Reading the data: load curves and capacity factor

Every analytical conversation in this business starts with one of two charts. The load curve answers what happens when. The duration curve answers how often each level happens. And one number, the capacity factor, summarises a whole year of a generator in one digit.

Get fluent with these three and you can follow almost any meeting about energy strategy.

The load curve: what happens when

A load curve plots demand in MW on the vertical axis and time on the horizontal axis. A winter weekday in Sweden has a very clear shape.

flowchart TB
    A([Overnight, 00:00 to 05:00<br/>around 10,000 MW<br/>people are asleep])
    B([Morning ramp, 06:00 to 09:00<br/>around 16,000 MW<br/>heat, lights, showers])
    C([Working hours, 10:00 to 15:00<br/>around 14,000 MW<br/>gentle midday dip])
    D([Evening peak, 16:00 to 20:00<br/>around 17,000 MW<br/>cooking, lights, EVs])
    E([Wind down, 21:00 to 23:59<br/>around 13,000 MW])

    A --> B --> C --> D --> E

    style A fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    style B fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    style C fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    style D fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    style E fill:#dbeafe,stroke:#1e40af,color:#1e3a8a

A load curve answers questions like when is the peak today, how fast does the system ramp, did we hit a constraint at any moment. You read it left to right, like a story.

The duration curve: how often each level happens

Now take a whole year of hourly values and sort them from biggest to smallest. The shape that comes out is the duration curve. The horizontal axis is no longer time. It is number of hours per year where demand was at least this high.

flowchart TB
    T1([True winter peak<br/>around 26,000 MW<br/>50 hours per year])
    T2([Strong winter mornings<br/>around 22,000 MW<br/>500 hours per year])
    T3([Average winter day<br/>around 16,000 MW<br/>3,000 hours per year])
    T4([Shoulder season<br/>around 12,000 MW<br/>3,000 hours per year])
    T5([Summer trough<br/>around 9,500 MW<br/>2,000 hours per year])

    T1 --> T2 --> T3 --> T4 --> T5

    style T1 fill:#fecaca,stroke:#b91c1c,color:#7f1d1d
    style T2 fill:#fed7aa,stroke:#c2410c,color:#7c2d12
    style T3 fill:#fef3c7,stroke:#a16207,color:#713f12
    style T4 fill:#dbeafe,stroke:#1e40af,color:#1e3a8a
    style T5 fill:#dcfce7,stroke:#15803d,color:#14532d

A duration curve answers very different questions. How much firm capacity do I need to invest in. How many hours per year does my peaker actually run. If I shave off the top 50 hours, what happens.

A simple rule: load curve for operations, duration curve for investment.

Capacity factor: one number to compare generators

The duration curve shows the whole shape. The capacity factor shows just the average.

capacity factor = actual energy in a year / what it could have made if it ran flat out

A year has 8,760 hours. If a 1 MW unit produces 4,380 MWh in a year, its capacity factor is 50 percent.

flowchart TB
    N([Nuclear<br/>80 to 90 percent<br/>runs almost always])
    H([Reservoir hydro<br/>35 to 55 percent<br/>held back on purpose])
    W([Onshore wind<br/>30 to 40 percent<br/>limited by wind])
    O([Offshore wind<br/>40 to 55 percent<br/>steadier sea wind])
    S([Solar in Sweden<br/>10 to 12 percent<br/>short winter days])
    P([Gas peaker<br/>5 to 15 percent<br/>only runs when prices are high])

    style N fill:#dcfce7,stroke:#15803d,color:#14532d
    style H fill:#dcfce7,stroke:#15803d,color:#14532d
    style W fill:#dcfce7,stroke:#15803d,color:#14532d
    style O fill:#dcfce7,stroke:#15803d,color:#14532d
    style S fill:#dcfce7,stroke:#15803d,color:#14532d
    style P fill:#dcfce7,stroke:#15803d,color:#14532d

This is why 1,000 MW of new nuclear and 1,000 MW of new solar are not the same product. The nuclear unit produces around 7,500 GWh per year. The solar plants produce around 1,000 GWh per year. Same MW on the press release, seven times the energy from the nuclear unit.

Capacity factor is the number that lets you turn a MW headline into the MWh that actually matter.

Next

The same MW vs MWh split, but for storage. See Storage on the grid.