How to Read a Generation Mix Chart
A generation mix chart shows what electricity was made from over some period. Reading one correctly takes three questions: is the vertical axis energy or share, what period does each slice cover, and is anything missing from the total. Get those wrong and the same chart can support almost any argument, which is why the same picture is used to prove opposite things in public debate.
Energy or share
Charts on this site come in two forms and they answer different questions.
Energy, in terawatt hours. A terawatt hour is a billion kilowatt hours. This axis shows the absolute quantity of electricity generated, so the height of the whole stack rises and falls with total generation. Use it to ask whether coal burning went up or down, or whether a country generates more electricity than it did ten years ago.
Share, in percent. Every year adds to 100, so the stack is flat topped and only the proportions move. Use it to ask what the electricity was made from.
The two can move in opposite directions, and this is the most common way electricity charts mislead. If demand grows by 5% and solar grows faster, the solar share rises while coal generation also rises in absolute terms. A share chart alone shows progress; an energy chart alone shows a problem. Looking at both is the only honest reading. Our country pages show them side by side for this reason, for example on the Germany page.
Stacked areas and stacked bars
Most mix charts are stacked: each fuel’s band sits on top of the last, and the top of the stack is the total.
Read the thickness of a band, not its top edge. A band whose top edge rises may be no thicker than before, because something underneath it grew. This catches almost everyone at least once.
The order of the bands is a choice, not data. Putting fossil fuels at the bottom and renewables at the top is conventional, but it makes the renewable bands harder to compare across years, because they float on a moving base. Where a single fuel matters most, a separate line chart of that fuel alone is clearer than any stack, which is why our fuel pages plot one fuel at a time.
Bands for small fuels can be invisible at chart scale even when they are significant in absolute terms. The hidden data table underneath every chart on this site exists partly for that: anything too thin to see can still be read as a number.
What is not in the stack
A generation mix chart shows generation. Several important things sit outside it.
Imports and exports. Net imports are a flow, not a fuel. A country importing a fifth of its electricity has a mix chart that describes only the four fifths it made itself, and the carbon intensity of what it consumes may differ from the intensity of what it generates.
Demand and losses. Generation is measured at the power station. Some is lost in transmission and distribution, and some is consumed by the energy industry itself, so generation is always larger than what customers use.
Storage. Charging a battery is consumption and discharging it is not new energy. Datasets treat this differently, as described in our note on how electricity statistics are collected.
Capacity. A mix of installed capacity in gigawatts looks quite different from a mix of generation in terawatt hours, because different plants run for different fractions of the year. That gap is what capacity factor measures, and confusing a capacity chart with a generation chart overstates wind and solar and understates nuclear.
Annual against hourly
This site carries both, and they are not interchangeable.
Annual charts, on the home page and every country page, are compiled statistics. They are complete, comparable between countries, and months or years behind the present. They are the right tool for trends, and they say nothing about whether the lights stayed on last Tuesday.
Hourly charts, on the US grid pages such as CAISO, come from grid operator readings. They show the daily shape: the solar rise and fall, the evening ramp as the sun sets while demand peaks, the night time floor. They are provisional and revised. They are the right tool for understanding how a grid actually operates, and a poor tool for comparing one country with another.
A grid that is comfortably low carbon on an annual chart can still be running gas at 7pm in January, and only the hourly view shows it. The pattern this produces has its own name and its own explainer, the duck curve.
Small print that changes the reading
Different end years. Countries publish on different schedules, so one may have data through last year and its neighbour only through the year before. Comparing a country’s latest year with another’s latest year can compare different years. Our comparison pages hold the year constant for this reason.
Revisions. Recent years move. A figure read today may not match the same figure read a year ago, and that is the statistical system working as intended.
Zero suppressed axes. A vertical axis that starts at 300 rather than 0 turns a 3% change into a dramatic cliff. Check the axis before reacting to a shape.
Rounding and negatives. Shares are published, not recalculated, so they may sum to slightly more or less than 100. A fuel can occasionally show a small negative value when its plants used more electricity over the period than they produced.
A short checklist
Before quoting anything from a mix chart: check whether the axis is TWh or percent, check which year or hour it covers, check whether the source is a compiled statistic or an operator reading, check whether imports matter for this country, and read the number from the table rather than estimating it from the picture.