Are ‘Baseload Generators’ essential?

The term Baseload is used to refer to a minimum amount of power that an electricity system must always provide (i.e. the minimum rate at which it must generate electrical energy to run the various devices attached to it).

In other words, the demand for power never drops below this level. Which in turn means that sufficient supply (i.e. enough power stations) must always be up and running to be able to deliver this minimum amount of power. Naturally, this minimum occurs sometime in the middle of the night, when most people are asleep; so 3-4 am or thereabouts.

Within most traditional power systems are plants that tend to run all day and all night at constant power. That power level is usually close to their peak capability, which means those plants run at high capacity factors. Nuclear power plants are a good example. For reasons of both relatively low fuel cost and limited capability to turn output up and down quickly, it makes economic sense to keep operating these plants all day and all night, even at times of minimum (baseload) power demand. More expensive and flexible plants can be turned off.

As a result, constantly-running plants like these are sometimes referred to as “baseload” generators, or described as “delivering baseload”.

By contrast other supply options turn on and off during the day – either because of variations in natural resource (e.g. wind) or because they are better suited both technically and economically to changing their output to meet demand as it rises and falls (such as gas). The next couple of chapters of this course will focus more on demand.

So far, none of the above should be particularly contentious!

Understanding Reliability in a Power System

Where debate can become more heated is when discussions turn to the “reliability” of power systems – in particular “keeping the lights on”.

It’s not uncommon to read commentary that implies an equation between “baseload generators” and reliability in an electricity mix.

The argument goes that since nuclear power plants (for example) operate on a constant basis, they provide more reliable power than “intermittent” generators like wind or solar (for example), which are hostage to the weather. While you can say the former provide more “predictable” power than the latter, it’s important to recognise that this isn’t always the same as “reliable” power.

It’s possible to be predictable without being a determinant of reliability. Equally it’s possible to envisage future electricity mixes which are reliable despite high proportions of weather-dependent supply.

That’s because keeping the lights on isn’t about the performance of any single power plant; it’s about the performance of the overall system that they are a part of.

Within any power system, what goes out (demand) needs to match what comes in (supply) at any moment in time. As will be discussed over the next couple of chapters, the biggest challenge here is usually in having sufficient supply available at times of maximum (peak) demand, not when it is at a minimum (baseload).

In truth though, even the size of demand at any point in time isn’t what matters; it’s the ability of the system supply to follow it as it changes.

In other words, if demand suddenly rises, the aggregated supply of the system must rise at exactly the same time. Or if demand falls, supply needs to fall too. Not in half an hour, or an fifteen minutes, but now.

In other words it is flexibility of supply – the ability to react on the same timescale as demand – that creates reliability.

Beyond “Baseload” – Towards “Flexible”

So-called “baseload generators” such as nuclear power may be able to operate predictably for twenty-four hours a day at high capacity factor, but if demand changes quickly and they can’t react at the same pace, then the system still has a reliability problem. It is therefore not any magical property of these steadily-operating plants that is guaranteeing reliability. It may well be other fast-reacting supplies in the system (hydro, gas, storage etc.) that achieve this.

* Aside/Important Note #1: it’s also possible for baseload plants to suffer break downs, which are by their nature unpredictable, causing them to drop off a system at very short notice. Since they tend to be large plants, these events can leave sudden, large holes in the required supply mix – which is obviously very bad for system balancing. Power system operators plan for these eventualities by having backup supplies ready and available (and have done so long before the arrival of sources such as solar or wind).

* Aside/Important Note #2: Equally, it’s not the case that because a plant operates as a “baseload” generator, it cannot behave in a flexible, “load-following” way (load-following being another bit of terminology that means they follow changes in load/demand). In particular during this course, you’ll find that I generally talk about nuclear power as a “non-dispatchable” source of supply; one that doesn’t ramp up and down rapidly and/or by large amounts as demand changes. In some operating designs there are specific, technical constraints to load-following at rapid rates. However there also designs of nuclear power plant which are better able to load follow than others. France has a very high percentage of nuclear electricity generation (>75%), which would not be possible unless these plants were able to operate in “load-following mode”. In fact, from a technical perspective, recent designs allow excellent flexibility of operation.

However that doesn’t mean you would build a new nuclear plant to operate like a gas one! That’s because, in practice, the non-dispatchable, steady-generating nature of nuclear is driven mostly by economics. Turning the output of nuclear power plants down hardly saves any money (since fuel costs are small). And nuclear is expensive to build, so when not generating (or having turned down its output) an operator will still have to cover very large capital cost repayments and other fixed operating costs (staff running the plant and so on). With less income to pay for these fixed costs, the business model risks becoming untenable.

So, given the blurred lines between different sources, why talk of “baseload” at all?

In my opinion, rather than talking about baseload vs. intermittent generators, better would be to talk about inflexible and flexible ones. Supplies can be inflexible because they are dependent on the weather, or because, while suited to constancy of output, they are unable or unwilling to vary this output at a rapid, demand-matching (“load-following”) rate.

In either case, the reliability of the overall system relies on other, flexible sources within the mix.

So are baseload generators essential?

The short answer is no. There are many ways to create an electricity supply mix.

It’s perfectly possible to imagine one which could meet demand reliably by utilising weather-dependent sources when available, so long as these were complimented by a sufficient quantity within the system of flexible (“dispatchable”) supply to fill the gaps when they weren’t. What constitutes a “flexible” supply may involve some technical constraints though, along with a large dose of economic considerations.

Consider this hypothetical demand curve (in red) and supply mix (the shaded areas):

The y-axis on the left is in GW, with time of day across the x-axis. The grey “baseload” generation provides a steady output throughout the day. Solar, predictably, provides its maximum contribution at noon, with none overnight. Wind varies much more through the day, from zero at 11am up to 5 GW at 10pm.

Filling the gap above these baseload and variable supplies is the green area – some combination of “flexible” supply which can be controlled to ensure that the red demand curve is always met. The y-axis on the right is in GW per hour, and refers to the “ramp rate” curve (the black dotted line). This shows by how many GW this flexible supply has increased or decreased its output from the previous to the current hour. So, for example, the flexible supply ramps up sharply in the morning as demand rises, wind is quite steady and solar not contributing; and it ramps down (negative ramp rates) from about 5pm, as demand falls.

In this scenario, the green area (energy generated by flexible sources) is 182 GWh, nearly 45% of total energy demand during the day.

In future, let’s say we triple capacity of installed solar and wind and remove all our baseload supply. On a day with the same weather conditions (i.e. same solar and wind resource), we could meet demand by this mix:

There is one period, between, 10-11pm, when we have enough wind available to fully meet demand. In fact we are at risk of curtailment here (a situation, discussed again later in the course, where we have too much supply). Elsewhere in the day, we still require flexible generators to fill the gaps up to the red demand curve.

In total we actually use less energy from flexible generators here: 163 GWh, down from 182 GWh. In other words our increase in solar and wind has replaced not only the baseload supply, but also a chunk of the flexible supply too.

However the ramp rates of these flexible generators have increased: from a maximum of +2.5 GW/h and minimum of -3.0 GW/h in the previous scenario, to +4.3 and -5.0 in this one. The business challenge for our flexible generators is that they have less opportunity for energy sales, and are being asked to turn up and down (or on and off) more rapidly; a less efficient way to operate. That may not be an attractive business case!

Then of course, there’s the question of what happens in the above scenario, but on a less windy and/or sunny day!

The answer of course is that the yellow and/or blue areas will shrink, and the green will have to expand to make up the difference. So on a day like that, the flexible generating capacity will have to be sufficient to replace the baseload that’s been removed, plus any missing wind or solar energy (when the weather is unhelpful).

The conclusion?

Baseload certainly isn’t essential, but without it we do need to plan sufficient flexible supply into a system, to cover the range of variability of whatever else we are relying on. So we are likely to need both investment in a greater capacity of flexible supply (for when the weather isn’t helpful) combined with a business case which enables it to run in a much more variable way: turning on and off or up and down at greater rates.

It’s a question of Economics more than Reliability

So we can say that “baseload” plants are not essential. However whether such generators, which operate steadily, at high capacity factor, are a desirable part of the mix is a different matter. They could well continue to be.

That depends on the particular economics of the market being analysed and the sources available. It isn’t fundamentally an issue of either “reliability” or meeting “baseload”, it’s an issue of comparative generating and other system costs.

A high capacity factor means generating lots of delivered energy from a relatively small installed capacity (when compared to a low capacity factor source). By contrast a supply mix built entirely from a mix of variable and flexible sources is likely to require a lot of installed capacity, all operating at relatively low capacity factor (the variable sources because of limited resource availability, the flexible sources because they turn off when they aren’t needed).

Which situation – or which mix of steady, intermittent and flexible supply – makes economic sense depends on the various capital costs, fuel costs and other economic factors in the scenario considered. It depends on how demand changes in a market, on what is already installed and how long it will last, on how a system connects to others; along with assumptions on how the economics are expected to change in future (e.g. fuel, battery and other costs).

Most of all, it depends on the business models created by the revenues available within a given electricity market structure. Those revenues may be governed by free market supply and demand, or may be adjusted by policy mechanisms aimed at favouring particular outcomes.


(1) You’ll see a range of terms used to describe “variable” renewable sources: intermittent, unpredictable, unreliable and so on. Being clear on what we actually mean by these different terms (and hence making sense of what these sources really are) is important. This article attempts to clear some of the confusion!…

(2) During this course, I generally talk about nuclear power as an “inflexible” source of supply – one that we won’t turn up and down often and rapidly. Along with the important note in the text above, if you want to read a little more about the load-following capabilities of nuclear power plants, here are two pieces of further reading: a short summary and a longer report. Key point: flexibility/inflexibility is a property which depends not just on technical capability, but economic constraints too.