes from power × time, and average power comes from total energy ÷ total time. Questions supply a table or a graph and ask for an energy total, an average or a comparison.
This lesson builds on comparing devices over an equal task and is part of power and energy resources.
How do I read the data?
Start with three checks before calculating anything.
- Axes and headings: is the quantity power or energy, and what are the units (kW, MW, MWh)?
- Time intervals: is each value an instantaneous reading, or an average over a block of time?
- Stated limits: some questions give the maximum possible output (installed capacity). Keep it separate from what was actually generated.
On a power-time graph, the area under the line is energy. For a table with steady blocks, each block is a rectangle: power × time. Where readings are rounded, your final answer should not show more significant figures than the data supports; the bounds and rounding explainer shows how to judge that.
Worked example
A solar farm has an installed capacity of 5 MW. Average power output in six 2-hour blocks of one day is shown. (Invented example data.)
| Time | Average power (MW) | Energy (MWh) |
|---|---|---|
| 06:00 to 08:00 | 0.5 | 0.5 × 2 = 1.0 |
| 08:00 to 10:00 | 2.0 | 4.0 |
| 10:00 to 12:00 | 3.5 | 7.0 |
| 12:00 to 14:00 | 4.0 | 8.0 |
| 14:00 to 16:00 | 2.5 | 5.0 |
| 16:00 to 18:00 | 0.5 | 1.0 |
Total energy: 1.0 + 4.0 + 7.0 + 8.0 + 5.0 + 1.0 = 26 MWh.
Average power over the 12 hours: 26 ÷ 12 = 2.17, so about 2.2 MW.
Share of the maximum possible over those 12 hours: maximum = 5 × 12 = 60 MWh, so 26 ÷ 60 = 0.43, about 43%.
In joules: 26 × 3.6 = 93.6 GJ.
Check: average power × time = 2.17 × 12 = 26 MWh, which matches.
The mistake to watch for
A common slip is to add the power column and call the result energy.
Mistaken answer: 0.5 + 2.0 + 3.5 + 4.0 + 2.5 + 0.5 = 13, so “13 MWh”
This adds rates and ignores that each block lasts 2 hours. The 13 is in MW, and the true energy is 26 MWh.
The correction is to multiply every power by its time first. Here every block is 2 h, so the answer is twice the power total, but the shortcut fails as soon as block lengths differ. Always show the multiplication.
Check yourself
1. From the table, which block produced the most energy, and how much?
Show answer
The block 12:00 to 14:00, 8.0 MWh.
2. A wind turbine outputs a steady 1.5 MW for 4.0 hours. How much energy is this, in MWh and in GJ?
Show answer
E = 1.5 × 4.0 = 6.0 MWh. Then 6.0 × 3.6 = 21.6 GJ.
3. Using the solar farm table, what share of the 26 MWh was produced between 10:00 and 14:00?
Show answer
7.0 + 8.0 = 15.0 MWh. 15.0 ÷ 26 = 0.58, so about 58%.
Where this leads next
Next, distinguish resource availability from reliability, which uses the same kind of data to ask whether supply met demand. The practice set mixes all these skills.
Students who can calculate but struggle to describe what a table shows can get targeted help in online one-to-one Physics tuition.